Bassman 50 volume issue

[nsubulysses]

I have a Bassman 50 that makes volume even with the volume all the way down. Then when you turn it up a little bit it actually loses a little volume and then comes back up around 3 on the knob. Also when its turned all the way down it seems the EQ is not engaged -- it is very thin and trebley. Once you turn up the volume and get to about 3 it comes in way fuller and bassier with the EQ affecting tone.

It actually sounds kind of awesome because it sounds like a Univibe when turning the knob between about 1 and 4, but this seems abnormal, right? The amp works fine otherwise.

And I should add that there was some play in the front to back motion of the volume pot so I tried a replacement but it was not the right answer.

[Enzo]

Classic crosstalk.

The parts after the volume control are picking up and amplifying signal radiated from earlier stages.

The goes down until 3 thing is because the radiated signal is out of phase with the signal at the control, so the little bit leaking through is progressively cancelled as you turn up. At 3, the real signal is stronger than the crosstalk, and so up from there it works normal.

[g1]

Also the crosstalk can be between 2 halves of the preamp tube, so you may get a bit of variation swapping V1 or V2, whichever channel you are using.

[Chuck H]

Has the amplifier been worked on or modified as far as you know?

http://www.studiosoundelectronics.co..._bassman50.pdf

[nsubulysses]

I got the amp a few months ago and to me it seemed to be original, except for the preamp tubes. It had two old 70s mismatched 6L6s so it got new power tubes, screen grid resistors, and one cathode bypass cap replaced. Could use a recap but it works. Now it got banged up in some gig loading in/out scenario and the volume knob was almost broken off, so that's why I replaced it, hoping it would be right after that.

I will run a signal through it tonight with volume all the way down and move some wires around and also try different preamp tubes and see if I can get the crosstalk to vanish.

Thanks for your input

[Chuck H]

The reason I asked is because the two channels are actually in phase. There should be no phase cancellation between channels. It could still be some sort of crosstalk between out of phase circuits, but not complete channels. In this case I would look for grounding faults or daisy chained grounds combined with failed or failing decoupling/filter caps.

Once you've repaired the physical damage and install good tubes of course.

It could still be a bad preamp tube too. I've seen tubes do some weird things. What you describe wouldn't surprise me.

[nsubulysses]

Here is a pic I have of it when it came in originally on Feb 3.



Wonder if those coax cables right by the coupling caps are the cause. But then I guess it was like that from the start and I didn't notice? And the customer didn't notice til the amp got banged up? that doesn't really make sense.

Power supply caps are a little bulgey I was amazed only one of the Mallory cathode bypass caps was bad when I got it. Seems like those never stand the test of time. I will check through all this thanks!

[Enzo]

Wonder if those coax cables right by the coupling caps are the cause

Well, they are shielded cables, so why would they?

[pdf64]

Suggest that you try different 'known good' 12AX7 in the 1st stage of the relevant channels before looking for problems / tinkering around with things.
As RG says, they're in sockets for a reason.

[nsubulysses]

I clipped a cap over the preamp power supply cap and the volume at 0 is gone. Decoupling problem. "It could use a recap but still works" 4 months ago but not anymore. Before I did this i tried all new AX7s and and moved a few wires around sporadically with no results.

Thank you for your input everyone. I wouldn't have thought power supply caps, especially since the problem came about after the front panel got banged up in loading

[Chuck H]

Carefully check all factory grounds too. It sounds like you've got It

[nsubulysses]

Will do, thanks Chuck!

[Enzo]

if you look up "logical fallacies", on the list you will find this one: post hoc, ergo propter hoc. Which is Latin for essentially, since it came after this, it must be caused by this. it happens, but it is not valid reasoning on its own. it is tempting to say, gee my panel just got banged up, so it must be the reason I have this new issue. And you found out why temptation should be avoided.

Decoupling failure is another way to get crosstalk, or sometimes instability, depending on the circuit. In the case where the signal is getting past the decoupling, you would probably be able to see the signal riding on that B+ node with a scope.

[Chuck H]

+1 to all!
I may not have thought to check the rail for coupling, but I WOULD have changed bulgy filters . I don't do repairs so this sort of if/then lesson is really good for me.

Regarding the if/then of "since it came after this, it must be caused by this." I totally get that. I learned that lesson in my youth because I was a motor head. Being a kid I had a few "not nice" cars. It amazed me how often what seemed like the obvious problem wasn't the problem. And more so, how often fixing a failure that seemed like the cause didn't entirely fix the problem. So there were analogous failures. It happened enough that it stopped surprising me about the time I started to dislike working on my own car

[nsubulysses]

coincidentally I was searching for info about line outs from the OT as per my other thread and then I found this page -- Doug Circuits

It seems to say all the things you all have been talking about, and just exactly what I was experiencing.