1968/69 Fender Bassman AA864 - Oscillation Issue

[GussyLoveridge]

Hi Folks -

I have an AA864 Bassman Amp on my bench. It came in for a recap.

There was a mod done to the amp to have a triode/pentode switch done previous to my work.

After recapping, cap forming, cleaning sockets and pots, a little burnishing of the grounding 1/4" jacks, and bias adjustment I am having an issue with a low frequency oscillation somewhere between E3 and Eb3 or approximately 165 Hz and 156 Hz.

It seems to follow both channels with an instrument plugged in or not. But it only seems to happen in Pentode mode.

I haven't started pulling tubes yet nor have I hooked it up to my scope. I figured I'd just post here to see if anyone has any ideas. It's a constant tone. Not the motor boating sound I've dealt with before.

As always, I do appreciate any help you have to offer. Surely someone has come across such and issue before.

Best,

Gus.

 

[emrr]

No ideas, but that seems a very odd frequency for oscillation in a tube amp. Grounding various grids would be my first move.

 

[dmp]

But it only seems to happen in Pentode mode.

I'd guess there is something wrong with how the triode/pentode switch is wired. Or at least something to check first.
You could draw it out, and then check it. Assuming it is a dpdt switch it might be only doing one tube in the push-pull pair, which lets in unfiltered B+
The power tubes have less filtered B+ since it cancels in push-pull. The switch should be moving both screen grids from B+ to plates.

 

[soapfoot]

I assume you’ve taken the obvious step of pushing wire leads around with a chopstick to see if anything changes?

 

[sodastereo]

remove that modification, leave it original, you can search for the scheme and layout in google.
If the wobble goes away, that's your problem. If not, it could be the proximity of cables, a valve, a defective component, oscillation due to transformer induction, etc.

 

[GussyLoveridge]

Hi everyone - I of course have a copy of the schematic and layout and will do all the regular trouble shooting. I guess what I am asking about and what has surprised me is the frequency of the oscillation and wondering if anyone has come up against this. Doesn't really seem like 60/120 Hz or high frequency oscillations that I have dealt with in the past. The customer enjoys the triode/pentode mod and does not want to remove it. I could of course temporarily disconnect it to verify that it isn't that mod which is causing the trouble - this is a good idea - thanks.

 

[sodastereo]

The ideal would be to use an oscilloscope, it would be much faster to find the cause, sometimes visibly everything is fine, but not inside. If after leaving it original, the fault is still there, you could take some photos, I would be happy to help you

 

[CJ]

I never had that problem, what kind of caps, f and t? You might try bridging the old cap across the new cap,

Check the grounds coming out of the dog house, there are two of them, one might have worked it's way loose when you soldered in new caps.
CBS used to wrap a bunch of leads with a strand of wire for a makeshift shield as you have no doubt seen before, maybe it got removed? Finding the source of these problems is one of the toughest things to figure out but if you persist you will fix it.

 

[GussyLoveridge]

I really appreciate the help folks.

Yes, I used F&T for everything in the dog house and the negative bias supply. (I did use 100 uF 100V here instead of 50 uF 50V)

I used Spragues for cathode bypass 25uF/25V.

For coupling I used Mallory 150 series.

There is an additional 16 uF/ 475V cap along with the rest of the main filters under the dog house that does not show up on the schematic but looked stock and feeds the first stage.

 

[GussyLoveridge]

When I am plugged in to the normal channel (Channel 2) V1 should have its grids shunted to ground via the shorting jacks at the input and it should amplify nothing as the signal is at ground or there is zero signal.

It seems as though my issue is that V1 is very microphonic and on pentode mode at higher volume levels and in the area of E and Eb the head is vibrating enough to cause V1 to amplify its own vibration/noise and oscillate. The tube seems to have some resonance at around those frequencies. (Is resonance the correct word here?)

I have never run into this problem before. I had just a minute or two to frig around with this last night. Tonight I will pull that tube and put a known good specimen in its place.

Weird.

 

[soapfoot]

I’ve seen that behavior before—I suspect you’ll solve it with a good tube

 

[CJ]

New tube sockets right? Shields installed? Yeah that's a weird one,

No wait. This is not the AC 15 thread so old socket. Maybe re tension the socket. I take an old small screwdriver and put it on the belt sander or grinding wheel and make an ice pick out of it which might come in handy tomorrow because we are about to freeze ourass off

 

[GussyLoveridge]

Old socket. Tension was good. Shields were installed.

Bad sockets on AC15s?

 

[GussyLoveridge]

New tube - problem solved.

The old tube was an Amperex ECC83/12AX7

It was replaced with a JJ ECC83S I had in stock.

Funny how the mind goes to places that are complicated when the problem is often very simple.

 

[emrr]

I might guess some banging around or a drop of the amp rearranged some internal elements of that tube.