Parasitic oscillation problem untraceable

[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...

[JMPGuitars]

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...

Awesome. I'm thinking that getting the snubber circuit in there may help with the remaining carbonation. Certainly worth a shot.

[Bieworm]

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...

Awesome. I'm thinking that getting the snubber circuit in there may help with the remaining carbonation. Certainly worth a shot.

Yeah, but I need some parts... have all but the 8M2 resistors.. + I used the 1k5 5W resistors for the screens.
But will try for sure!!

I also think It's the nature of the amps circuit to be nasty fizzy midrangey

[Bieworm]

Josh.. if you look closely at the schematic it appears some pots have a different mark for grounding point. Is the series grounding of pots with 1 wire to ground near the input likely to cause oscillation? It looks to me like all preamp components and the midbite pot should be grounded on the ground buss?

[JMPGuitars]

Josh.. if you look closely at the schematic it appears some pots have a different mark for grounding point. Is the series grounding of pots with 1 wire to ground near the input likely to cause oscillation? It looks to me like all preamp components and the midbite pot should be grounded on the ground buss?

That's not meaningful. One of them was done that way to not pass through the grid line (schematic clarity), and the other makes no sense not to just draw a line there, pretty weird, but still all of them mean the same thing. Good ground schemes are good ground schemes, if you followed what you know from the other builds, you should be fine.

Either way, not likely to cause an oscillation, and also not likely that you actually have an oscillation. You've got a gremlin.

From this question I assume you're still having an issue. I think you should be patient until you get the snubber parts you need. Besides that, you could try leaving the 12AT7 in the PI, and try a lower gain tube in V1. 12AT7 would be a good starting point if you have another.

[Bieworm]

Josh.. if you look closely at the schematic it appears some pots have a different mark for grounding point. Is the series grounding of pots with 1 wire to ground near the input likely to cause oscillation? It looks to me like all preamp components and the midbite pot should be grounded on the ground buss?

That's not meaningful. One of them was done that way to not pass through the grid line (schematic clarity), and the other makes no sense not to just draw a line there, pretty weird, but still all of them mean the same thing. Good ground schemes are good ground schemes, if you followed what you know from the other builds, you should be fine.

Either way, not likely to cause an oscillation, and also not likely that you actually have an oscillation. You've got a gremlin.

From this question I assume you're still having an issue. I think you should be patient until you get the snubber parts you need. Besides that, you could try leaving the 12AT7 in the PI, and try a lower gain tube in V1. 12AT7 would be a good starting point if you have another.

Did the snubber yesterday. It only darkened the tone. I put the 1000pf back in because it didn't solve anything.
Even tried a 12au7 earlier on in V1. No change, just less gain.

I have a few other suspects OTOH. Looking for the common part of the circuit . V1 tube socket malfunctioning? I tightened the pin holders, but to no avail. What if the power supply caps are the origin of the problem? I changed nearly all electrolytic caps of the preamp yesterday too.

But anyway.. I don't think I dig the OD on this amp. I don't like the midbite circuit after the tone stack. It wipes the total tone control from the tablet. I was thinking of removing everything between the plate of V1 where the OD circuit branches off until the .047uf after the plate of V2b and insert the circuit of the tremolo TMB with the modern classic tone stack. (sans tremolo) I'm drawing a schematic that I will show you if it could work and what needs to be changed perhaps.

[JMPGuitars]

But anyway.. I don't think I dig the OD on this amp.

In that case, turn it into a Tremolo TMB Reverb 6V6 amp. I'll send you the docs in email if you want to make the first one.

[Bieworm]

But anyway.. I don't think I dig the OD on this amp.

In that case, turn it into a Tremolo TMB Reverb 6V6 amp. I'll send you the docs in email if you want to make the first one.

Nice! With use of the deluxe reverb transformers I hope?

[JMPGuitars]

But anyway.. I don't think I dig the OD on this amp.

In that case, turn it into a Tremolo TMB Reverb 6V6 amp. I'll send you the docs in email if you want to make the first one.

Nice! With use of the deluxe reverb transformers I hope?

Possibly. Email me the transformer info for what you have and I'll see if it will work. Also send a good deluxe reverb schematic including voltages.

[Bieworm]

In that case, turn it into a Tremolo TMB Reverb 6V6 amp. I'll send you the docs in email if you want to make the first one.

Nice! With use of the deluxe reverb transformers I hope?

Possibly. Email me the transformer info for what you have and I'll see if it will work. Also send a good deluxe reverb schematic including voltages.

OT is a hammond 1750H
PT is a classictone deluxe reverb replacement type without heater center tap
I will look for the schematic and voltages

[JMPGuitars]

OT is a hammond 1750H
PT is a classictone deluxe reverb replacement type without heater center tap
I will look for the schematic and voltages

You could make that work.

The OT is only 8ohm output, but that's okay as long as you match your speaker.

The PT- did you do an artificial heater CT on this build?