Parasitic oscillation problem untraceable

[JMPGuitars]

No a 12ax7
I know I should get other tubes. The ones in there are new ehx 6v6gt
Maybe the OT is faulty.. that would be a bummer.
This is a tricky thing to find ...
NFB? Isn't that bad for nice overdrive?

I said to switch to 12AT7 for a reason. Try it out and see what happens. If not better, raise the plate voltages.

NFB is kinda bad for the distortion we prefer, but it's good for some issues.

[Bieworm]

I said to switch to 12AT7 for a reason. Try it out and see what happens. If not better, raise the plate voltages.

NFB is kinda bad for the distortion we prefer, but it's good for some issues.

Ok here is with the PI 12at7
Plates 147V
Grids 47V
Cathodes 73V

PI 12at7 with 56k (input) +68k load resistor ( that's what I have left in the box
Plate (input) 175V
2nd plate 171V
Grids 53V and 57V
Cathodes 85V both

NFB tested with 100k and then 820R

Same fizz

Still want to lower the plate resistors more?

10k:
Plates 230V
Grids 84V
Cathodes 112V

Same fizz

[geoff 1965]

have you tried a 47 or 100pf snubber across the phase plates?

[geoff 1965]

have you tried a 47 or 100pf snubber across the phase plates?

[Bieworm]

have you tried a 47 or 100pf snubber across the phase plates?

There is already 100pf there

[Bieworm]

I tried tungsol 6L6 STR . Didn't check the bias, but same fizz

[JMPGuitars]

PI 12at7 with 56k (input) +68k load resistor ( that's what I have left in the box
Plate (input) 175V
2nd plate 171V
Grids 53V and 57V
Cathodes 85V both

Same fizz

Set up the PI so the voltages are on target to what I sent you with the 12AT7. You will need to lower your tail resistor, try 22K. This won't likely fix the problem, but it will be setup well for when the problem is resolved.

Considering everything we've tried, it's likely the fault lies within the tubes or the OT. Either way, you should still try the snubber circuit. At this point I'm leaning towards a defective OT.

Thanks,
Josh

[Bieworm]

PI 12at7 with 56k (input) +68k load resistor ( that's what I have left in the box
Plate (input) 175V
2nd plate 171V
Grids 53V and 57V
Cathodes 85V both

Same fizz

Set up the PI so the voltages are on target to what I sent you with the 12AT7. You will need to lower your tail resistor, try 22K. This won't likely fix the problem, but it will be setup well for when the problem is resolved.

Considering everything we've tried, it's likely the fault lies within the tubes or the OT. Either way, you should still try the snubber circuit. At this point I'm leaning towards a defective OT.

Thanks,
Josh

I have news. I put the signal from point x into the poweramp of the modern classic. Guess what? Fizz.
So it's somewhere in the preamp...

[Bieworm]

I'm gonna try the reversed and put the preamp of the M classic into the output ofthe imperial. If the fizz is gone it's certainly the preamp.

[JMPGuitars]

So it's somewhere in the preamp...

Are the preamp tubes new? Post some photos of your build.

Try adding a bypass cap to R3. .68µF to 2.2µF. You can also try increasing that resistor value to between 1.5K and 2.2K.

[Bieworm]

So it's somewhere in the preamp...

Are the preamp tubes new? Post some photos of your build.

Try adding a bypass cap to R3. .68µF to 2.2µF. You can also try increasing that resistor value to between 1.5K and 2.2K.

Personally.. with these values now it's no more fizzy than my modern classic. It's also much more gainy. I'm going to try the CR on V1 like you proposed. Put 22k on the PI plates? Should I up the cathode resistors to lower the cathode voltages? Like 470K perhaps?
Here are pics. Have to tidy things up ofcourse..

20210109_161348.jpg

20210109_161400.jpg

[JMPGuitars]

Personally.. with these values now it's no more fizzy than my modern classic. It's also much more gainy. I'm going to try the CR on V1 like you proposed. Put 22k on the PI plates?

No, 22K TAIL resistor. 470 ohm (not K) cathode resistor.

PI change:

PI-Tail.jpg

Preamp change:

V1-Input.jpg

[Bieworm]

Personally.. with these values now it's no more fizzy than my modern classic. It's also much more gainy. I'm going to try the CR on V1 like you proposed. Put 22k on the PI plates?

No, 22K TAIL resistor. 470 ohm (not K) cathode resistor.

PI change:
PI-Tail.jpg


Preamp change:
V1-Input.jpg

Did the changes.. but I can't test on volume because my daughter is asleep..
But here's the chart. I should raise the plate load resistors of the PI?
Bias of OT is 78%

20210109_225212.jpg

[JMPGuitars]

Did the changes.. but I can't test on volume because my daughter is asleep..
But here's the chart. I should raise the plate load resistors of the PI?
Bias of OT is 78%

Don't make any more changes until you listen to it at volume.

[Bieworm]

Did the changes.. but I can't test on volume because my daughter is asleep..
But here's the chart. I should raise the plate load resistors of the PI?
Bias of OT is 78%

Don't make any more changes until you listen to it at volume.

Played it. Sounds pretty fine that way. Fizz, but yeah...
Doing some resoldering of the power supply right now. Who knows...
And oh yeah, wired a 250R cathode resistor which brings the bias to 89%. Seems more in line with the design...