• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Yaqin MC-5881A amplifier improvements

Lift one 500 Ohm from each pair. This will get the bias down from 70ma to 40 mA. Right now each tube dissipates 26W (overspec). You will measure around 40VDC on each 500Ohm left in circuit, and feel the heat backing off seriously enough. Do that and let me know how many cables go from PCB to chassis and from where off the circuit gnd points.

I've had the exact same heat problem with my 5881A.

I measured 6,7/6,8 heater voltage and similar voltages to the 500R's. Lifted one of the resistors made it run cooler but still a bit hot.

Do you think it is worth while to try to drop the heater voltage as well? If so what resistors do I add where?
 
Update. I mounted moderately used WBT-0210 cu nextgen RCA socket purchased for mere 20EUR. On the other side of the interconnects, on tube DAC is Eichmann PhonoPod HC-XTC RCA female chassis socket.

Interconnects are my own brandname, Mantra, Strato audio standard, skin effect free, microscilations free, solid core wiring gold plated pure silver and silver-copper alloy. Plugs are Eichmann Silver Bullets.

A month before i made digital BNC Mantra Space audio standard cable prototype on Oyaide SLSB BNC Plugs for my tube DAC. I have other Oyaide products, mainly on mine Mantra Space power cables, so i know their products philosophy very well. Like Eichmann, on the softer side of the sound with a lots of clear, sweet sounding microdetails- if they are tuned exactly with the good audio cables and inlets. Without tunning, still good and listenable, but their potential will be not fully used.

WBT Nextgens(forget non Nextgen WBTs) are different story. Neutral, not soft, not hard sounding, exact("punktlich" in German language, that is their philosophy from the times of the Bismarck on)

Gaofei Rhodium RCAsockets kicked out in the first listening test, i wont waste words for comparision. Still usable, but only for subwoofers, like CMC silver sockets. Best of the three in that test should be- Neutrik RCA socket. They offer certain satisfaction for blending the sound of average components, their sound-filtering characters are very different.

WBT-0210 Cu Ms Nextgens are, surprisingly, everything that WBT claims in their datasheet and more. Compared to the excellent Eichmann Phonopods, there is a good listenable nuance of properly presented and positioned stage with WBT sockets more. They are horrific component. Not just a socket, they "act" and sounds like superb audio component. They need a week to burn in.

Combo of those sockets is marriage made in heaven. Next logical step was ordering a pair of Wbt - 0210 Ag Ms Nextgen Silver. 0210 Cu model is horrific, there is nothing bad to say abut it, i cant even imagine better RCA socket.

But. WBT itself have Ag model to compare with Cu, which is "very easy to live with", so i wonder what Ag model has more to offer? 0210 Nextgen model are builded technically the same as Eichmann built his revolutionary Bullets. To "act" like a wire. But Eichmann Phonopod and also "classical WBT" sockets are not exactly builded like Bullets! Only WBT Cu Nextgens s are, if we believe(and i do) to what WBT claims between the lines in their techincal specs! And listening and comparing combo of that on the good tuned wires, make this simple technical fact obvious.. Combo Cu-Ag is very promising, but... Listener cant say nothing in forward. Listen to the music and forget plugs, sockets, wires and comparision and after weeks, or months, your brain will inform you, if you like it or not, about the differences of the technical items you installed.
 
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Here are some improvements I received via Ballad Audio.
 

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I realise this is an old thread, but I just purchased one of these. They must have changed the design somewhat as there is no sign of hum or hiss even at high volume. This was tested with my DUNE HD Smart H1 connected and with nothing connected. The valves heaters do look like they a running a bit hot, so maybe they are still a bit over voltage, I'll check at the weekend. Overall I'm pleased with the way it performs for the price, but although it has less power, I still think I prefer the sound from my Lafayette LA-224B with its Russian gold grid quad set. I'm not going to use it again until I have checked the heater voltage. When I do I will take some pics to compare the build to those already posted.

Question:

When testing transformer heater voltage output, do you test it with all the valves in or out? I ask because pulling one of the 6E2 magic eye tubes and testing pins 4 & 5 only shows 2.9V with all the other valves in place, despite this low reading all the valves glow somewhat brighter than they do in my Lafayette LA-224B and the amp seems to work okay.

As I'm not working today, I decided to investigate further. On removing the bottom cover it became clear that the 6E2 valve heaters were fed from the PCB not directly from the transformer as the other valves are. That low voltage may be by design for some reason. On removing the valve guard and pulling one of the 6N1 valves, I got 6.94V which is just about 10% over, should I try to bring this down to 6.3?

The first difference I noticed to the O/P picture is the change in the valve guard design. Looking inside, they have changed the orientation of the centre transformer by 90 degrees and added stiffeners. They have also done away with (though still mentioned in the manual) the Speaker/Headphone changeover switch. The Output valve bases now mount above the board not through it as before. There is also a 110/240V changeover switch, I'm not sure if this was in the original design?
 

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i purchased this amp a back week,i wave many tuner/signal/vhf valves.
I will share information on this thread

Well, it's not the best idea I've had lately than buying this amp :/
he was massacred by his previous owner who had to read a lot of things on the internet and who implemented them
one night strongly alcoholic ...
it works, but it's a miracle.
I'll post a photo or two later for the pleasure of the eyes :)
 
Transformer Cover Overheating

For what it is worth I have a Mingda MC34-A that has a physical layout similar to the Yaqin.

The transformer covers would get too hot to touch. I too assumed that it was a problem with the transformers or circuitry.

I contacted Anatoliy (Wavebourn) and he told me the tubes were too close to the covers. I put a fan on the side of the amp, turned it on, let it run for an hour and the transformer covers were cool to the touch.

He told me to put stainless steel mirrors between the tubes and the covers. I got the mirrors from Amazon and fixed them to the covers using 1/4 inch thick cork rounds.

That solved the heating problem. The mirrors get too hot to touch and the transformer covers remain cool to the touch even after many hours of operation.

Steve
 

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actually but in the case of yaqin, it is rather because the bias is too high original, and it literally cooks the tubes and unnecessarily heat the sheet and the transformer.
the fact that the power transformer is not up to European standards for primary, it does not help things ...
personally, and for having worked on both brands, I prefer mingda / meixing to yaqin
 
With regards to the bias point being the cause of the overheating maybe it is maybe it is not.

I don't see any posts where the user said I changed the bias points or filament voltages and the transformer cover is now cool to the touch.

Put a fan on it and see if the transformer cover heating problem goes away.

That is what I did and the solution I proposed eliminated the problem.

FYI: the tubes in my amp were biased at too low a plate dissipation when I got it.

Steve
 
I did not say that the mingda had a bias problem, but the yaqin yes, and not just a little!
I really like the old mingda in kt90!

I just finished it and I'm very happy with the result!
especially when you know the starting point ...
it is now equipped with 6bq7 / 6n1p-ev / 6p3s-e sorted
and paired pseudo triode mounted with low noise resonances of 470ohm and without feedback.
I added a first b + with 50uf + 100ohm 40w, black wall
and perfectly silent.
I changed the connecting capacitors by Pio, for the price, it's an absolute happiness !!
 
I am trying to make the two 6E2 level indicator valves work at a lower volume output by adding a voltage doubler.
Can anyone tell me if this should work, also is the location I have placed them correct? I have attached the original and modified schematic for comparison.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
 
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