Russian tube power amp repair

[Bernhard]

Hello, a friend asked me for help with a grey box Russian tube power amp.
One channel starts to give loud 100Hz noise few seconds after music starts after warm up.
We have swapped tubes, the two 200µF caps and 22µF are ok.
When both small tubes are plugged in, I measure 14 Vac on the coupling cap between the two small tubes.
When one of the two, no matter which one is unplugged, the amp is quiet and no AC voltage.
Some kind of oszillation ? But there is no global feedback ?
When >I disconnect the input pot, the amp starts pumping on the bad channel.

Any idea ?

The noise is still there when the amp is switched off untill caps discharge.

[the_manta]

Hi !
Is it a priboy ? What tubes does it have ? Do you have any name or a schematic ?

PS: kennen wir uns ?

[Bernhard]

All written in russian output tubes are 6P3C1

Light grey box, something like NPUGOU50 and hifi written on it.

[Bernhard]

It is a priboy.
Do the wires that go from the transformers to the anodes have to be parallel on both sides or crossed on the right side ?

[soundbrigade]

Does it look the amp at the end of THIS page???

Schematic

PCBs

You need DejaVue to view the files.

[Bernhard]

Thanks, yes it is that amp.

Looks like there is global feedback on pin 7 00C ?

[soundbrigade]

the schematic is slightly complicated, but yes there's a lot of NFB I guess.

[the_manta]

Here is another very good page

Hope this will help you

Regards from Ottobrunn, Simon

[KaDe]

Do the wires that go from the transformers to the anodes have to be parallel on both sides or crossed on the right side ?

Bernhard, did you try to swap these anode cap wires ?
- they may be connected in wrong phase -

[soundbrigade]

The OP-tubes are very special and easy to brake. If that happens a change to EL34 or Russian GU50 is a good choice.