Bassman Volume Control Issues

[ID Crisis]

Hoping someone has seen this before. I am working on a Fender Bassman Blackface. It appears to be an AC568 circuit. It has Bass and Normal channels each with Volume, Treble, and Bass controls. The Bass channel has a Deep switch and the Normal channel has a bright switch.

The issue occurs on both channels. As you turn the volume control down the volume decreases, but as soon as you turn it all the way down, rather than going silent, the signal comes back as if it were turned to 2 or 3.

I tried replacing the volume pot on one of the channels, but it still does it. One of the preamp tubes tested bad so I replaced it, but the problem still exists. It's got new filter caps. I'm ordering some of the other caps because they measure out of spec on my capacitance meter. Could they be the cause of my problem?

Thanks in advance!

[Jazz P Bass]

I have seen pots do that.

If you measure from the center pin of the pot (the wiper) to ground, you should see the wiper grounded when you the pot is turned all the way down.

Try it & see what readings you get.

[bsco]

For what it's worth, I use an analogue meter to check pots for proper resistance tracking....the needle gives a visual indication of the tracking of the pot....and if there are any dead spots you will see the needle move erratically......I check the pots from the center terminal to each outside terminal....hope this is of some help to you.....

[52 Bill]

It could be that both pots are bad in the same way, but I'd check the grounding scheme first.

[Chuck H]

Is this amplifier stock? Or has it ever been worked on by anyone who was only marginally qualified? Not you'll know these things for sure but perhaps you can tell.

[pdf64]

Some 12AX7 have excessive coupling between their sections, which can cause this effect.
So eliminate the tubes in V1 and V2 slots (depending which channel).
Pete

[soundguruman]

There's nothing wrong. That's normal.
It happens in a lot of Fender amps.
The grounds are not all zero ohms continuity. There is some resistance in between them...however small.

That is correct, there is coupling between sections despite the volume settings. Good Point.

It is not necessary to have the volume go to "zero," there's going to be be little bleeding.
Check just about "any" blackface Fender...they pretty much all do the same thing.

[km6xz]

If it is a BF and AC568 wiring, it is not stock.

[km6xz]

No a good 12AX7, the section isolation is very good, at least 60db down so it is unlikely to be a problem with signal cross coupling. Besides, they are out of phase so the any cross-coupling would lower volume, not raise it. There are lots of other paths for signal but the most logical would be a signal ground return on the pot being poor. The bottom of the pot is the return so is there is a high path resistance either in the carbon trace on the pot or signal ground path. A way to test which is just short the wiper pin to the grounded pin.
Listening to the unmuted sound with the pot will tell you where it is coming from. If it is muffled and bassy, it could be power supply decoupling cap dried up, or if primarily just high frequency and noise it could be a bad tube with cross coupling. If it sounds normal in tone but just weak, look at the signal ground the bottom of the pot, or worn out carbon trace in the pot.

[g1]

For grounding issues, always check where the star washer meets the chassis on the pots and jacks of these type amps. Also make sure the nuts are not loose. Oxidization at the star washers can create grounding problems that can cause all kinds of weird symptoms.