Shortcuts to killing oscillations in PA to guitar amp conversion?

[Carlsoti]

Trying to track down HF oscillations that occur at only the highest volume settings. Amp had ground bus, can cap, isolated input and output jacks, and NO NFB. Lets assume I've checked grounding, lead-dress, cap health, etc.

What are some tricks to kill oscillation feedback locally to help try to isolate the location of the problem?

Thanks.

[Dave]

Don't turn it on.


Just kidding.... Go from stage to stage with an O-scope until you find the problem. THat's a good trick.


Dave

[tubenit]

I have 7 thoughts for you....................


1)  We don't drive cars with the gas pedal floored all the time. Same with amps. I think we should strive to avoid oscillations
     but I don't personally worry about oscillations that might occur with every potentiometer dimed at volumes that I will
     never ever come close to playing at. One of my favorite amps will oscillate if everything is dimed. I think it sounds lovely
     even at volumes much higher than I'll ever come close to playing at. 


2)  Try lower gain tubes like a 12AY7 in V1  or a 12AT7 (or even 12AU7) in the LTPI


3)  Smoothing caps  (plate to cathode) like on Dumble amps sometimes help.  I consistently use an "enhance cap" across
     the plate resistor of the 82k LTPI plate resistor.  You can do a search on the forum.  I find this very useful to me on
     eliminating high frequency harshness/hash. I don't use "snubbing caps" across the LTPI plate to plate. To my ears these
     are tone killers and mute the sound way too much.


4)  Some tubes are simply prone to oscillation even sometimes new ones. Try different tubes.


5)  Sometimes shielded wire especially on long runs can be useful.


I have on occasion, stopped oscillation by moving the heater wires. I took a non-conductive chop stick and gently moved the heater wires around to see if it changed the sound.


I've twice had a silver mica bright cap across a volume pot be a problem and replaced it with a ceramic cap of the same value eliminating oscillation.


With respect, Tubenit

[pdf64]

Consider the possibility that your layout is poor, eg proximity between earlier and later stage circuits being too close, leading to unintended coupling, positive feedback and oscillation.

[Carlsoti]

Thanks for the inputs, guys. I threw together the OG post before leaving for work this morning, so it didn't include some vital information. Sorry for that. I was more interested in various tips and techniques beyond the typical troubleshooting used for ANY amp, not just this one in particular. Tubenit's point #3 is right in line with that. Every time I looked up "snubber" it seemed to lead to info regarding the RDH snubber circuit from the 30's or the Dave Zimmerman article that discusses the same thing. Does OPT ringing show up as/sound like oscillation? The way I read that article makes it seem like that's more of a fizzy HiFi thing, so I've somewhat left that idea behind. It could just be that terminology has changed and solidified since then, and I'm not steeped deeply enough to know that etymology.

With that said, this is the same Allied/Knight 93 SZ 682 PA amp conversion I posted about last year. After I returned it last time, the owner said he liked it so much he was going to tour with it, which I quickly shot down. As it was then, it still had NO fuse, just as it was designed. There's a tell-tale burn scar inside the amp indicating that what passed for "Good Enough!" in the 50's just doesn't cut it these days. That, coupled with the oscillation at high volume is what made me say "NO!" When he finally got back into town, I offered to make it tour-able, because I feel an artist should be able to rely on their tools.

On pdf64's point, I KNOW that the layout of the amp I'm working on is poor, but the tubes and pots are where they are and there's not really any more space to be had in between. I have considered moving the rectifier the to old "phono plug" octal in the "middle" of the amp, which would allow me to align the pre-amp, PI, and power tubes as it's typically done these days. I've been hesitant because moving HV AC to the middle of the amp and closer the pots just seems so wrong, but it would put more space between the mixer tube(V2) and one of the the power tubes.

It's not a particularly high gain amp, which is why it has me so flustered. With all 6SL7s, it howls just above halfway on the volume. With all 6SN7s, it's just at the very extreme volume settings, but the tone suffers quite a bit, and as expected, voltage drops considerably across all points. Mixing and matching Ns and Ls falls in between, in line with the expected gain and power consumption.

On the note of Dave's advice, when it oscillates, it's pervasive, literally everywhere in the system, which makes me think it's power supply related, though subbing in individual filter caps for the original can offers no respite.

I've shielded the signal runs from the tubes to the pots and back. Chopsticked like mad to no avail. I've read so many different guides on grounding; RDH, Blencowe, Aiken, etc, that I'm considering buying a new shovel and setting forth on permanently grounding myself in the hole I've dug. 

The original design had 3 6SC7s feeding a pair of cathode biased 6L6G output tubes. The smaller bottle sockets have been switched to the pinout of 6SN7/6SL7s for better road-serviceability. I've swapped through a half dozen of each of the pre-amp tubes, which might not be a big enough sample size. It has 6L6GCs in the output. I keep thinking the extra load from the stronger power tubes at high volumes might have something to do with it, but as I'm doing this fix gratis, I'm hesitant to buy a pair of unknown, low-power 6L6 tubes that may just end up sitting around if that doesn't fix the issue. Additionally, I'm running mains to the 130V tap to bring down the voltages a bit, but it's nothing that I would expect to cause these issues.

Is it possible that the more powerful tubes are pulling down the voltage at extremes, setting up the oscillation in the power section, making the oscillation pervasive, which spikes the voltage, masking the voltage drop from the 6L6GCs due to DMM sampling rates not being high enough?

I'm running out of love for this old beast, but I also don't want to let my buddy down. At lower volume settings, it's really nice. I just want to bullet-proof the thing.

On tubenit's first point, what failure modes could be expected if this is run "right at the edge" for too long? I'd assume smoke and flames, dead transformers, etc. I don't want that to happen. If it were my own amp, I'd accept the risk, but that's not the case.

SAMS docs included so you can see what was originally there.