Weird sound on Silverface Bassman pots.

[tele_savales]

I got a 1970 Bassman a while back which had some elements of an AA270 and AA371. I replaced the filter caps before even powering it on, and eventually reworked the power section to AA864 spec. The only changes I made to the preamp section were to replace the cathode bypass caps, remove the plate load resistor bypass caps, and I replaced most of the plate resistors w NOS 1 watt carbon comps. Also replaced the bias cap and reverted it to fixed adjustable bias. I put Tung Sols in the preamp and JJ 6L6's in it.

There's a strange sound that I don't recognize on the normal channel pots the increases and decreases w knob movement; it doesnt sound like frying bacon, it sounds like way more pronounced and thicker white noise than the standard plate resistor hiss. It doesn't do this on the Bass channel.

The caps themselves are the original "brown turds" of the era, which it seems most people here feel reasonably good about. I have checked for DC leakage on the coupling caps w the amp powered on and in circuit; and w one leg of the cap lifted. I haven't seen anything more than .005 vdc. I recently rebuilt a Heathkit C-3 Condenser Checker and the caps test perfectly out of the amp. The cap values themselves are pretty much right on the money. I've checked random spots all over the board for random DC and there isn't any.

I'm kind of stressing myself out for no reason, but I really don't want to shotgun all the coupling caps and plate load resistors. OTOH, I had a drip edge Bassman that I bought, buffed out, and sold, and that thing sounded epic- it had ALL the plate loads and most of the coupling cps replaced w brand new ones when I bought it...

Any other suggestions as to tracking down the source of the noise would be appreciated.

 

[dan40]

If you rebuilt the power amp, is it possible that you got the NFB phasing wrong. This would normally cause a squeal or a howl but occasionally it can cause other weird noises to pop up. Disconnecting the NFB wire from the output jack would be a quick way to test this theory.

 

[tele_savales]

When I put it to aa864 nfb values the trace on my scope started doing insane things that I've never seen before w the amp plugged into the dummy load with the volume barely up. Flipped the OT secondaries and it went away.

 

[schmee]

Well, if it only does it on one channel it's not in the power section right?
Unfortunately you dont know if it was there before your mods. I would always suspect the mods first. But cant tell you where to start.
I love the AA371 circuit as it was for 'clean and loud'. But could see changing it for better lower volume use. Doubt it's the turds, but you never know.
I have found modern CC resistors to be WAY off sometimes. I have several that are 40%+ out of spec. I wont use CC anymore.

 

[tele_savales]

Yeah it's still out on the bench so I think I'm going to re check all the resistors that I didn't already replace. I did do some poking w a chopstick with it on the scope and didn't see any spikes. The other thing I don't fully understand is when I'm zoomed way in on the scope the trace looks fat and fuzzy when probing the grids, but it's clean and perfect looking at the output jack and goes into clipping where you'd expect.

 

[tele_savales]

So I found that the 250pf ceramic cap in the normal channel tone stack was significantly noisy, so I replaced it as well as the tone slope and plate load resistor there as well. Just over 20 percent out, and I dug thru my stash of nos 1 watt carbons and found a few that were right on the money.
Also V3 which was a brand new Tung Sol generated quite a bit of noise so I tried a couple JJ's and that helped too.
I'll attach a few shot of the trace I'm getting with and without signal in a bit.