Fender 5F6A Bassman Clone Problem - Treble = Volume?

[nash076]

Hey folks, I have a weird issue and was looking for some assistance ...

Recently, I had to replace the filter caps on my 5F6A due to it blowing fuses. The filter cap replacement solved the problem, and the amp is running again, but something bizarre started happening: the Treble knob, when turned up, kills the volume.

So if I turn the treble knob toward 11, the volume from the entire amp goes dead. If I turn it back to zero, full volume, no problems. Bass, Mid and Presence all appear to be behaving properly, as are the Bright and Normal volumes.

Before the filter cap replacement, when the amp was running everything was fine. Any idea what could be causing this?

Also, if it makes any difference, my preamp tubes are all EH 12xa7's, the power tubes are Tung-Sol 5881 re-issues, and the rectifier is a JJ GZ34.

Any help or insight would be greatly appreciated.

[martin manning]

You mean the treble pot right? All of the preamp signal passes through that pot, so if the wiper fails to make contact with the element you lose the signal. Did you disturb that pot in any way?

[nash076]

You mean the treble pot right? All of the preamp signal passes through that pot, so if the wiper fails to make contact with the element you lose the signal. Did you disturb that pot in any way?

Herp derp, yes. Treble. Edited to reflect that ...

Not that I'm aware of, but I'll double check the connection to be 100% sure ...

[martin manning]

An easy way to check it is when the signal has been lost, jump the clockwise and middle lugs with an insulated tool like a small screwdriver.

[nash076]

An easy way to check it is when the signal has been lost, jump the clockwise and middle lugs with an insulated tool like a small screwdriver.

Tried it. When I short the lugs, the signal dies entirely.

[martin manning]

I think the top of that pot (clockwise lug) must be shorted to ground then.

[nash076]

I think the top of that pot (clockwise lug) must be shorted to ground then.

Well, I didn't *see* a short ... but to be on the safe side, I just went through and re-soldered every joint on the board and the tubes that was part of the signal chain.

Must have either had a cold joint or something going where it shouldn't, because once I finished, the treble pot's working normally again.

So that was probably it. Thanks.