Fender Bassman: sound cutting out

[Jonny toetags]

I had a friend ask me if I could tidy up his 57 Bassman (original) that someone had messed with through the years.
I basically disconnected the polarity switch, changed some no-name coupling caps to Mallorys and put in new tubes.
Once I power it on, it sounds great for a min or two, then the volume slowly fades away. If you wait a bit, it slowly comes back.
What could be the cause?

[The Dude]

Could be any number of things. I'd start with power supply. Do your supplies stay up and running when the amp dies (B+, filament, etc.)?

[g1]

Did it have this problem before you did your work, or not?

[Jonny toetags]

I'm doing some digging...
This time I turned it on, all tubes seem to slowly light up, I plug in...no sound. I notice all the tubes werent glowing or hot at all, except the rectifier.
Then all of a sudden, the sounds slowly comes back, all tubes light up

[mort]

I'd be looking for a loose connection on the heater loop, rectifier gets it's heater from a different source. Something is going sideways with the 6.3v track

[The Dude]

I could be wrong, but if memory serves, the heater winding of the PT goes first to the power indicator lamp. Does the lamp light up? If it does, but no filament voltage gets to the tubes, you likely have a connection issue on the lamp socket or the wires from the socket to the first tube in the parallel filament "chain".

[Chuck H]

It could also be oxidized or dirty tube sockets or pins. You mention putting in new tubes. Suspect them too anyway. And if you didn't replace them all, suspect the older tubes more.

[Jonny toetags]

Thanks guys.
As of posting last night, it cut in and out once more then was fine for the rest of the evening. I couldn't get the symptoms to reproduce, even after turning the amp on/off, standby on/off, tapping connections, etc...
I'm going to get back into it tonight. It was doing it with the 'stock' Fender branded tubes that were in it and the new Tung Sols, so I'm thinking it's not the tubes.

-Dude, yes, lamp lights up, voltage is on spec.

[Chuck H]

Also, flex the board, whap on the amp in operation, tap and wiggle tubes in their sockets while listening for any weirdness.

[robrob]

I agree with mort, since all the tube filaments are losing power and going dim you have a 6.3v heater supply problem. The fact that the rectifier tube's heater doesn't fade that implies the power transformer primary is not the problem.

I would reflow the 6.3v solder joints at the pilot light and first power tube and see what happens.

[Jazz P Bass]

I agree with mort, since all the tube filaments are losing power and going dim you have a 6.3v heater supply problem. The fact that the rectifier tube's heater doesn't fade that implies the power transformer primary is not the problem.

I would reflow the 6.3v solder joints at the pilot light and first power tube and see what happens.

Ahh, but does not the rectifier run on a separate heater winding?

[mort]

Ahh, but does not the rectifier run on a separate heater winding?

I think he was just pointing out that if there was a problem somewhere in the primary, that ALL of the filaments would go dim, narrowing the search to the 6.3v circuit and not a general power supply loss.

[Drewline]

Have you checked the standby switch? I've seen what you are describing occur with a flakey standby switch. Once you have verified the heater circuit check the standby switch.

[robrob]

The standby switch doesn't affect the heater filament power or "tube glow."

[Drewline]

rorbrob, I do understand that the standby switch doesn't affect the heater filament power. I've edited my post above. Still the most common cause of intermittant sound I've seen in these amps is the standby switch getting intermittant as a result of many years of use,