• 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

Whilst Salas is sorting the schematic, this picture may help you. Note that this is not the best way to lay these things out, but it's functional. Someone with more time and skill (or if I was doing it again) would do things differently.

An externally hosted image should be here but it was not working when we last tested it.


The circled parts are where modification has occured...

1. top and bottom left - some ground layout work (this may be trial and error - but the key thing is making all grounds go to chassis at the same point
2. middle left - one of the pairs of 500 Ohm 5 watt bias resistors removed
3. up and to the right is the resistor in the CRC filter. I used a 270 Ohm 20 watt which meant that I could leave the original 450v rated cap in place as it dropped sufficient volts to bring it into spec.
4. right of that is the 100uF 500V cap that makes up the CRC filter.
5. right of that is the other 500 Ohm bias resistor with it's friend removed
6. right of that is the pair of 0.33 Ohm 10 watt resistors that drop the heater voltage to 6.3v
7. The other circle at the bottom right shows where I replaced the original 0.22uF coupling caps with (cheap) polyprops. The fitted originals were only rated at 275v, although the schematic suggests 400v. They only see that kind of voltage with the valves out of circuit, but they may be worth changing anyway. Note that all the yellow cylindrical caps are replacements - I just haven't circled them all!

Hope this helps,

Dan
 
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Heaters fix.
 

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sorenj07 said:
another quick little fix might be to stick a CL-90 on the transformer primary, assuming there isn't some other slow-start device implemented. i'll calm down any current rushing into your heaters, and drop a few volts, which eases the strain on your PT. exceeding the voltage spec on the windings is not a good thing.


Hi Folks,

Does anyone know of a UK supplier for the CL-90 other than RS Components? I fed up of spending £15 to buy a £0.88 part with them folks... Maybe I'll try giving the UK distributer a call for a couple of samples!? :)


Cheers,
Chris.
 
DanDini said:
Whilst Salas is sorting the schematic, this picture may help you. Note that this is not the best way to lay these things out, but it's functional. Someone with more time and skill (or if I was doing it again) would do things differently.

An externally hosted image should be here but it was not working when we last tested it.


The circled parts are where modification has occured...

1. top and bottom left - some ground layout work (this may be trial and error - but the key thing is making all grounds go to chassis at the same point
2. middle left - one of the pairs of 500 Ohm 5 watt bias resistors removed
3. up and to the right is the resistor in the CRC filter. I used a 270 Ohm 20 watt which meant that I could leave the original 450v rated cap in place as it dropped sufficient volts to bring it into spec.
4. right of that is the 100uF 500V cap that makes up the CRC filter.
5. right of that is the other 500 Ohm bias resistor with it's friend removed
6. right of that is the pair of 0.33 Ohm 10 watt resistors that drop the heater voltage to 6.3v
7. The other circle at the bottom right shows where I replaced the original 0.22uF coupling caps with (cheap) polyprops. The fitted originals were only rated at 275v, although the schematic suggests 400v. They only see that kind of voltage with the valves out of circuit, but they may be worth changing anyway. Note that all the yellow cylindrical caps are replacements - I just haven't circled them all!

Hope this helps,

Dan

Hi Dan Im complete a newbie! I have disconnect the earth wire from body to front selector pcb and it made a great difference to the amount of hum thanks.

Now i don't want to destroy my amp but would like to know if removing the 2 auto bias resistors on there own would bennefit, or should this only be done following the heating mods? And is removing the bias resistors as simple as cutting or pulling after having heated the solder?

I own a soldering iron thats still not been used yet from new!
:eek:
 
DanDini said:
You can safely remove one of the bias resistors on each channel. This will probably be enough, the over-voltage is not that significant in some ways. See how you go with these initial simple mods.

Dan

Oh dear i think i have just killed it!

After Removal of the two bias resistors shown in picture and turning on and running for about 5 seconds i hear a pop and now nothing....

I have tested plug fuse and is ok so something bad has happend inside?
 
More mod suggestions:

Fit 100nF/630V film caps across the 22uF/450V electrolytics feeding the input stage and the driver stage (b.t.w. Svetlana 6N1P are lovely as an upgrade to the chinese tubes).

Audio Jewelery option - replace the output tube cathode bypass cap (100uF/100V) with a Blackgate, could try increasing its value to 470uF/100V as well

More complex mods:
Make up a little "matrix" board with a cascode transistor current source (or ring of two transistor current source) to replace the 30K/2W tail resistor in the driver stage and make the 2 anode loads the same - both 47K would be good.

Use separate cathode bias resistors and bypass caps for each 5881.

Cheers,
Ian