Yaqin MC-5881A amplifier improvements

[Mikie]

Yaqin MC-5881A

I bought one of these off ebay, and as soon as it comes, I'll trace the circuit, measure the voltages etc. and ask you knowledgeable chums for advice what components to change/improve. Not a major rebuild, just obvious mistakes if any and similar.

Anyone that already has the circuit, perhaps? This thing is also known as Yaqin (or Yaoin) MC-6L6 and Markhill MC-6L6.

[DanDini]

I have one of these too. Did you ever get anywhere with it? Mine sounds ok but has noticeable hum.

What's yours like?

Dan

[DanDini]

Does anyone have experience of calming these things down a bit? Mine runs very hot - both transformers and valves get to scorching temperatures.

I don't need 23w per channel and would be happy to drop some output in favour of a cooler office.

I'm also interested in other people's experience of dealing with the hum these things can make. Is it likely to be the chinese valves, the transformers or just the design?

It appears well put together internally but it's a shame I haven't tracked down a circuit for it yet.

Thanks,

Dan

[Salas]

I know nothing about the amp (well, no more than it has Greek letters in its logo ), but that is what I would do in your case in order to relax the heat, so to preserve the amp's longevity and even avoid fire hazard if it got forgotten long unattended:

Firstly, see with a multimeter, the AC voltage on the heaters. There is a thick yellow twisted cable pair feeding all the valves as I can see on the linked pics, so probe there. If it is significantly more than 6.3 VAC there is an over-healthy transformer secondary at work when in British mains. It can be tackled with small resistors.

Secondly, see for the resistors that are connected from cathode to ground, one for each output valve. They maybe the gold metal box ones on the pic. I don't see any bias pots so maybe its auto-bias. In any case measure the DC voltage drop on those between cathode (pin8) and GND, and write down their resistance too. Then we can calculate the working bias. If its more than 45mA, better back it off. If there are trim-pots that I cant't see its easy, if its auto-bias, the resistors are going to be in the hundreds of Ohm range, and you can up the resistance by changing cathode resistors.

I post a 5881 octal base guide, so you can easily locate the cathode resistors. If the resistors are 1-10 Ohm there must be trim-pots somewhere.

As for hum, a little bridge rectifier and a couple of caps are going to help if its AC heaters on the small valves too as I suspect.

[DanDini]

Thanks! - I'll do that tonight or tomorrow night depending on what time I get back from work. One thing to add:

On the advice of my friend, I've tried it without the 6N1 valves in, the noise is still the same, so it appears to be generated in the output stage.

Thanks,

Dan

[Salas]

It can even be a ground loop rooted in the layout or you may need to lift the ground to chassis (if it has one) with a 10 Ohm sandcast resistor. Have a first look so we get a general idea of how the amp is implemented, one step at a time.

[DanDini]

OK, I haven't done any more testing yet, but I do now have the schematic. Please see the linked file.

http://seis.bris.ac.uk/~chdma/files/mc-5881.jpg

Thanks,

Dan

[Salas]

Yes. Autobias (500R//500R), AC heaters, lifted chassis (most probably).
With given circuit data, not away from early assumptions, the initial suggested test procedure and report stands as described.

[sorenj07]

just a bit of two-cent advice. if there's at all any room in the chassis, PLEASE consider putting a smallish choke into the B+ supply. it's almost shameful to tap the B+ right off the filter capacitor as there are scandalous amounts of PSR spreading to your whole amp. Even a lowly Hamond 159T (2.5H 300mA 43 ohms) ought to make a significant improvement. Splitting B+ and using one choke for each channel might make even more improvement, like two 159Q's at 7H/150mA/100 ohms ought to be very good. In addition, the little bit of voltage drop from the chokes might well help the amp to run cooler. Do you have any pictures of the inside of the amp? How much free space is there inside?

[Salas]

Good suggestion Soren. Would have liked to bring it up too, but when I saw the innards, I skipped suggesting due to lack of free sq inches. Auricaps instead of the gray block caps may bring some benefit for other issues. Tonal mainly.