Bassman Head Silverface Hum

[misterc57]

Now wondering about the Bass Channel Volume pot related to the noise.

I bent the solder pins at V1 slightly and the noise diminished a tad but is still there.

Also I noticed some problems with the Volume pot tied to V1. Dialing the pot to zero or "1" does not cut the guitar signal, it is still loud. Dial the pot from "1" to around "3" and the volume drops off, then starts to increase in volume after "4".

When tapping the wires around V1, volume pot at "1" is loudest noise, dial up and noise level diminishes, pot at "10" the noise is almost gone.

Pot was sprayed out with cleaner. BTW, all V1 tubes pins make noise but pin 8 is the loudest offender.

[Leo_Gnardo]

Now wondering about the Bass Channel Volume pot related to the noise.

I bent the solder pins at V1 slightly and the noise diminished a tad but is still there.

Also I noticed some problems with the Volume pot tied to V1. Dialing the pot to zero or "1" does not cut the guitar signal, it is still loud. Dial the pot from "1" to around "3" and the volume drops off, then starts to increase in volume after "4".

When tapping the wires around V1, volume pot at "1" is loudest noise, dial up and noise level diminishes, pot at "10" the noise is almost gone.

Pot was sprayed out with cleaner. BTW, all V1 tubes pins make noise but pin 8 is the loudest offender.

I've sometimes noticed much the same problem especially with old Fender pots. Sometimes the resistance track has gone wonky. Or maybe was that way from day one. Easy enough to check. Disconnect the hot & wiper leads on the pot, and monitor the resistance as you dial it up from minimum. You should see a steady rise in resistance between the ground terminal and wiper as you turn clockwise. If that's not happening, replace the pot. These days I use Alpha. All the spray in the world won't help a dodgy resistance track that's asking to be replaced.

Another possibility is crosstalk from the first pre stage into the second. Or is it vice versa? Or verse visa? Whatever... Sometimes the crosstalk takes place within the preamp tube. A change of tube may be all that's needed. In those amps where cathode bypass caps are shared by two triodes, when that cap fails, there's a clear path for unexpected transfer of signal from one triode to another. But your Bassman doesn't have that. Nonetheless I'd have a very close look at the cathode bypass caps anyway. Even though they look to be Sprague Atoms, presumably high quality caps, check 'em with a cap meter anyway & also make sure they are properly soldered in. That solder "pile" where the 2nd stage triode lands along with a bunch of other components could hide a dry connection. Pin 8 is the cathode of your second-stage preamp triode.

[SoulFetish]

As the tubes already have cathode resistors, you can use the cathode voltages (pin 8) to determine idle currents. Just divide cathode voltage by the (measured) resistance value between cathode and ground. Both cathode voltages should be identical for least ripple hum.

If you use 1R resistors, they won't see more than 0.25W.

Unless a tube shorts, perhaps. In that case, I suppose you could approach that scenario in two ways: You could use those 1Ω resistors as fuses, since Fender didn't include secondary fuses back then.
Or, you could spec them to survive a tube short, and rely on the mains fuse blow before there is any immediate damage to the transformers.

[misterc57]

I found the noise problem and I have a few questions.

Comparing the schematic wiring to the Bass Channel amp pots I found that the treble pot wiper was feeding the volume pot wiper. Last person working on this amp crossed the two wires on the volume pot. Corrected this and the nasty noises are gone.

I noticed that both channel volume pots behave the same with what I described earlier; Neither pots cuts the signal when dialed to zero. Both act the same when dialing up the pot, slight decrease in volume followed by increase in volume.

The Bass Channel pots (where the issue was)... The treble pot affects the volume of that channel. As you dial up the treble pot the volume diminishes. You can make adjustments to dial in a desired tone and volume, but that seems weird. Is this normal for this amp?

Thank you. Below is a before picture of the pots.

BTW, the range of the Bass Channel Vol pot with hot and wiper disconnected from circuit was about 26 ohms to 1.2M

[Helmholtz]

Unless a tube shorts, perhaps. In that case, I suppose you could approach that scenario in two ways: You could use those 1Ω resistors as fuses, since Fender didn't include secondary fuses back then.
Or, you could spec them to survive a tube short, and rely on the mains fuse blow before there is any immediate damage to the transformers.

Yes, and I'm sure this has been discussed before (just as the power rating of screen resistors).
Now what is your recommendation?

(A 1R / 0.25W cathode resistor would allow for a cathode current of 0.5A. This is more than 4 times the cathode of current of a 6L6 at full power and would correspond to a power draw of around 200W - but probably not enough to blow the mains fuse.)

Actually I think a standard 0.25W resistor doesn't make a reliable fuse, as there is no guarantee that it will safely open (no arcing, no flames etc.), so a higher power rating and relying on the mains fuse might be safer.