Hilgen B2502 Basso Profundo Tube Combo Amp - help eliminate noise!!!

[misterc57]

Alnico 15" speaker, (2) 12ax7 pre tubes, 5y3 rectifier tube, (2) 7591 power tubes.

This amp has two input channels, bass and instrument. Each input channel has two inputs.

Bass input channel works great!

Instrument channel makes noise and sometimes does not work.

If the amp has been off for a long time. The Instrument channel works fine. After several minutes of operation it starts to make a noise (like a bad ground crackling type of sound).

If I tap on the chassis I can trigger the noise.
If I tap and probe around the preamp tube connections with a Sharpie, I can trigger the sound on/off, usually.
The noise can be made louder by increasing the volume pot.
None of these things will trigger noise when using the Bass channel inputs.

Rotating the top of the preamp tube has no effect (I was thinking loose tube pin holders).

I have swapped the preamp tubes, problem persisted in the Instrument channel.
I have sprayed all the tube sockets. Tested all the tubes. Sprayed all the pots and input connectors. Tightened the input connectors.

I have no schematic and do not know where to find one.

I would love to get this resolved on my own if possible. Looking forward to advice and things to try!

I have had this amp for sale on CL and have had a lot of responses. I am on the fence about selling it.

Pictures can be seen at 1960s Hilgen B2502 Basso Profundo All Tube Combo Amp

Thanks in advance.

Mark

[Jazz P Bass]

I would look for leaky coupling caps.
They will be attaced to pin 3 & also pin 6 of the pramp tubes (assuming 12AX7).
The coupling caps keep the previous plates high voltage out of the next grid circuit, while passing the ac signal.
There should be very little Vdc on the next grid.

[misterc57]

Turned out to be three bad grounds where some terminal strips are rivited and soldered to the chassis. Problem resolved.

Thanks, Mark