Whistling oscillation - interesting find

[pjd3]

Hello,

I offered to attempt a fix on another guitarists Bogner Alchemist. He had noticed that one of the preamp filter electrolytic caps was leaking along with a moderately loud high pitched steady oscillation that made the amp unusable.

I replaced all 4 of the 22uF/450v preamp filter caps suspecting that would resolve the issue since one was leaking badly.
This did not fix the oscillation so I began the usual process - pulling V1-V5 one at a time first of all.
Pulling V3 was the 12ax7 that ceased the whining so I looked at the schematic to see that this tube was part of the 2nd channel, the overdrive channel. Substituting tubes did nothing to resolve the issue so I reflowed some solder joints - tube sockets, switch pcb lugs, etc, tapped around on components etc... but to no avail.

Finally I noticed that exercising the 2nd gain stage pot caused all kids of sonic havoc. But, as I excersised that volume pot, the oscillation slowly started to diminish. Eventually, this seemed to make the oscillation disappear which I was precariously grateful for.

But, just before that I decided to take a look at the voltages present on the contentious V3. What I found was that when probing the plates (and maybe other elements of the tube) with a Fluke meter, the oscillation would completely disappear.
Think about what was happening here, without being certain, I hypothesized that the impedance from the Fluke meter was creating some kind of low pass filter that was killing the oscillation, or somehow changing the impedance of the circuit enough to no longer create an instability.

Can you comment on what you think was going on with that? It was almost tempting to look up the Flukes input resistance/capacitance and make a little RC to imitate the Fluke input and strap it to what ever pin on the tube socket that I was looking at. I nearly certain it was one or both of the plates on the sensitive V3 tube.

Thanks for your feedback!
Best,
Phil Donovan

[Stevem]

Does that pot have a grounding wire soldered to it from other the other pots?

Daisy chaining grounds is not a good thing!

If so is that pot and all the others nice and tight to the chassis?

That gain stages cathode bypass cap could be leaky also.

[pjd3]

Hi Stevem,

Those pots and switches are installed a separate board that I haven't removed as of yet. Perhaps I will unfasten that board and do some trouble shooting on it. The whistling did go away after many, many burnishing turns of the channel 2 gain pot but, we may not be out of the woods yet. I'll need to get that board out to see how Bogner/Line 6 actually configured the ground wires to the pot.

Its not clear to me how say, a dirty pot could be part of creating an oscillating gain stage. I know tube grids need to see some path to ground or they can become problematic, and wondered if a volume pot wiper effectively being lifted from the resistive substrate could have been akin to a "semi-floating" grid. I dunno. But I certainly found it interesting that the presence of a voltmeter looking at the plates removed the oscillation. Had to wonder what that was insinuating.

Thanks for stopping by. I'll try the amp again tomorrow and see if anything has changed, and if I can bring back that sensitivity/oscillation.

Best,
Phil Donovan

[pjd3]

As I look at the schematic I see that the originally sensitive Pot (2nd high gain channel volume pot) has its wiper going right to the grid of a second channel V3 grid. There is a branch of what looks to be a couple of pF size capacitors in a feedback circuit from the 2nd tube stage plate back to the grid of the first stage (never saw that before - oscillation prevention?).

So, without being certain yet, I'm suspecting that a dirty-not-used-much pot was mimicking a lifting wiper that could effectively be lifting the grid on the first V3 gain stage. That might explain why a vervant effort in exercising the volume pot gradually but completely caused the whistling oscillation to vanish.

Perhaps its time to get some good contact cleaner and give a shot or two to all the pots in that amp. why not?

Thanks all,
Best,
Phil Donovan