Control pots issue, help please

[Helmholtz]

Thank You Helm
I still have half of 12au7 unused. I can made a cf to put in front of, between tone control and 12au7 gain stage. Do You think it solve my problem, please ? Thanks.

That sounds like a promising idea. But first I would try to verify that lowering grid impedance actually cures the issue (see my proposals above).

[J M Fahey]

Thank You,
Without nfb the amp is stable, meant have not the issues mentioned before.

Then by all means do NOT use that much NFB.

Back it down to 12dB so you have better than 6dB stability margin.

DonŽt band-aid a poor design choice.

In any case, why do you need so much NFB on a *guitar* amp?

All youŽll get is sharper edged, flatter topped, symmetrical halves squarewaves when overdriving, plus lower output impedance (higher damping), all undesirable, you might as well make an SS amplifier, not kidding.

[catalin gramada]

I tied 23pF ( 2x47 series) from grid of 12au7 to ground. It osscilate on very different waveforms , depends by treble pot position

[catalin gramada]

Then by all means do NOT use that much NFB.

Back it down to 12dB so you have better than 6dB stability margin.

DonŽt band-aid a poor design choice.

In any case, why do you need so much NFB on a *guitar* amp?

All youŽll get is sharper edged, flatter topped, symmetrical halves squarewaves when overdriving, plus lower output impedance (higher damping), all undesirable, you might as well make an SS amplifier, not kidding.

Hey JMF , it is boomy, actually found 9-12 db tame it enough and let me keep enough low end. This happen with cabinets I have. I made a sealed one with 158cb from eminence and found also to be the best rapport
12db is what I use and issues was related to that. It is a amp for bass guitar

[catalin gramada]

I tied also DC coupled the 12au7 to 12at7 PI and is the same. Please let me know what did from posted picture a bad design related to my actual issues? I.m pretty interested to know what kind of predictable mistakes the circuit have, please. Thanks.

[catalin gramada]

So the problem is related by the tone control and input of12au7 , that is correct ?

[Helmholtz]

I tied 23pF ( 2x47 series) from grid of 12au7 to ground. It osscilate on very different waveforms , depends by treble pot position

This at least shows that there is an influence of grid circuit impedance. Next I would connect a variable resistor of 500k or 1M directly between the 12AU7 grid and ground (short leads!) and vary/lower resistance until the amp is stable. Of course this will reduce preamp gain but the result will show if a passive solution would make sense.

I don't see something obviously wrong in your circuit but please post complete schematic. The main issue seems to be a relatively high feedback ratio for a given non-ideal layout and OT and probably insufficient frequency compensation.

BTW, cabinet boominess could also be adressed by acoustic cabinet damping - or by a high damping factor SS power amp.

[Helmholtz]

So the problem is related by the tone control and input of12au7 , that is correct ?

The settings of tone and volume pots change the grid circuit impedance.

[catalin gramada]

Schematic and grounding layout,please

[catalin gramada]

Some mentions are necessary as follow:
1.I already buit an amp exactly same which not perform any issues and work flawless...who knows why...?
2.tried 1M grid leaks did not changed nothing
3.The problem is the same AC or DC coupling between 12au7 and PI
4. power stage work without problem with signal from generator, even with 20db feedback

Heard a noise like arcing when power stage clipping. I don't know if commin from OT or power tubes. The problem is related when treble pot is in max position, or ,if is at max when volume pot goes to max, and all time when power stage goes to clipping

[Helmholtz]

Schematic and grounding layout,please

[ATTACH=CONFIG]54394[/ATTACH]

[ATTACH=CONFIG]54395[/ATTACH]

Power tube grid drive/biasing is not clear. I don't see coupling caps.

[catalin gramada]

That,s happen when try to do some in a rush, I,m very sorry. Sent the additional Infos in attach





It is mess due to multiple desperate soldering interventions, but You can get an ideea. Thank You

[catalin gramada]

And a pic with the cab built for it: 3.5cf sealed cab with Eminence 158cb



Found 10-12 db nfb to balance very well with tight and low end enough for 4 strings.

[catalin gramada]

Amp output and input of 12au7 with signal from generator



and clipping



12db nfb

[catalin gramada]

With signal generator connected in front of tone controls ( both treble and bass at max)



and clipping



12db nfb