help troubleshooting

[metamp]

I have a 2 channel guitar tube amp
it has 6 preamp tubes 4 power tubes and 2 rectifier tubes
here is the schematic
The Free Information Society - Mesa Boogie Tremoverb Electronic Circuit Schematic

now when I switch to one of the 2 channels the a weird roaring hum noise is coming out of the amp, sounds like a little motor, fish tank pump or like a when a fan blades hitting against something

this sound is coming out from the unit not from the speaker connected to it

I switched the power tubes around but that didn't help
I took out the 2 rectifier tubes and the noise is gone

I read in one article that it is some sort of arking ..
I think the preamp tubes are too small for making such a noise and I don't see any sparks in the preamp tubes

this is happening on one channel only so Im not sure if it's the power tubes
also when I switch to that channel that hums the power tubes glow little more

it is an old amp and the capacitors are original
also this amp wasn't played for years

can someone advice where to start looking for the problem
any help appreciated

[HollowState]

From what you have said, it sounds like either severe hum or oscillation called motor boating. And because it's not coming from the speaker, this leads me to think the speaker is bad. An open voice coil or totally bad connection between the speaker and amplifier. Don't let this continue because it's extremely stressful for the output transformer and can damage it beyond repair.

Removing the rectifier tubes will cut all power to the circuits and will naturally kill the noise. An increased "glow" in the output tubes is an indication that the output stage is working and this glow is normal.

Since you haven't said that the other channel works normally and sounds ok through the speaker, please verify the speaker is good. With the amplifier turned on, put your ear right up against the speaker opening and listen. You should hear some kind of low level hum or buzz with the volume turned up, but with no signal applied. This is where we begin.

[famousmockingbird]

Sounds like you got some motorboating going on. If it's just one channel I would put money on preamp B+ decoupling capacitors not decoupling properly. Changing electrolytic capacitors in an old amp is a good idea anyway.

[famousmockingbird]

Lol, in retrospect if is not coming through the speaker it's probably the tremolo switched on. I didn't look at the schematic but their might be an optocoupler that makes that type of noise when engaged.

[Enzo]

Since he specifies it only happens on one channel, I have to think the speaker and power amp are OK. In guitar amps, "channel" refers to the preamp.

And if a circuit is motorboaring, I don;t see how it could happen without coming out the speaker. After all, the motorboat is the signal path responding to the problem.

MY first reaction was perhaps a relay coil buzzing. If the relay supply lost filtration it would be left with pulsing DC. Don;t recall if there are relays used in that one.

If there are any small relays, press your finger onto the top of each to see if any are vibrating. Of course be careful of the lethal high voltages in the live circuit.

[HollowState]

Originally Posted by Enzo

Since he specifies it only happens on one channel, I have to think the speaker and power amp are OK.

And if a circuit is motorboaring, I don;t see how it could happen without coming out the speaker.

metamp states in his third paragraph that it isn't coming from the speaker.

Any sound, motorboating or otherwise, at full amplification will cause the output transformer to produce a reasonably loud sound if operated unloaded. I fear this is what he may be hearing. If so, the output transformer is under great stress and in danger of internal arcing.