help bassman '59

[JC@]

The amp is a reissue Bassman, Bass Breaker, model '59, 5F6A circuit
When I strum a big chord, volume at 6 or 7 and the amp being hot, there is a scratchy "bad contact like" noise that comes sometimes, when the sound is at it's peak level.

I have looked everywere but found no clue. Then I tried again today, and suddenly the whole sound went out leaving an "output tube shorted" like hum, then the sound came back again as I switched the standby on and off again inmediatly trying to catch/hold the noise again.
I didn't have the time to look at the ampmeter so I am not sure if it went up or not looking for an excess in current draw.

It could be two things going wrong, because at some point of the troubleshooting I tried a new set of output tubes, and I am pretty sure the mentioned scratchy noise was there, so I plugged the old tubes again.

I would appreciate any kind of input here

[Mars Amp Repair]

Loose connection or loose filter cap that is getting vibrated by the cab at high volume perhaps? Have you tried tapping around with an insulated screwdriver handle. g

[JC@]

Loose connection or loose filter cap that is getting vibrated by the cab at high volume perhaps? Have you tried tapping around with an insulated screwdriver handle. g

I have a problem here because the expected load is 2 ohms (2 x 4 ohms speakers conected in parallel) and my cab for testing is 8 ohms

One more thing, this amp is using one of those plugs instead of a rectifier tube, but I am not sure if it is working properly or not, as I don't know what components are inside this thing.

edit: I just checked the schematic and it is not a bridge rectifier inside this plug, just two diodes inside it. I get 1.1volts across each diode instead of the usual 0.7volts, but I think this is okay

[stokes]

Check the plate load resistors on the preamp tubes,a common problem on some '59 RI Bassman amps is that they used 1/4 watt resistors on the plates,1/4 watt is too small and they burn out after some time,sounds like they could be starting to go.The solid state rectifier will raise the volts and hasten the failure,turning the amp up also increases the heat,and heat is actually what causes the resistors to fail.