Yaqin Tube Valve Amplifiers

Comment from James
Time: June 18, 2008, 4:55 pm

Good info Dan.

I Look forward to getting hold of mine – currently on a slow boat !

Comment from Paul Dixon
Time: June 27, 2008, 5:26 pm

So what do you people have these hooked up to? I’m interested in a low cost tube amp but am having problems getting info on all these chinese designs. Antique Soundlab, Yaqin, Dared, Music Angel, etc the list goes on. Any advise?

Comment from ove m
Time: July 1, 2008, 6:28 pm

I hav my mc 10 l paird with the epos es 14 and it is no les than midblowing.I also use a rega p3 and i must say i am more than satisfied.tight bass and a awsome midrange with a swet topp..

Comment from bert geddes
Time: July 21, 2008, 6:38 am

Hi Dan,I have a markhill mc 10l which appears to be the same as yaqin,can you please advise ?.

Comment from Dan Ashby
Time: July 21, 2008, 1:42 pm

Hi Bert, as discussed in the wikipedia page, Markhill is a rebadged yaqin – http://en.wikipedia.org/wiki/YaqinAudio

Cheers,

Dan

Comment from Harry S.
Time: October 3, 2008, 2:11 am

I am an owner of a Yaqin MC10L integrated. Have had very good experience with it right out of the box. After about 200 hours I decided to try new output tubes — Electro Harmonix EL34 valves from NewQuestSound. The improvement was noticeably positive and well worth the expense and effort. Then a further 200 hours later I purchased Sovtek 6N1 tubes from Triode Electronics and they too markedly improved the sound. I would say that this tube complement is easily more musical and highly recommended over the good stock Chinese tubes that came with the amp. This is my first valve amp ever and it has been a very pleasant experience thus far.

The event that started this all was my decision a year ago, to re-discover my vinyl record collection by building a Bottlehead Seduction 6922 based tube phono preamplifier. I included the CS4 upgrade at the very outset of the project. Then I bought a used manual Nitty Gritty record cleaning machine (really works and is truly a must!). Next I bought OFC interconnects, started to prowl the used LP aisles at Princeton Record Exchange and the rest they say is history…

Comment from Dan Ashby
Time: October 3, 2008, 9:14 am

Hi Harry S. That’s great to hear your positive experience with a Yaqin product. I must investigate the Nitty Gritty machine as I’ve also recently rediscovered my vinyl. I must confess that I’ve been listening through a rather nice homemade opamp phono stage though, which may offend true valve-heads!

Dan

Comment from Harry S.
Time: October 8, 2008, 10:49 pm

A few points of clarification are requested…in the bias adjustment guide offered by this web site, the following text is found, “So for example, if you measure 420mV across the 10 Ohm resistor you can calculate that 420/10=42mA (or perhaps more clearly 0.42 Volts/10 Ohms=0.042 Amps). Based on the desired anode current for the valves supplied and the operating voltage, around ~40mA is spot on.”

What is the desired anode current for EL34 tubes when the mains voltage is 120VAC? Is it 40mA or some other value? Is the “operating voltage” referenced in the guide, the measured voltage across the 10 Ohm resistor?

Are other power tubes such as EL84, KT77, or KT88 safe to substitute in this amp provided the correct anode current adjustments are made?

Any help in this regard would be appreciated.

Comment from Dan Ashby
Time: October 9, 2008, 5:51 pm

Hi Harry.

The desired anode current is the same regardless of the initial supply voltage (in that sense, current is current, as it were). The mains transformer does the job of supplying the correct voltage based on the supply. Yes, I’m referring to the measured 420mV as the operating voltage. FInally – absolutely do not just stick any old valve in. You need an electrical and pin equivalent valve – and you would still need to check what the anode current should be. More info on the EL34 is found here http://en.wikipedia.org/wiki/EL34 and many other places on the web.

Dan

Comment from Harry S.
Time: October 18, 2008, 2:51 pm

Not to be thick…but do all EL34 tubes have the same desired bias regardless of manufacturer? i.e., do Electro Harmonix El 34 tubes, Mullard EL34 tubes, etc., have the same measured 420mV bias expectation across the 10 ohm resistor?

Comment from Dan Ashby
Time: October 18, 2008, 6:37 pm

Hi Harry,

Ideally check the data from the manufacturer – they are certainly not all the same.

Dan

Comment from colin
Time: November 4, 2008, 9:54 pm

I Have for the last 4/5 weeks been using one of these mc 10l things .power in dropped through an auto transformer to the required 220v , hence keeping the valve heater voltage to the correct 6.3v.Make sure you do this or use one with the modyfied transformer for 240v operation.I am using a linn axis/g1042 combination using the pre amp stage from a nad 3020a(exceptional preamp design) , the mc 10l and roger ls4a’s Ihave not yet been able to catch it out amazing detail ,bass mid and treble well balanced.have also used wharfdale valdus 400 foor standers , with these ok but bass loose and treble tinny without separation(ie could no longer dishtinguish different symbol types when used. currently waiting for lone unit of thr larger kt88 amp for a comparison ,together with the same manufacturers phono stage.

Comment from Dan Ashby
Time: November 5, 2008, 11:42 am

Hi Colin,

Good to hear that you are enjoying the MC-10L. I’d like one again if I had the space!

Dan

Comment from Colin Jones
Time: November 24, 2008, 8:23 pm

Time to retire the mc-10l. Have now been able to review Yaqin mc-100b a kt 88 dual mono block amp.
Replaced the rogers speakers with Linn Isobariks ,still using Nad 3020 as phono stage and Linn Axis turntable.
The amp has had specially wound power transformers suited for 245 volts.Using pro input on amp for phono stage.This amp and speaker setup is heads and shoulders above any system I’ve ever heard.All I need now is a good valve phono stage to become street cred(the Nad will have to go ,even though its good).Use 4 ohm output on amp for isobariks as their impedance can drop low at some frequencies.The only thing left to do with this setup is to listen and listen ,hence enjoy the music because all the detail comes through.

Comment from Dan
Time: November 30, 2008, 1:31 pm

Hi,

I’m thinking about getting this amp but I’m confused about voltage – I live in UK – can I use it straight out of the box in UK or do I have to tweak something? Thanks!

Comment from Dan Ashby
Time: November 30, 2008, 3:34 pm

Yes, you can use it out of the box. I found the model I had more or less in spec for 230Vac.

Dan

Comment from Rys
Time: December 2, 2008, 3:28 pm

MC-10L hum -advice please
I have had the MC10l for nearly two years now and I am very pleased. However it has, over time developed a hum in the right channel only. Its quite audible with no music playing; adn I suspect it does deteriorate the overall sound with music on.
Any suggestions on how to solve this? (it is not plugged in via a mains conditioner and has its own plug)
Thanks
Rys

Comment from Dan Ashby
Time: December 2, 2008, 5:04 pm

Hi Rys. I have no specific advice, but of course it is always sensible to begin by swapping the valves about to see if the problem either disappears (bad connections) or moves to the other channel (bad valves). You should be able to isolate it to the power tubes (or not) by removing the 6N1s first and running it up without them in. Next carry out a thorough visual inspection. Look for weak/dry joints or components showing signs of excessive heat etc. Beyond that it’s time to get the multimeter and schematic out.

Dan

Comment from Rys
Time: December 2, 2008, 6:01 pm

Thanks Dan Appreciate your time on this. I’ll give your suggestions a try

Comment from ah luo
Time: December 11, 2008, 6:17 am

I wonder if anyone has tried using KT88 on MC10L ? From the circuits between the MC 10L vs the MC100B, it appears that the MC10L will take the KT88 tubes with some bias re-adjustment ?

Comment from Martin Hann
Time: December 28, 2008, 3:07 pm

Hi
I am interested in either a mc10l or mc100b.I now live in France could I use either amp straight out of the box,the rest of my system is Rega P5/Quad 57’s .Could you advise me on both amps, thanks . Regards Martin

Comment from Peter
Time: January 2, 2009, 7:03 am

Anyone know how to get a clearer copy of the circuit diagrams for the MC100B? I am interested the value of the the capacitor in the global feedback. I can’t read it in the diagrams that I have found on line.

Comment from ah luo
Time: January 2, 2009, 9:24 pm

I think it is 500UF and 22K connected in parallel.

Comment from ah luo
Time: January 2, 2009, 9:28 pm

Have anyone tried using a 6922 tube replacing the 6n1 on the MC10L. Though I have read in two postings that it works, I wonder if the 6922 will be operating at a too high the plate voltage ?

Although the heating elements draws less current by the 6922, its max plate voltage is much less than the 6N1. When looking at the MC 10L schematic, it appears that the top 6n1 of the SRPP pre-amp has a plate voltage of ~170V across !

Comments ?

Comment from Sal
Time: January 6, 2009, 8:49 pm

I have replaced EL33s with KT88s on my MC-10L. The overall sound and especially the base is much better. I believe the sound of original EL33s were pretty thin. I did not readjust the bias. MC-10L has been blowing fuses these days and it has gotten pretty bad since it just blows the fuse when it is turned on. I will have to find someone in the area to fix it.

Comment from Sal
Time: January 6, 2009, 10:03 pm

Correction,

I replaced EL34s with JJ KT77 and not (EL33s with KT88).

Sal

Comment from Serge Tremblay
Time: January 7, 2009, 4:23 pm

I am driving a MC-10L since 2 years. From a couple of weeks I am earing lots of distortion from the amp. yesterday I openned the amp to measure the BIAS. One tube on each channel is ok with around 0.37VDC. but others are amazing 78.00VDC and 38.00VDC. what is the problem?

Comment from ah luo
Time: January 11, 2009, 4:27 pm

Suggest swapping the tube positions and re-measure to see if it goes with the tube. Tube problem or capacitor leakage.

Comment from Sal
Time: January 11, 2009, 6:55 pm

Has anyone seen a complete bias adjustment document with pictures for novices. I need to check the bias (and possibly change) on my MC-10L.

Comment from Dan Ashby
Time: January 12, 2009, 9:59 am

Sal – what is missing from the bias adjustment guide above? – if you can’t follow that you probably ought to seek help from someone who can.

http://yaqin.slickpepper.org.uk/wp-content/uploads/2008/06/mc-10l_bias_adjustment.pdf

Thank,
Dan

Comment from Sal
Time: January 13, 2009, 6:58 pm

Hi Dan,

Being a nivoce in this area, I wasn’t too sure where to connect the crocodile clips. Reading the article again, I believe they have to be connected to the resistors? on the bottom of the screen.
Thanks,
Sal

Comment from Dan Ashby
Time: January 14, 2009, 10:03 am

Hi Sal, I haven’t explained it that well, but in fact the brass hex posts to the right and left of each resistor are connected to the legs of the resistors so they make the ideal place to connect, you could connect directly to the leg of the resistor if your croc clips are small enough. I hope that makes sense?

Dan

Comment from Sal
Time: January 14, 2009, 6:26 pm

Hi Dan,
As you know there is not much information on these amps. I really appreciate what you have done and also your informational site explaining these amps. Your last post made it very clear to me what I need to do in order to complete the task.

Thank you,
Sal

Comment from Dan Ashby
Time: January 14, 2009, 10:43 pm

Hi Sal,

It’s been my pleasure to help you. Hope you visit again sometime.

Dan

Comment from Ron
Time: January 16, 2009, 5:55 pm

I have also a Yaqin MC10L (new version). I want also to replace the EL34 Shuangang valves by Electro Harmonix EL34EH . Is it necessairy to change the bias? Or can it be the same.

Ron
Netherlands

Comment from HSTOM
Time: January 16, 2009, 6:02 pm

I have the upgrade bug and would like to know whether it is permissable to offer my MC10L for sale in a comment. Let me know, thanks!

Comment from Dan Ashby
Time: January 16, 2009, 6:08 pm

Hi Harry, I’ve made a FOR SALE page – please comment on there…

http://yaqin.slickpepper.org.uk/for-sale

Dan

Comment from ah luo
Time: January 17, 2009, 5:34 pm

Be careful of lethal high voltage if you have the older version and need to open up the enclosure.

Do go to the DIYAUDIO forum, there is a long threads of discussions (with photos, schematics, etc.) on modifying and tweaking the MC10L.

Comment from tony
Time: February 1, 2009, 9:05 pm

Hi, Can anyone help I’ve just got my self a second hand MC-10L amp which is the mark one version. Being a complete novice to the world of valve amps, I’m not sure if my Mordaunt short 902s are going to be a good match. The speakers are 89db
Would be grateful for any feed back
Thanks Tony

Comment from Les
Time: February 6, 2009, 3:09 pm

Many folks think that changing valve types will not mean re-adjusting bias. This is false, you should always check the bias when fitting new valves or simply just moving them around. It is quite easy to do with a multimeter, decent connecting leads and a trimming tool. This latter item may be hard to obtain for the older MC10L, the newer designed MC10L comes with a trimming tool. As it is tricky to connect the multimeter across the monitor resistors, especially the older amps, I use a special monitor box I made myself. The circuit is here http://homepage.ntlworld.com/lez/YAQIN%20MC10/
I have some of my re-drawn circuits here as well, the original Yaqin one on graph paper has errors.
Les

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Time: April 21, 2009, 8:51 pm

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Comment from Alfonso Calvo
Time: June 10, 2009, 9:22 am

Hi, a view which seems to me that I have explained badly or not at all well, the amp has the electronics technician who can fix it by welding the tracks with copper wire but it is a “fudge”, and also expensive, what I want is that it is like when you buy and this is achieved by changing the motherboard and that is the problem getting the dichosa plate, placing it as an example if I buy a new car manufactured in China and 2 years is damaged, it took but the mechanic did not find the part to return to work, but says it can fix a botched job and doing that of course will not be as before, specifying what I need to know is how to get the motherboard.

Comment from Dan Ashby
Time: June 10, 2009, 9:34 am

Hi Alfonso, I would try the suppliers on ebay. On the other hand I don’t see that reparing it is a bodge – if done properly it should have no effect on the performance of the unit.

Good luck,

Dan

Comment from dragonX
Time: June 30, 2009, 9:28 am

Got two of them, one the new 2009 model, one from ~ 2006. Reached plenty of sound-improvement by changing the caps to Wima MKP10 and changing V1/V3 tubes (ECC85) , V1/V4 (6N1P-EV). Now I reached the X-Effekt with Spendors s8. – Also the cathode-resistors should be changed (<2009′) only for security reasons. The Chinese have changed them too in the 2009′ model. The main transformer has also changed- now i found better 6V in the heating circuit (it was 5.8-5.9V) – Also try both speaker-connectors, some improvement may happen. You may try this, but never try to drive the amp without any resistor at the speaker-connector. If the amp doesn’t ’see’ any, damage may happen to the output-transformer. – So at all I’m very pleased with the amp(s) after some ‘improvements’. They don’t reach the level, a bewitch 6550 provides, but this is another story…..
I’ve tried also different powertubes (EL34 EH, 6CA7 EH, E34L JJ- all matched quarts) would recommend the cheapest (EL34EH) if you are on the ‘warmer’ side and 6CA7EH for more details, tighter bass and fine highs…..
Sorry for my bad english!
So, if there was a recommendation for the ‘old’ MC10L, you shouldn’t hesitate to buy the 2009′- right out of the box it sounds almost as good as the ‘improved’ 2006′ or older model. Add two Ecc85 and two russian otk-tested 6N1P-EV (better not the 6N1-P) and you will obtain a very lively sounding amp (with enough punch to drive almost every speaker without effort). – Don’t forget to adjust the bias- most important if trying 6CA7!! I would recommend to put it to minimum before changing! After warming up a little put it to rather ‘cool’ 30mA and after breaking in some hours you may try different settings.

Comment from Same64
Time: July 2, 2009, 11:05 am

HI dragonX .
I ‘ve got at the moment 6n1P-ev .Can i safely put ECC85?.
I thought about it but not realy good at electronics.
Also i was looking at bewitch 6550 and Audioromy Fu13 .What would you recomend?
regards

Comment from dragonX
Time: July 4, 2009, 8:30 am

@Same64:
You can! (change 6N1P-EV to ECC85) . Some of the 5881/B.Melody (6L6/5881) come with ECC85 today!
I’ m not sure which of the two, Bewitch 6550 and Audioromy Fu13 I would prefer. (because i have only the bewitch myself and haven’t seen the A. . – Let’s have a look. – The A. is a rather hot beast. ~900V a rather difficult bias adjustments, if necessary- and if you buy an amp from china you have to check first.
Transmitter tubes in the power section….. if you get them easily??? The 6550/bewitch: no prob.
Have a look youself: http://www.diyaudio.com/forums/showthread.php?threadid=139464 (Sorry most of it in german)
Some had rather weird probs with the Fu13, but they say the sound is outstanding.
Which speakers you have connected to the MC10L and which driver you want to use with the new amp?
Have you changed anything in your MC10L? Better Caps, feedback adjustment, …..?

Comment from Ryan
Time: November 25, 2009, 10:26 pm

Can someone tell me what I re-bias to when I swap in KT88′

Comment from Josef
Time: December 4, 2009, 7:54 am

Hello. I just purchased an MC-10L and it should be here next week. I expect it won’t come with any instructions and I am concerned that I will end up having to guess where to install the tubes and such. Also I think I can manage the bias adjustment but is there any other sort of primer or beginner’s guide for owning a tube amp? I just read through all the comments and will check the DIY audio guide as well. One other question. In looking at pictures of this amp I seem some with a sort of “floral” pattern that appears to be etched into the copper (or is it copper?). Is this standard or is that a custom job? Thanks again.

Comment from Dan Ashby
Time: December 4, 2009, 2:07 pm

Hi Josef,

You simply put the valves in the sockets they fit. They are not set up per valve. My experience was that they were labeled anyway – though for no reason other than peace of mind.

No floral pattern as standard and it is not copper – it’s just coated mild steel.

Best of luck,

Dan

Comment from Les
Time: December 4, 2009, 6:21 pm

Hi Josef, you will need a multimeter for checking the bias, preferably a digital one. When the unit arrives you will find the tubes packed separately, one set of small 6N1’s and another set of larger EL34’s, each tube in each set with numbers 1 to 4. So EL34 number 1 goes into the V1 position and EL34 number 2 goes into the V2 position etc. You do the same thing with the smaller tubes. You would have now placed the tubes in the exact positions they were in when the MC10L was adjusted at the factory. You should also find in the packing a mains lead and a long trimming tool with a hexagon end, this is used to adjust the bias as and when it is required to be re-set. You will find a set of two test point access holes near each of the EL34 tubes and these will allow your multimeter to measure the bias current of each valve by way of each valves cathode resistor so you need to set your multimeter to Volts, probably the best range will be 2V (or 2000mV) DC. You should expect a reading of approximately 0.35V (or 350mV). Most folks would be happy just to get the readings almost identical with each other and whilst it does not give the ultimate perfection it is a reasonable standard to adopt. My Yaqin is an early Mark 1 and has given faultless operation so far though it is a pain having to remove the top each time I check the bias, something you will not have to do with the newer model. My best tip for you is this:- If you accidentally or otherwise switch the amp Off to do adjustments, allow a few minutes for the tubes to cool before re-applying power. Failure to observe this rule causes a much greater then normal in rush current to the output valves (still awaiting bias supply to come fully up) and hence to the mains transformer and may cause premature fuse failure or other damage. Also make sure your speakers are connected to the rear terminals, unlike solid state amplifiers, tube amps do not like being driven with no load on them.
You may want to add a remote controlled volume and switch box later, I could not live without the one I knocked up and by feeding the switched outputs into just one of the Yaqin inputs, you will not get the cross coupling problems you get with the Yaqins simple input switching arrangement.
With proper care and feeding you will be rewarded with a long and happy time with your new amp, you may now sit back and hear rock guitars as they were recorded…via a tube amp. Enjoy!
Les

Comment from Ryan
Time: December 4, 2009, 7:08 pm

Les could you provide more info on your remote volume control setup, website or pics would be great. Cheers

Ryan

Comment from Les
Time: December 4, 2009, 9:52 pm

Hi Ryan, Well the basic remote controller was supplied by Evolution but they show it as currently not available, maybe they are updating it?
The info is available at:-

http://81.134.141.187/epages/Store.storefront/?ObjectPath=/Shops/Store.TechSupplies/Products/PPM060

I have used this module to make a few of these controllers, no doubt it could all be done by a PIC, I just used what I had in the junk box and it was originally just intended as a one off.
The complete write up I have placed on the space provided by my ISP:-

http://homepage.ntlworld.com/lez/IR%20Remote%20control/

I am toying with the idea of making my own IR receiver/decoder and control when time allows, maybe using an arduino board. I think I would also use just a row of LED’s for indication as the hexadecimal device is way too expensive even though it looks cute.
Cheers and hope the links work ok
Les

Comment from Les
Time: December 4, 2009, 10:41 pm

I note that some peeps have been experimenting with replacing the 6N1′’s in the front end with ECC85 tubes.
The instructions I have read refer to V1/V3, this is WRONG! The front end tubes are V1 for the Left channel and V4 for the Right channel. i.e. the two outside 6N1s. Be careful however as the heater/cathode limiting values are a little lower for the ECC85 and I have had oscillation problems with some ECC85 tubes I have tried.
With regards to performance, I did notice a very slight increase in HF but I would not like to comment further without proper test equipment replacing my 60+ year old lug holes. Maybe there is more but I cannot hear it LOL.
Les

Comment from same64
Time: December 5, 2009, 12:24 am

Hi there..
From my expierence i just did replace 6n1 with russian tubes EB and after a months it was like magic.
Chinese El tubes actualy are very good..I’ve got Audioromy now but still miss that yaqin sound.
poeple keep forgetting that not tubes are the same.In Audioromy guys tried to replace 813 chinese tubes with western versions and all of them were sounding worse.
As for the Yaqin i think better caps and russian EB tubes is just enough.

Comment from Ryan
Time: December 7, 2009, 3:40 pm

Les, outstanding project and writeup. Have you modified the yaqin to bypass the volume pot or do you have it set at a fxed position.

Comment from Rob
Time: December 16, 2009, 7:35 pm

Hey guys I’m looking to get a yaqin 10L, 300b or 100b.
This is a question iv posted elsewhere just woundering if you guys can help.

Hey guys. Just after abit of advice.
I have just bought my first home. It’s a bungalow, I have installed 4 ceiling speakers around the place to use as background fillers.
I want to be able to power these as well as 2 decent speakers which will
be in the living room. The inputs I’ll be using will be my iPod, dab radio, and maybe laptop.
The ceiling speakers do not have to be amazon quality but I want the sound from the two speakers in the living room to be amazin!

I have no idea really on the ohms etc for a speaker. I have not purchased the 2 speakers for thr living room yet so advice on that would be good to. Don’t want to spend stupid amounts either.
The Amps iv been looking at are the yaqin’s
mainly the 300b, 100b and the 10L.
Would these be up to the job etc.

Thanks for your time and I hope you can help.
Rob

Comment from Ryan
Time: December 17, 2009, 12:56 am

Any of those amps will do 8ohm or 4 ohm. If you wire your ceiling speakers in parallel to the floor standers you will have 4 ohm (assuming both are 8ohm). There may be better ways to wire this; if done in parallel you limit your floor standing speaker to the power rating of the ceiling speakers. You may be better off without the ceiling ones, and build yourself a nice set of floor speakers. Or install an A/B switch box so you can choose either one.

Comment from Mike Stevens
Time: January 4, 2010, 8:45 pm

I am new to the tube amp; purchased the 10L and it blew both fuses at about 10 hrs. each, powered continously; put a 4 amp slo-blo in and got a flash out of the corner of my eye and a pop across the speakers, turned it off. Yaqin said send the tubes back and they would replace; I opted to try some tung-sol in the meantime. Does the (B) suffix on an EL 34 tube mean anything significant? or can I use any EL 34? Thanks, Mike

Comment from Dan Ashby
Time: January 5, 2010, 4:46 pm

This page seems to cover you question, Mike:

http://www.voc-rock-guitars.com/Valves-Tubes/Shuguang-EL34B-valve.html

Comment from Les
Time: January 6, 2010, 9:48 pm

Sorry for late reply Ryan but I have been away, I have not altered the Yaqin in any way. I just feed the switch box into one single pair of inputs and set the front panel volume to max. All volume settings now done by the remote box. I found quite substantial cross talk across the MC10L inputs, it is a pity that Yaqin did not have say 470 Ohm feed resistors on every input and arrange for unselected inputs to be grounded. This would kill the crosstalk and the 470 Ohm resistors would protected the grounded inputs. I was going to fit an alternative wafer switch with a combination of shorting/non-shorting wafers but in the end I left it to the relays in my remote switch box to do same. The main problem is that Yaqin did not use separate screened leads, all four inputs are lumped together in one cable for each channel, Terrible!
Some mods I want to do is to fit a screen resistor to each EL34 g2 and I have seen High end amps using 10 Ohm cathode resistors like the MC10L but each fitted with a 220uF 63V electrolytic. Not sure if this will have any audibly noticeable effect but no harm in trying, will post here if there are any results, good or bad.
P.S. Having taken the amp apart I found a dry joint on g3 of EL34 (V2) – Right Hand Channel. It is a short link between valve holder and PCB and thankfully it was not on g1 which would have caused lots of damage. I checked all the links (3 on each valve holder) and made sure they were good soldered joints.

Comment from Les
Time: January 9, 2010, 11:33 pm

Well I can report a very favourable outcome to the changes I have just made.
I fitted 270 Ohms resistors in series with each output transformer tap that goes to Pin 4 of each EL34. I used some heat shrink sleeves to cover the resistor and the joint with the transformer wire. I next added a 220uF 63V axial electrolytic across each 10 Ohm EL34 cathode resistor, there being plenty of room under the board for these to be attached. I then replaced the ceramic 100pF feedback capacitor (C106/C206) with a mica version and fitted a variable 10k trim pot in place of the 5k1 resistor (R117/R217), setting the trim pot to 5k1 before installation. After setting up the bias on each valve to 35mA, I tried adjustment of the Left channel trim pot but was disappointed to find that it gave very little effect on the shape of the 1kHz square wave test signal I was applying. In fact, the square wave looked pretty reasonable, the trim pot having more effect on the amplitude. Setting the trim pot to a very low value gave some parasitic oscillation on the output so the trim pot was reset to its original position. I then adjusted the trim pot so that the maximum output on the 8 Ohm speaker terminal (8 Ohm dummy Load terminated) was 20dB with an input of -10dB. I then set the other channel trim pot to give the same readings on my RMS Voltmeter.
I swept the constant output generator on both channels from 20 Hz to 30 kHz and saw virtually no change on either channel.
Putting it all back together and yes man! The bass has definitely improved and the heart beats on Pink Floyds Dark Side of the Moon are testament to that.
I invited someone with far more critical ears then mine who was very familiar with the set up before and he was quite impressed with the results.

Comment from stephen.
Time: January 16, 2010, 12:42 am

Hello,
Ive brought a yaqin mc10 l amplifier which I love but a couple days after i first used it, Its developed a noticeable hum in the left speaker side The right side starts to hum slightly about 20 seconds after the amp is first switched on but it then quietens up but the left side stays noticeably louder.
If i remove the el 34’s from the left side; it stops but I’ve tried swapping the right valves over thinking that it maybe faulty el34 valves but its still the same
I was just wandering if you have had any experience with this fault before.
Best regards;
Stephen.

Comment from Les
Time: January 16, 2010, 5:33 pm

Hi Stephen, check (by removing all inputs) that you do not have an earth loop existing between the amplifier and other units. It might sound silly but an earth loop can sometimes catch you out in thinking there is a fault in the amp. If the amplifier still hums in isolation, even after changing the valves round, check the bias setting for each valve because if this is allowing too higher valve currents it places extra strain on the power supplies smoothing ability. The bias reading is so important and tells you how healthy the output stage is. Turn volume control to minimum and set each valve for 35mA which is 0.35V as measured using a digital multimeter across the bias test points positioned close to each output valve.
You may find one or more are drawing too much current (in excess of 0.5V for example) and may be a sign that one or more valves are miss-behaving. Hum is not a usual problem with these amps, they are very quiet. Also try to determine if the hum is 50/60Hz as opposed to 100/120Hz as this will show where the problem is,the higher frequency pointing towards a power supply fault whereas the lower points to earth loop or general pick up of the mains supply somewhere. Hope this helps you to narrow down the fault, get back if you still have problems.
NOTE: My MC10L is now almost 3 years old and no hum problems except when I connect my computer sound card due to a small earth loop. Too small to really notice but easily cured using an audio isolating transformer.

Comment from Les
Time: January 21, 2010, 8:25 pm

Following on from my early experiments, I made up some tag boards from standard tag strip. This is the stuff that has right angled earthing tags for screw fixing inter-spaced by two insulated tags. Cut it in short lengths (4 needed) so that there is an insulated tag either side of the mounting one. I fitted the tag boards to each EL34 position using the outer screws of each valve holder as this avoids disturbing a lot of the wiring. The insulated outer tags now take an 820 Ohm 5W Wire wound resistor with one side going to pin 4 of each EL34 and the wire that was already there is directed to the other end of the new resistor. Not only is this mod more technically correct for the amp but I swear it has improved the bass response to a level that gives a more pleasurable listen. You have nothing to lose as everything can be reversed if you want to do that but somehow I think you will not want to do that

Comment from Mick J
Time: January 31, 2010, 7:56 pm

Hi guys, I bought an MC10l last year and was using it tonight quite loudly and there was a spark from behing the right hand rear valve and the power went off! Sounds pretty bad. Has anyone else encountered this before?
Thanks.

Comment from Les
Time: January 31, 2010, 9:46 pm

Gee Mick, sorry to hear of that. Did the spark come from inside the valve or outside? The rear of the right hand EL34 has pins 1,2,3 and 4 which are G3, heater, Anode & Screen G2 respectively. I cannot see how anything could have arced here unless there was a trapped wire perhaps whose insulation finally broke down. It may have been a breakdown inside the valve? Is the right hand speaker connection ok as if it went open circuit a very high voltage would have appeared at the anode of the el34 and caused a flash over. Open circuit speakers can seriously damage valve amplifiers. While you have the top off, check the board fixing screw to the right of the output connector (X2) that feeds the right hand output circuit. Remove it and fit an insulating washer to it before replacing. On some boards this screw can accidentally place 300+ Volts onto the grid circuits of the right hand EL34’s. While you are at it, do the same to the centre screw between the small valves V2 and V3. This can likewise short out, not so dangerous but it takes an anode of one of the phase splitters done to ground. You MUST discharge the HT line before doing any investigation, the capacitors in the HT circuit can give a mighty punch! Check if there is any voltage on them before probing about, you can use something like a well insulated 1k resistor to discharge them, should take just a second. Look for any carbon deposits where an arc may have formed. Treat the valve as suspect unless you can find something else that exonerates it. Remember there is a fuse located on the supply input connector. DO NOT be tempted to increase the rating of this fuse. Try the amp with both right hand EL34’s pulled and see if it works ok on left channel, get that right before trying to find out why right hand channel failed. Hope this helps, Les

Comment from Mick J
Time: February 1, 2010, 12:07 am

Hi Les, thanks for your advice, it did appear to come from the valve it’s self. I will strip it and try what you have suggested. I will get some fuses and try it without the right hand valves in place. Speaker connections appear fine as I used a camb audio A1 tonight for a couple of hours and the speakers seem/are fine. Thanks agian for your input.
Kind Regards,
Mick.

Comment from mike stevens
Time: February 1, 2010, 3:12 pm

Mick J…. I had the exact same spark on the right rear valve area, caught it outa the corner of my eye, there was also a pop on the speakers. I pulled the factory tubes and put some tung-sol in; no problems anymore. I originally had a problem with the 2 supplied fuses both blowing after about 10 hrs apiece; got a 4 amp slo blo and solved that problem.

Comment from mike stevens
Time: February 1, 2010, 3:37 pm

This is my first tube amp; I bought a simple radio shack pocket size multimeter; with it set on DC 2V the readings are jumping all over the place…one second .23 then .67, etc…a large useless range. I have tried on all 4 valves with the same result; does anyone know what the problem is… amp or a defective meter?

Comment from Dan Ashby
Time: February 1, 2010, 4:10 pm

Measuring AC on a DC scale?

Dan

Comment from mike stevens
Time: February 1, 2010, 4:45 pm

I am posotive I have read over & over that the bias is done in the dc scale; I just assumed there was a conversion in the amp; in not a tech…. am I wrong?

Comment from pete
Time: February 1, 2010, 7:31 pm

Hi, can anybody help, I brought a second hand mark 1 MC10L a while back. It was running fine for about 6 months, but while listening one day the amp make a loud bang and smoke started to come out my new MA RS6 speaker on the left hand channel. When turning off the power to the amp I noticed that two of the small tubes 6n1’s had blown on the left hand side. I want to get the amp up and running again but do not want to blow more speakers, does the amp just need the bias adjusting?? or is there more to it?
Any advice would be appreciated

Comment from stephen
Time: February 2, 2010, 10:35 pm

hello Les,
Thanks for the help; I am pulling my hair out with the hum!!! I cant determine the frequency (no testing equipment other than a high quality digital/analogue multimeter me and my friend took the amp apart the other day looking for any earth / ground faults and all we could conclude was that if it was not a badly made valve set then all we could do was maybe change all the capacitors on the power supply side as the hum is now identical to the left and right channels and being one power supply to the output channels etc. we did notice however that f we tapped a output (el34)valve It can be heard quite clearly from the respective speaker being supplied almost as if the valve is situated too close to the supply transformer I also am using 6 ohm speakers plugged into the 4 ohm pins (i’ve tried the 8 ohm side which was identical) I will point out it is a quiet hum which does not alter with the volume or ruin the lovely clear sound. But is quite uncomfortable when listening in a quiet room.
I don’t know what else to do

Comment from Les
Time: February 4, 2010, 1:56 am

Wow! Quite a lot of faults here. To Mike,try the meter on a single AA cell and check it works ok. If it is working ok (Yes the range you need is 2V DC) and you still get erratic readings on all 4 bias points, suspect a faulty bias supply or one of the EL34’s misbehaving and affecting the other 3. Try pulling the EL34’s from one side at a time and see if the bias steadies, it might give you a clue.Remember too that the readings will change if there is any input so make sure the volume is turned down. Make sure the amp is off and cooled down before pulling the EL34’s.
To Pete, I am very surprised with your fault, I cannot see how high DC voltage caused the problem as the transformers provide some degree of isolation, DC overloads normally come with solid state amps but you can put speaker fuses in those, you CANNOT do that with valve amps as they must be loaded at all times. I rather fancy that it was a high output low frequency pulse that may have damaged your speakers. I would replace the 6N1’s that blew, pull the 4 output valves then try the amp. If no explosions, check the voltages around the now vacant EL34 valve holders, take care, around 450V here! Check that you have this voltage on at least 2 pins of each valve holder. This will be on pins 3 and 4 but as you are looking at the sockets from above, count ANTI-CLOCKWISE from the key way. You should also see on pin 5 a negative voltage, around -35V.
If this appears correct then try replacing output valves one side at a time. You must switch the amp off when removing/replacing valves as otherwise you will get high powered crackling on the output. I would recommend you replace your speakers, for fault checking, with two separate dummy load resistors. These should be 4 to 8 Ohms wire wound resistors rated at 25Watts, this will prevent you having to take chances with your speakers. If things appear stable and you can set up the biases, then try your speakers, maybe an old set just in case lol, but at this stage I would have to assume that all should be ok. I would normally use an oscilloscope for monitoring the output.
To Steve, try the amp with all four 6N1’s pulled before switching on. If you still get hum then yes it could be power supply ripple due to a bad cap. What is the HT? If it is a bit on the low side then maybe one of the diodes in the bridge rectifier may be open. You could also have hum on the bias supply (as this would appear on all 4 output valves) which would be easier to check if you had an oscilloscope or could borrow one. I fitted 100 k Ohm 1 Watt resistors across each of the large upright electrolytics as this helps balance the voltages across them. I think Yaqin fitted them in later models. If the hum disappeared when the 6N1s were pulled then suspect an earth fault as I doubt if a poor 6N1 would cause hum on both channels unless you have more than one faulty and here I would suspect a heater cathode leak. When you set the bias on each channel, make sure you set both valves to the same figure then by listening to the speaker on that channel, try adjusting one of the two bias pots for minimum hum. Adjust it only slightly as you do not want to deviate too much from the 0.35V setting point, this is why I use twin meters wired in series with the valves so I can keep an eye on the currents. I cannot adjust for minimum hum on my amp because I cannot hear it! I have to use a separate test amplifier with 100Hz filter feeding the oscilloscope, that is how quiet the Yaqin should be.
Hope this helps, Les

Comment from stephen
Time: February 4, 2010, 10:09 pm

Hello, les,
Thanks again for the advice, The hum is no different with the 6n1’s out; only with the el34’s out. Would you replace all the caps/rectifier first or would it be worth investing in other el34’s before I touch the inside of the unit?

Comment from Les
Time: February 4, 2010, 11:53 pm

Hi Stephen, If you get hum on any single channel even using any combination of the four EL34’s you have then it would appear more likely that you have a faulty capacitor or rectifier. The caps are easier to change than the rectifier. I recently bought a spare rectifier to fit if ever the original fails and took the opportunity to get one with a higher PIV (peak inverse voltage) rating, I think it is a 1300V 3Amp device. Due to their physical size, you may have trouble finding the caps, again I acquired 4 spare ones many moons ago, they were EPCOS B43504-A2477-M, 470uF at 250V. It is very wise to fit the 100K resistors I previously mentioned! I tried to order my caps from EPCOS who replied that they did not deal with small orders and sent me 4 for free! You could try same lol!

Comment from Robert
Time: February 5, 2010, 11:59 pm

I was considering getting one of these amps. I have seen problems and the like in this forum. Are these amps reliable? I have never had a tube amplifier before. The closest was a Mac 1700 and that was tube tuner. Any advice would be appreciated.

Comment from Les
Time: February 6, 2010, 1:18 am

Hi Robert, There must be 1000’s of these amplifiers in use and giving lots of pleasure. The fact that you see some posts on here or other Forums about problems does not necessarily mean that they are inherently unreliable. There are many more posts around the internet extolling the performance of this amplifier. The amp is a very reasonable entry into valve amplifiers considering the cost and after 3 years of faultless operation I am very happy with mine. All valve amplifiers need some TLC at times, valves age slowly and require periodic adjustment of their operating conditions. The latest MC-10L’s appear to have many improvements over my much older model. Like most other things, there are some changes you can do to improve reliability. The only downside I find of these amplifiers is that they are fairly compact and thus give little space for adding refinements such as easy replaceable extra fuses and circuitry for reducing high inrush current at switch on. However, my amp has faired well with very little changes except those that try to bring it up to the same standard as the new model. If you shop around you may find some bargains, it all depends on how much work you want to do to your purchase and perhaps looks may be important as well as sound. I recently acquired a £1500 Copland 501 for a friend, it was a bargain at just £50 plus a bit of metal bashing to get it up and running. But built into a 19″ case it is not very pretty for the lounge whereas the Yaqin and many offerings from other manufacturers are.

Comment from Les
Time: February 11, 2010, 1:22 am

Grasping straws here Stephen but did you say the EL34’s were micro-phonic? There is little gain on this stage and I would not expect micro-phonics to be a problem. I doubt if the magnetic field of the mains transformer would induce anything due to the shielding but I was just wondering if the mechanical vibrations from it are being picked up by the structures within the EL34′’s. This really is a long shot and borders on the insane. I have never heard of this happening, have you at hand a couple of EL34’s of different manufacture you could try. Les

Comment from Pat
Time: February 23, 2010, 6:07 pm

Hi all,
I recently received my Yaqin MC-10L from HK (e-bay seller), and was pretty much pleased with the sound out of the box. Unfortunately after a week of listening to it for an average of 4 hours daily there was a loud crackling noise coming from the left channel together with a red flash from the el34 v3 tube and the fuse blew. After replacing the fuse with the spare I checked the bias on both valves on the left and it went from the original 0.37 setting to over 10(both). It was still playing though the sound was duller, i tried to set down the bias all the way to the left but it did not budge a bit on either valve, right side was fine at 0.37 both. When trying to trim the bias on the left the potentiometer emited a loud high pitched noise, concerned that i might be causing more damage I unplagged the unit and decided to kindly ask for your more experienced advise. What do you think happened? and what should I do?
Please advise. Pat

Comment from Les
Time: February 23, 2010, 11:48 pm

Hi Pat, because the other channel can be set correctly it would point to the bias supply being correct.
I am of the opinion that one of the left hand EL34’s is at fault. Try removing one and see if the other can be set.
If still wrong, swap it for the other one and try the bias setting again. If this is now ok you know the first valve tried is suspect. If both give a fault symptom then maybe both are faulty but I think this would be unlikely or just bad luck. You could of course run the amp with both left hand valves removed and check for correct bias at the valve tube base sockets as detailed in an earlier post. Just be careful as there is 450V lurking on some of the valve sockets. One of the valves may have a damaged screen grid, possibly shorting to the control grid and hopefully you will find the faulty one by substitution or removal as I have detailed.
Les

Comment from Pat
Time: February 24, 2010, 2:58 am

Thank you very much Les. You have a very nice thing going on here, sharing is so much appreciated.
I will follow your suggestions and post the results, when I have some spare time. As for this postings, they really help.
Pat

Comment from Pat
Time: February 24, 2010, 5:37 pm

Hi Les,
I followed the steps you suggested, there is no doubt the right channel is fine, the bias there went up to 0.39 from the previous 0.37, but now I got a better meter and that might account for the difference. The left channel valves are both defective, maybe one went first and caused the other to fail as well. I measured voltage at the sockets and it is 504 volts at pins 3 & 4 and -42 volts at pin 5. This is more than the 450 and -35 you mentioned, do you think it might be damaging the tubes? if so, what can I do to bring it down to acceptable levels?
I have ordered a new set of EL34s (Tungsol), I hope they behave better than their sino counterparts. I also took several good pictures of the uncovered chassis of my MC-10L, it is the latest model I think. The transformers are not toroidal, just rounded covers. If you, or anyone here, is interested in the pictures just let me know how to share them and I’ll be happy to oblige.
I wish to, once again, thank you all for your dedication to keep this forum alive, I know it is not easy to find the time to do it, and as I have learned throughout my years that time is our most valuable asset.
Cheers!
Pat.

Comment from stephen
Time: February 24, 2010, 10:53 pm

Hello, Les. re. mc 10-l hum
Thanks for the previous advice. Ive changed all the awkward capacitors under the supply transformer apart from the 4-pin rectifier diodes i’ve brought some 1000v 1.5A ones which I was a bit worried about replacing as although on the mc10-l schematic it says 800v 1A the ones in my amp (2009 model ) seem bigger than the ones i’ve brought. The amp still has a quiet but annoying hum from both speakers like a mains hum which seems strange as you can hear a similar sound from the amp itself I’ve ordered the 4x 470 uf and the 4 22uf 250v capacitors for the other board which will be easier to change but I’m loosing faith any ideas?

Comment from Pat
Time: February 25, 2010, 12:28 am

I think these are bad news, I tried the two EL34s from the right channel, which are good and fine on the right channel, and the levels on the left are still skyrocketing, I think they are close to 100v I had to cut the power around there because it kept increasing. So far the right channel is fine, verified. The left channel, even with working valves, shows extremely high currents and cannot be biased. Any ideas guys?
Pat.

Comment from Les
Time: February 25, 2010, 2:01 am

Hi Pat, 100V is impossible as the valves would have to pass 10 Amps to get that across a 10 Ohm resistor!
But if the 10 Ohm cathode resistors were defective then you may see this. Check the 10 Ohm resistors are OK as the previous measurements you made would seem to rule out any problems with the bias supply. Set the bias controls to give maximum negative voltage on the grid pins, the valve currents should be low. My amp achieves 35mA with the bias controls giving -38 to -40V. The only other thing that can cause loss of bias is leaky coupling caps and I cannot believe both have failed. You can unplug the input to the left hand amp section to rule out any problems from the preamp/splitter. Check for any dry joints beneath the left hand amp section too, especially the little wire links that go from valve holder pins to the pcb, even if they look good give them a little tug to make sure.
To Stephen, I am at a loss here, you really need to find out if the hum is 50Hz or 100 Hz (60Hz or 120Hz in the States). The frequency will point you in a clearer direction as at present you may be chasing a non existent supply filter problem when it may be due to conducted mains from somewhere.
Les

Les

Comment from Les
Time: February 25, 2010, 2:26 am

Forgot to mention:
To Stephen, make sure there is no ripple on the bias supply as this controls the currents in the output valves. Any ripple on the negative 50V supply will be produced in the output due to modulation of the bias supply.
This will affect both channels!
To Pat, I am a little concerned about your amps high HT level, it should not be as high as 500V as the mains transformer should be giving 320V which after rectification will provide around 450V. Is the 6.3V AC heater rail high as well? If it is close to 7V you may have a 220V model and therefore an Auto transformer may be needed to convert 240V to 220V. There are other ways to drop the voltage but the Auto transformer is for me the preferred method and for a little extra get a high VA version, 250VA should be really fine.
Les

Comment from Pat
Time: February 25, 2010, 3:25 am

Sorry, I forgot to mention I am in the US, voltage is 120.
Pat.

Comment from Pat
Time: February 26, 2010, 10:10 pm

Hi guys,
Like many others here I am a neophyte in the realm of hifi circuitry, this thing was getting too heady for my current level of knowledge and my urge to just listen to the sound of tubes. I packed the MC10L and off I go to the local tube technician, I will let someone who knows more to bring the amp back to life, then, listening placidly to the warm tunes, I will invest some time reading and learning about the subject, better to be prepared, I intend to continue this hobby. For the time being this forum has been of great help, a very nice initiative.
I will post the tech’s findings once my rig is up and running again.
Thank you all.
Pat

Comment from mike
Time: February 27, 2010, 2:34 pm

Is there any problems with connecting a powered sub (carver hrs 12) to the amp? Would I stay on the 8 ohm terminals?

Comment from Dan Ashby
Time: February 27, 2010, 2:59 pm

Hi Mike, powered subs have such a high input impedance that they are effectively zero load. You can connect it however you want. It will make no difference. For convenience you could use whichever terminals you currently have free. Dan

Comment from stephen
Time: February 28, 2010, 12:15 pm

Hello,
Can anyone help Ive brought a mc10-l which is a beautiful looking amp the problem I have is an annoying mains type hum coming from the mainly one channel Ive changed the smaller capacitors under the mains transformer which did not help. All the bias readings are perfect so I presume The rectifiers are working o.k I have limited knowledge and no oscilloscope (even if I could work it!) So I decided the best option would be to just replace all the electrolytic capacitors on the bias supply
Ive brought some capacitors of equal value for the bias supply the large 470uf 250v ones I’ve brought are aluminum electrolytic’s I think and are made by rubycon and I’m not sure what make the smaller 22uf 250v ones are but I’m worried about swapping them over as I’ve noticed that both ones are physically smaller than the original ones fitted are they a poorer quality than they should be ?

Comment from marc
Time: March 1, 2010, 2:23 pm

HI Les, i was reading your mods to the Yaqin MC-10L i have the 2009 model, you stated pin 4 from the o/p tubes to the trasnformer now has a 820ohm 5w resistor in series and across the bias resistors you have a 220Uf @63V. i read you made slight adjustments to the feedback circuit, but it sounds like the capacitors and resistors around the EL34’s did the trick is that correct.
regards marc

Comment from Les
Time: March 4, 2010, 12:30 am

Sorry for the late replies but I have had problems getting to these later posts, each time I visit, the last post shown is for July 2009, crazy! Anyhow tonight I managed to get back in so that I can leave a comment.
To Stephen, as you only have hum on one channel I would not suspect a problem with the bias supply as this would affect both. Have you tried swapping valves from one channel to the other? Just in case you have a rogue valve. This hum problem that others also have found puzzles me somewhat as the MC10-L is such a very quiet amplifier and I look with interest for your eventual findings for this problem.
To Marc, yes I fitted the components as stated. What happened here was that I had to repair a £2k high end amplifier that had these components fitted and yes it also had the same value cathode resistors that the MC10-L uses. I had read of many occurrences where EL34’s had flashed over in the MC10-L and I just put this down to a probable faulty EL34. However, some research and subsequent reading of Philips (Mullard) data for the EL34 gave me the info that the EL34 should have a 470 Ohm to 1k Ohm screen resistor. The manufacturers of the high end amplifier had obviously gone half way and actually fitted 750 Ohm resistors so I took the next best value of 820 Ohms. After doing the additions, I am afraid that the MC10-L still did not have the gorgeous bass of the high end amplifier, probably due to the very expensive multiple interleaved windings of its output transformers. But the bass was definitely improved on the MC10-L, quite noticeable on some CD’s, it is not a lot but I think worth the effort. Also as I said before, you have nothing to lose as the extra parts are of little cost and you can always revert back to the original circuit later if you so wish.
All the best, Les

Comment from marc
Time: March 4, 2010, 10:22 am

Dear Les,

Sorry i wanted to ask the Yaqin MC-10L mk2 uses the 6N1 high gain tubes. V1
and V4 are the pre-amp tubes and V2-V3 are the drivers if im correct.

Can they be replaced with 6992 tubes, all information seems to show there
identical, though on the Yaqin from what i can see in the circuit i had to
re-draw as there circuit is wrong, the 6N1 are driven a bit harder.
If you can said any light on the subject, it would help just to have a nicer
pre-amp stage.. Even if i only replace V1 and V4 tubes and put V2*3 to
6N1PEB (EV ) same tube

regards

Comment from Les
Time: March 6, 2010, 12:54 am

Hi Marc, I did exactly what you propose to do, keep 6N1 in the V2-V3 positions but fit alternative types in the V1-V4 positions. I tried ECC85’s but they were not happy here even though they appeared to sound sweet. Micro-phonics were a problem and for some strange reason the right channel would oscillate at around 14kHz. I tried GE, Mullard and Mazda tubes but the micro-phonics remained a problem so I went back to using Svetlana 6N1 golds which I think are the best versions of this tube. These seem to have dried up now and the much heralded Svetlana winged C have been a disappointment to me. I have not tried ECC88/6992 but whatever you do take care of the cathode to heater ratings as the 6N1 does have a higher spec than some other possible tube alternatives. I notice that some MC10L owners are raving about the fitting of 6P3S tubes to replace the EL34’s in the output. This is dangerous! The 6P3S has a maximum anode (plate) voltage of around 250V yet the MC10L has a B+ supply of around 450V, I don’t think I would ever consider using 6P3S tubes for this very reason. It highlights how careful you have to be, always check up on each valves maximum ratings before opting for an alternative.
The cathode to heater voltage ratings of the ECC88 seem to be good enough, if you try them let us know if you have any instability or micro-phonic problems with them.
Les

Comment from marc
Time: March 8, 2010, 3:38 am

To Les..I put the resistors on Pin 4 of every El34 Tube = G2 between the Socket and the windings of the TX ( transformer ) with our putting the 220Uf@63v cap over the 10Ohm reistor, As i tried a experiment if the resistor goes O/C and things go wrong you could end up with close to 100V there, so i belive the caps should be 220Uf @160V. Any way with out the caps yes i agree there is a slight low end tonal increase, i put heat shrink over the resistors to protect the circuits .

The 6992 experiment.. I can report this, some of the China 6992’s are Rubish, to much microphonics… How ever the Amperex ( bugle Boy ) sound warm and inviting, though expensive, I agree with one of your testings i found using Wing “C” 6N1P EV they where not to good, Infact this may help one of the people with slight hum on one channel. when using the Winded “C” i found that on one channel i was getting a hum, Swapping the tupes ( driver stage ) around made the hum decrease, i found itn was poor quality of the Winged “C” and the CVurrent draw was a bit higher, i noted on the “CRO” that there was ripple on the power lines using these tubes. Took them out and went back to Russian 6N1EB all okay or 6N1 tubes..
I also have used on the output of the MC-10L RFT El34 Tubes hard to come by though a great over improvment in sound quality down low, very nice and warm. good detail and something you can listern to for ages.. Sadly there NOT made any more though you will still find them around.
marc

Comment from marc
Time: March 8, 2010, 3:39 am

To Les..I put the resistors on Pin 4 of every El34 Tube = G2 between the Socket and the windings of the TX ( transformer ) with our putting the 220Uf@63v cap over the 10Ohm reistor, As i tried a experiment if the resistor goes O/C and things go wrong you could end up with close to 100V there, so i belive the caps should be 220Uf @160V. Any way with out the caps yes i agree there is a slight low end tonal increase, i put heat shrink over the resistors to protect the circuits .

The 6992 experiment.. I can report this, some of the China 6992’s are Rubish, to much microphonics… How ever the Amperex ( bugle Boy ) sound warm and inviting, though expensive, I agree with one of your testings i found using Wing “C” 6N1P EV they where not to good, Infact this may help one of the people with slight hum on one channel. when using the Winded “C” i found that on one channel i was getting a hum, Swapping the tupes ( driver stage ) around made the hum decrease, i found itn was poor quality of the Winged “C” and the CVurrent draw was a bit higher, i noted on the “CRO” that there was ripple on the power lines using these tubes. Took them out and went back to Russian 6N1EB all okay or 6N1 tubes..
I also have used on the output of the MC-10L RFT El34 Tubes hard to come by though a great over all improvment in sound quality down low, very nice and warm. good detail and something you can listern to for ages.. Sadly there NOT made any more though you will still find them around.
marc

Comment from marc
Time: March 8, 2010, 3:55 am

HI Les,
I put the resistors in only being 820Ohm @ 5w from pins 4 G2 to the transformer and it did make a slight diffrence, i used a tone generator to see what it was before and afterwards, using a ( CRO ) that takes pricture print-outs, your right a bit of a diffrence not much, but something is better than nothing. I had not put the Capacitors in and i belive the V-rating should be a bit higher as if the 10Ohm resistor goes O/C then they may be more than 63V there at worst. ( Hope not ) This may help one of the readers i used Winged “C” 6n1P tubes — The worst i ended up obntaining hum from one channel, with the 4 tubes inserted and swapping them all around, i found all tubes had a slight diffrent current draw, using the ( CRO ) taking 2 out and using Normal 6N1 tubes or Russian 6N1EB(EV) all okay.. I have used as well the 6992 Tubes the China ones where terrible with Microphonics. I used Amprex ( Bugle boy ) tubes *2 and there was a plesent tone increase, more a warmer sound, not harsh.. They matched well with my RFT-EL34 Seimens Tubes so the MC-10L sound like a better amp for it…. See how you go and what you think Marc.
If this Comment comes up twice, it stated did not upload, so had to re-write sorry to all !!

Comment from marc
Time: March 17, 2010, 12:32 am

Hi Les,
I put a chokes aross the mains on the input of the mains Transformer to dropp the 240Vac to 220Vac the choke was rated at 2 amps fused, and now the mains transformer is cool, the sound is more appealing, and there is a slight more detail…people should try this, and its cheap as well. Marc

Comment from Pat
Time: March 24, 2010, 7:17 pm

Hi Guys,
I was here a little while ago asking for help when my MC-10L blew a fuse and a tube. I gave up on the self help and went to the tech guy round the corner. The tube caused a short and blew the fuse, a couple of resistors and a pot. After the fact, I guess I could have figured that out myself with your generous help but being a newbie, and an impatient one to my chagrin, did not allow me much room to wait. So the problem was not so seriuos, it is now fixed, the amp has a new set of tubes and it sounds great. In short, I am out of a good amount of money for the tech fees, I know that the parts replaced (not counting the tubes) are less than U$10.00 and I have assumed that knowledge is not free. I paid for the lesson, I know what to do if it happens again and I am studying more about tube designs to troubleshoot myself if there is a next time.
Thanks for your help again.
Pat.

Comment from robert
Time: March 29, 2010, 9:42 pm

I have an older model of the mc-10l and yesterday I was hooking up a subwoofer and when I powered it up I heard a small click in the amp and the left side had no sound I shut it down and checked the fuse that is located below the power cord in a ltlle drawer and it was good. Is there another fuse elswhere and if not what do you think happened ? thanks in advance for any help.

Comment from Les
Time: March 30, 2010, 8:55 pm

Hi Robert: Total loss of sound on one channel is puzzling as usually there are sparks and smoke if anything major has failed in the output stage. It could be something simple like a disconnection on one of the speaker terminals or perhaps the wiring to the speaker has been disturbed. You can check the circuit through the output transformer with a multimeter set to measure resistance by connecting it to the speaker terminals and check for a very low resistance of just a few ohms. You must have the amp off and the speaker removed from the sockets while you do this of course. There is no other fuse inside the amp and the fact that one channel is operating suggests that high voltage is available. You can check if the output valves on the left channel are getting their supplies correctly. Pull the two output valves (EL34’s) noting their positions for later replacement. Using a multimeter with well insulated probes, turn on the amp and check that 400 odd volts is present on pins 3 and 4 of each valve holder with respect to the chassis (Ground). As you will be looking at the top of the valve holders you have to count ANTI-clockwise from the location spigot, take care as the high voltage is lethal! If these voltages are present, check for a negative 40 odd volts on each valve holder pin 5, this proves the bias circuits have not been damaged. If you measure a positive voltage here then you may have a faulty coupling capacitor but I am pretty certain this would have manifested itself in a more spectacular fashion!
If there is no high voltage on pins 3 and 4, suspect supply to the output transformer centre tap or the transformer itself. It is becoming rather alarming to find that more failures seem to be occurring on the left hand output stage than the right but maybe it is just coincidence. Did you have a speaker connected to the left hand channel at the time of failure? Valve amplifiers need to be loaded at all times and can suffer severe damage being operated without a speaker load. May I just mention here again the other amplifier killer which is turning the amp off and back on quickly. There is a massive surge of current as the hot output valves are free to suck whatever they like as the bias takes a little time to fully establish. This does not occur so much when the amp is cold as the bias gets established in plenty of time before the output valves get into operation, a kind of built in current limit. This problem is one of the downsides to fixed biasing and why some high end designs incorporate special time delayed high voltage circuitry. There should however be no problem if you remember the golden rule to let the amplifier cool down before switching back on.
Hope you are able to find the cause of your problem.

Comment from Mike Stevens
Time: March 30, 2010, 10:29 pm

Robert; I too asked for info on hooking a sub to my newer mc-10L a few weeks back; I did as told and connected the powered sub (I assume yours is powered as this makes a significant difference) to the unused terminals (4 ohm in my case); it works fine….previously I rolled tubes and use a slo blow fuse… Mike

Comment from brian brinker
Time: April 28, 2010, 4:37 am

Hello. I have a problem with my mc10l. While listening to the amp, the line fuse blew and now the right side has a lower bass output and a slight hum. When measuring the bias on the right channel, one of the tubes is reading 36volts! Suggestions on how to fix this would be great. Thank you, Brian.

Comment from Troy
Time: April 28, 2010, 5:57 pm

Hello… sorry…. I need to be walked through this like a 5 year old… to connect a sub (as per Mike’s comments above) I just connect the sub to whichever two unused speaker terminals I have? That’s an easier solution than I expected. Thanks for any responses you may have.

Comment from Dan Ashby
Time: April 28, 2010, 8:54 pm

…your sub MUST have speaker level inputs. If they are line level only (often only one in this case) do not connect your speaker terminals to it. You can make a suitable attenuator if you only have line level inputs.

Comment from Troy
Time: April 28, 2010, 9:21 pm

Thanks… I don’t have the sub yet, so I will make sure it has speaker inputs… thanks to you guys I won’t blow anything up. I appreciate the help.

cheers.

Comment from brian brinker
Time: April 28, 2010, 10:40 pm

I measured the cathode resistor and it is open. How do I check to see if the coupling caps. are ok? Thank you, Brian

Comment from Les
Time: April 28, 2010, 11:39 pm

Brian, see my post to Robert on March 30, 2010, 8:55 pm. Just check the negative 40 odd volts is available on pin 5, as stated in the earlier post. However I repeat the warnings:-
Pull the two output valves (EL34’s) noting their positions for later replacement. Using a multimeter with well insulated probes, turn on the amp and check that negative 40 odd volts is on each valve holder pin 5. Do not connect to pins 3 or 4 as there will be 400+ volts on them. All measurements are with respect to the chassis (Ground). As you will be looking at the top of the valve holders you have to count ANTI-clockwise from the location spigot. If you measure a large positive voltage here then you may have a faulty coupling capacitor.
In my opinion the problem you have may be due to a faulty EL34 but by all means check the pin 5 voltages and after you have replaced the open circuit resistor I would suggest you get the valves tested before installing them again.
Hope this helps
Les

Hi Robert: Tot

Comment from brian brinker
Time: April 29, 2010, 2:08 am

Thank you for your help. I will be checking this and hoping that it is just the resistor.

Comment from brian brinker
Time: April 29, 2010, 2:57 am

I measured the #5 pin a got a reading of -50v. I assume this is good? Now the resistor, is it a 10ohm 5watt or is it 10hm 2watt? I see the schematic shows 10ohm 2watt, and the pics of some show a round resistor when mine is the square ceramic sand type body.

Comment from Les
Time: April 29, 2010, 6:16 am

Hi Brian, the -50V is good and shows the bias is present and not being affected by anything like a leaky coupling capacitor. It is highly likely that the resistor in use is a metal oxide film component, similar to the round ones but doing the same job in acting as a fuse if anything goes wrong. I would suggest you fit a similar component and not a heavy wire wound job so that you maintain protection, Fitting the wrong type here could cause major damage to the circuit board from excess heat due to the heavy resistor acting like a small heating element. 10 Ohm 2 Watt Metal Oxide Film is what you want and yes these do come in the square white packages your amp has. As the bias is OK there remains three possible causes.
1) A switch on surge due to short term power interruption,
2) a defect (short circuit or flash-over) inside the EL34 or
3) Resistor just failed for no apparent reason. Definitely get the valves checked out first though, are they the originals or have you fitted other makes?
You could of course try out this channel using the known good valves from the other side.
Les

Comment from brian brinker
Time: April 29, 2010, 2:20 pm

Les, thanks again for your help and suggestions! The reason I am uncertain about the resistor value is because mine has NEO5, 10ohmJ printed on it. Doesnt that indicate it is a 5watt? Also, I have not had much luck finding the 10ohm 2watt resistor. Does anybody have a good source fors these?
Yes, these were the original chinese tubes. I have a quad of E.H. on their way, but I will be using the good chinese tubes to test this cannel when I get the resistor replaced.

Comment from Les
Time: April 29, 2010, 4:08 pm

Hi Brian, well the difference between a 5W and a 2W here is of no real worries except perhaps for reliability if a flash over occurs. When the amplifier is running with a bias of 35mA the actual dissipation in the resistor will be 12 milli-watts, very small indeed. Even with 150mA going through the valve the resistor will only dissipate 225 milli-watts or otherwise stated as 0.2 Watts. The problem with this site is that one never gets to see where each person lives and it poses problems when one has to suggest a supplier of parts. Here in the UK we have a number of outlets like Maplin and CPC who will deal directly with the public. I found these on the CPC site:-

http://cpc.farnell.com/_/mor03sj0100a19/resistor-3w-5-10r/dp/RE04276

At 3Watts they would certainly do the trick and £0.70 for 10 so one would have loads of spares. Most resistors of this type have a tolerance of +/- 5% and is a pity one could not get a 1% but even if available it would be a lot more expensive. But DO try to get metal oxide types and if you ever get round to building a remote bias checking box then the difference in values becomes insignificant as you will be monitoring the exact current. Not that 5% is a lot but two resistors at extremes could make almost a 4mA difference in valve currents. But all is not lost if you buy a bag of 10 then there is a good chance you will be able to get a pair very close to the ideal. Hope this helps and I am certain you will notice the difference with the new valves.
Les

Comment from brian brinker
Time: April 29, 2010, 6:35 pm

Once again, you are the man! Thanks for the explanation. I couldn’t source anything lower than the 5watt locally and I am getting extras, just because they are so cheap.
I am excited to get this going again with the E.H. power tubes, I hear good things about them.

Comment from brian brinker
Time: May 4, 2010, 8:07 pm

Hello again. I am happy to say that I have received the new Electro Harmonix tubes and the amp is up and running. I noticed a difference right off the bat, the new tubes are much smoother on the highs and mids as well as having fuller bass. What bias do those who run these tubes use?

Comment from Les
Time: May 4, 2010, 10:00 pm

Hi Brian, it is really up to you but I would recommend staying with the original 35mA or the measured 350mV measured across the 10 Ohm resistors. It will give you a longer valve life and also remember that the hotter you run the output valves then the hotter the power supply will run. The mains transformer already runs quite hot in these amps and I think it appreciates not having to supply too much of a standing bias, so stay with 35mA is my recommendation.
Regards, Les

Comment from brian brinker
Time: May 5, 2010, 2:22 am

Thanks again. I will stick with the 350mV bias. I had a friend over to listen to the new tubes, and again with only 15 hrs on them, they are a big difference in sound. His comment was, in reference to forte 2’s,”I knew those things produced bass”.

Comment from dinga
Time: May 6, 2010, 3:15 pm

MC10L uses 2 fuses of F4AL250V (fast, 4 Amp, 250 Volt).
Would it matter much if T4AL250V (slow) is used instead?

Comment from Charles Clemens
Time: May 27, 2010, 4:29 am

I bought a new MC-10L about 2 weeks ago.
It sounded fine at first; however, after about a 2 hour warm up, I started getting a lot of intermittent popping and snapping and staticcy noise in the Left channel only.
I tried rotating tubes. this had no effect.
I kept using the amp for 2 weeks, hoping that the problem would go away; but it always came back after the amp was on for 30 minutes to 2 hours. Once I even had a loud pop and lost the Left channel completely.
I even replaced all the stock EL34s with Russian EL34s and all the 6N1s with Russian “6H1N-EB” (Cyrillic). I had planned to do that anyway. It did improve the sound; but did not stop the noise problem in the Left channel.
Finally, I bit the bullet and took the unit apart. I examined every joint and component under magnification. Finally, I found a solder bridge that had formed between 2 adjacent pin sockets on one of the 6N1 preamp sockets. Apparently tube heat had formed it or caused it to expand to complete a intermittent short.
I removed the errant solder with a scalpel. Put everything back together and I know have beautiful music from both channels and wonderful silence from both channels when there is no input.
I’m now listening blissfully to Miles Davis (“Miles Ahead” CD). This amp has sensational sound and the price Sound ratio is worth the risk of having to do self repairs.

Comment from Les
Time: May 29, 2010, 6:30 am

Hi Charles and many thanks for sharing your experiences with this fault. My recommendation would have been to try pulling the front end valves to see if the fault cleared and maybe this might have given a clue as to which stage was giving the problem. Hopefully, as with my MC-10L, yours will now give you years of trouble free operation.

Comment from john
Time: July 24, 2010, 5:16 am

the schematic does not show how the plates of first stage gets b+…what is that…some kind of double amped phase splitter….odd

Comment from john
Time: July 24, 2010, 5:58 am

and furthermore why does the second stage show the cathodes to ground via 30k at 2 watts? i doubt more than 250 ma flow there.

Comment from Les
Time: July 24, 2010, 11:58 am

The first half of V1 consists of a voltage amplifier using its second half as a kind of infinite anode load (but by voltage values approximately 40k Ohms) with the overall current set by R104. This is approximately 3.5mA and biases the top halves grid at minus 3.5V with respect to its cathode. The circuit allows direct connection of the second valve V2, without any capacitor which could otherwise produce unwanted phase shifts. It also improves the low frequency stability when feedback is applied to R103 via C106 and R117.
The intermediate stage consists of a cathode-coupled phase-splitter often called a Schmitt phase splitter and provides a push-pull drive voltage for the output stage. It is necessary in a cathode-coupled phase splitter for the anode load of one side to be slightly higher than the other if reasonable balance is to be obtained thus one is at 51k and the other 47k. At low frequencies, the presence of C103 and R106 in the grid circuit of the right hand triode produces both phase and amplitude unbalance. The frequency at which the lack of balance becomes significant depends on the time constant of these components and in the MC10L it is less than 5Hz. The cathode voltage on the upper section of the first 6N1 (V1 Pin 3) determines the operating conditions of the phase splitter. The power dissipation in the 30k cathode resistor ((127V x 127V) / 30000) is approximately half a Watt and being fairly constant I guess Yaqin decided to fit a 2W component for long term reliability.
Hope this helps, Les

Comment from Harry
Time: July 24, 2010, 1:23 pm

Hi Les,

Any suggestions for replacing the volume pot on the MC10L? Mine developed scratchy noises in the left channel. Opening the cover revealed a sealed pot so I could not spray electronics cleaner into it. The schematic indicates a 20K unit but most attenuators I could find are either 10K, 25K, 50K, 100K, 250K.

I have the older version without the access holes and the capacitors sticking up through the top cover.

Many thanks!

Comment from Les
Time: July 24, 2010, 11:38 pm

Wish I could help Harry but so far I have not found a replacement. I thought the Yaqin used an Alps control but I have a feeling it is a copy due to the fact that it has a splined (knurled) shaft and I am certain Alps only made straight shaft potentiometers. The actual shaft length from the threaded boss appears to be about 20mm and a replacement has to be the same in order to fit the knob correctly. You may be able to find a dual control of a more conventional open style of manufacture from a supplier. I for one will keep looking as I think it may be a component that I myself may wish to replace in my Yaqin, which incidentally is the same early type as yours. The 20k value is not too critical and a 50k or even a 100k could be used, the control feeds the grid of the first valve which already has a 100k resistor on it. As stated in my earlier posts, I use a remote controlled motorised Alps control for my volume control so the Yaqin control is normally left fully on.
I think that if I was in a position where I desperately needed a replacement, it might be possible to fit an Alps control and extend its normal 11mm shaft with a coupler and make up the remaining 9mm using part of the old control. This would give me the splined interface back but careful measurement needs to be made first to ensure the process is completely viable. Also, don’t give up on the chance of perhaps getting a control from China and of course, keep a good eye on popular auction sites for a possible replacement.
Les

Comment from john
Time: July 25, 2010, 12:52 am

thanks Les for the explanation. I still see the schematic has no B+ shown for the plates in first amp. What is the voltage supposed to be?

Comment from Les
Time: July 25, 2010, 9:15 am

Hi John, I wonder if you have the schematic drawn by Yaqin on graph paper as there is indeed a missing HT connection between B+ and the 33k resistor feeding the upper valve. The Anode of this valve should be around 300V and the cathode will be about 124V. There should be about 3V drop across the 1k resistor to give 121V on the anode of the lower valve.
Cheers, Les

Comment from Jon Chia
Time: July 25, 2010, 11:43 am

Hi,
After reading this forum, I have gained enough confidence in buying the MC10-L. Have placed order and it should be here 2 to 3 weeks time. Never own a tube amp but you guys helped in my decision making.

Question:
About adjusting the bias, the amp should be switched on and speakers loaded, right?

I intend to order some backup tubes. Here’s what I have in mind:
6N1P-EV (for Pre)
6P3S-E (for Power)

Any comments or suggestions are most welcome.

Thanks for reading!

Jon

Comment from john
Time: July 25, 2010, 4:01 pm

Thanks Les, I see as well the top grid is going to the bottom plate (anode) directly. I would have to see an actual amp and do testing. I do not think this schematic is drawn correctly and wonder what else the company wants to play games with. Is there other bugaboos? I know in Hong Kong they play “gotcha” often as a revenge scheme.

Comment from Ian
Time: July 26, 2010, 4:13 pm

Interesting post by Jon Chia, I’m in nearly the same position. I have even ordered those same valves and am waiting on the amp, I’m looking forward to playing with this amp
Has anyone run one of these amps in a triode strapped configuration?
Cheers
Ian

Comment from Alun
Time: July 28, 2010, 6:43 pm

I’ve just got MC-10L and it’s grrrrreat !!!
The top valve (with the gate strapped to the lower anode) is correct, it’s effectively a high impedance constant current source for the incoming anode (as long as there’s PSU headroom), have used this myself a few times, so the actual stage should be treated as cascode, but with an anode C-I feed. the 1M res & 220nF cap then serve as both a bias corrector and a LF rolloff stage to the phase splitting long-tail pair.

Think the circuits right, but I would have included a grid-stopper in the upper triode’s gate.

Hope this helps

Alun