Parasitic Oscillation?

[Janglin_Jack]

My 50w is exhibiting what I think is Parasitic Oscillation. I have a PPIMV installed. With the MV cranked and turning up the bright channel volume, it starts to lose power at 8, gets thin sounding, then squeals, fizzes and cuts out. With the MV engaged, it doesn't squeal, but still loses power and thins out and sounds static-like between 8-10 on the bright volume. The Normal Channel seems to be operating fine. What to check?

Mike

[toner]

Set the bright channel volume on 10 with the MV on 0 and check for DC voltage at the input (1st tab) of the bright channel volume to see if the first coupling cap in that channel is leaking. If that's okay (0 VDC), try a different tube in V1 to rule that out.

Do you have grid stoppers on pin 5 of the power tubes? Try moving all the preamp grid wires (pins 2 and 7) around. Lift them up away from other wires.

If it's PO, the source could be anywhere, even after the mixer resistors, and the extra high frequency gain in the bright channel triggers it. You'd probably need a scope to find the source.

[Janglin_Jack]

I forgot to mention, on this amp, I have a tone stack switch wired in. So the board has the 56K/250pf and then on the back of the chassis there is a DPDT switch with 82K resistor and 250pf cap going to the tone stack. Those wires are bundled and shrink tubing. Maybe this could be my source? I don't really use the Tone Stack switch, I just go with the modern 33K/500pf setting, but maybe I should eliminate this first to see if that cures it???

The wires are bundled, but sort of floating over the bright coupling cap and the mix resistors.

Mike


I will check for DC Voltage at the input of the bright channel to see if the first coupling cap is leaking. Will mess with a variety of tubes to see if that helps.

I do have grid stoppers on pin 5 of the power tubes. Been poking around and nothing seems to help. But will keep trying.

Scope, I don't have an oscilloscope, so I would have to have a shop help with that. How expensive is a scope and what is a useful model to get or occasional use? I assume a shop is going to charge a bit to trouble shoot, and maybe that money can be used to invest in a scope, (if not too expensive).

Thanks,
Mike

[Janglin_Jack]

Got my amp back from my buddy who reported the issue. I decided to remove the tone stack switch, (since I use 33K/500pf tone stack anyway on this amp). Noticed I had a 33K slope resistor on the board vs the proper 56K. I think that gave me a 23K/500pf tone stack. I put that into the tone stack calculator, not sure it makes a huge difference. Anyhow, I moved the output jack and PPIMV pot so the output jack is next to the output ohm selector switch to shorten some leads and make sure the PPIMV and output jack didn't cross so much. My kids are sleeping, so I put the MV low and dimed both the normal and bright volumes, (jumpered). No issues. I will check tomorrow with my 4x12 and get the volume up and see if the amp cuts out like my friend reported. Hopefully these changes have the issue licked. Amp sure sounds awesome! Glad to have the amp back. Playing it with my 1960B with two Heritage G12M and two Silver Series V12-60. V12-60 has 96db sensitivity like the Heritage GBacks. To my ears, they have a Greenback tone and response, but more low end and more top end, (more even response). Great blend with the G12M.

Didn't check the DC voltage leakage as Toner suggested, just due to time, but will do so if the amp has any further issues.

Mike