Bassman 50 power tube socket problem

[Ten Over]

Funny thing is every possible schematic of my amp, AA270, AA371, and Bassman 50 all show the 2000 pf caps coming off the grid to ground. My amp doesn't have these caps. I am nearly certain it has never been modded, and no trace of solder on the chassis where these caps would have been according to layout diagrams. The amp has played fine for years.

A 1974 Bassman would have these caps mounted on the socket from pin 5 to pin 8 as opposed to pin 5 to the chassis.

[Stevem]

So you ordered new PI coupling caps because one tested leaky I assume?

[Smitty]

If you lift the 3k3 resistor from the bias supply board to the bias pot (doesn't matter which side) and then power up the amp without tubes in, you should have no voltage anywhere on your output tube grid circuit. If you've got positive voltage that means one (or both) of your phase inverter coupling caps is leaking DC and should be replaced. The small amount of positive voltage is added to the negative bias voltage and would account for the difference in grid voltage when you've got your bias pot in the center position. Don't forget to reconnect the 3K3 resistor before you put the tubes back in.

[Ten Over]

Or just measure the voltage across each 100K grid leak resistor with the circuit intact.

[bluesguitar]

Here is an update. I replaced the .047 capacitor feeding the grid on the suspect tube socket. The amp is working great now! My grid and plate voltages are perfect giving me very near 70% plate dissipation. One clarification is that I do have the 2000 pf caps on the grid to ground. They were mounted right above the socket bridging pins 5 & 8. Thanks again to everyone for your help!

[Stevem]

Yes, pin 8 is at ground.
Clip one of those out at a time and check if the amp still sounds normal / good?

It likely will so then leave them out .

[TUBEDUDE]

I have seen that also on some of these amps!
I think that at some point Fender / CBS engineers realized that the oscillation issue these caps where installed to shunt out really only took place in the 4 output tubes amps that had far longer wires needed off the PI section to get to the tubes next to the power transformer, and also in the guitar amps it was needed due to more of the preamp stuff like the reverb pot and the master volume pot being way over on that side of the chassis.

The bottom line as always comes down to layout and build .

And price point.

[Paul G.]

Here is an update. I replaced the .047 capacitor feeding the grid on the suspect tube socket. The amp is working great now! My grid and plate voltages are perfect giving me very near 70% plate dissipation. One clarification is that I do have the 2000 pf caps on the grid to ground. They were mounted right above the socket bridging pins 5 & 8. Thanks again to everyone for your help!

Fender used 400V caps there, barely adequate. If one is leaking, odds are the other will do so soon. I replace those as a pair, a couple of bucks and 5 minutes translates into peace of mind.

My two cents.

[pdf64]

...Clip one of those out at a time and check if the amp still sounds normal / good?...

That really needs verifying at at a variety of vol and tone control setting combinations, including max vol and treble, and any alternative cabs available.

...Fender used 400V caps there...

Yes, at power up initial voltage surge, and when testing with no valves in place, there will be >500V across them, ie full unloaded HT + bias VDC.

[Ten Over]

Fender used 400V caps there, barely adequate.

They were 600V.