Control pots issue, help please

[catalin gramada]

Hello.
I have some issue in my circuit regards volume/ treble control as follow:
When keep volume pot to max position I can deal with treble and gain pot without problem to entire range.
Turning down the volume pot little bit with treble in max position I cannot even open the gain pot without loud boom followed by loud noise and microphonic manifestation (ringing when I hit the chassis)
Turning treble down a bit from max and I can deal with volume and gain without problem even max dimmed
What I can do to get it stable no matter how I can deal with controls, please? Thanks.



Edit: my 12au7 seems to have its input extremely sensitive,

[nickb]

If that really is a 100uF cap in there it'll be leaking far too much current for this high impedance circuit. A 100nF film cap would be more like it.

[catalin gramada]

Hey Nick, thanks for answer. It is not any 100uF but 100nF instead, I made the sketch in a rush, sorry

[Helmholtz]

Hello.
I have some issue in my circuit regards volume/ treble control as follow:
When keep volume pot to max position I can deal with treble and gain pot without problem to entire range.
Turning down the volume pot little bit with treble in max position I cannot even open the gain pot without loud boom followed by loud noise and microphonic manifestation (ringing when I hit the chassis)
Turning treble down a bit from max and I can deal with volume and gain without problem even max dimmed
What I can do to get it stable no matter how I can deal with controls, please?

Any signs of RF oscillation (scope)?

[catalin gramada]

I dropped the volume pot and I tied 100k screen resistor directly to treble wiper pot. I get loud hiss noise when put treble pot at the end of the track. It act like a switch when put treble at max. The pot have nothing wrong, I changed couple, the same thing...

[catalin gramada]

Something seems wrong with firs pot, gain pot, heard a small pop noise when turn it up from 0 position even with tone pots down.Have to check

[Helmholtz]

Something seems wrong with firs pot, gain pot, heard a small pop noise when turn it up from 0 position even with tone pots down.Have to check

Does the plate voltage of the second triode change with turning the gain pot?

[catalin gramada]

Hey Helmholtz, no the plate voltage of second triode didn't change

[Helmholtz]

Hey Helmholtz, no the plate voltage of second triode didn't change

Anyway, as the ground reference of the grids in your circuit depends on good pot wiper contact it is always a good idea to spend separate grid to ground resistors (1M).

[catalin gramada]

I scoped the output of second triode, it flick a litle and come in back when it pops, is exactly the same

[catalin gramada]

Okay, maybe it not worth to bother with the pop of gain control. How can I get rid of noise when put treble pot at max ? It horrible switching in loud noise at the end of the track.

[catalin gramada]

When I keep the gain pot close I can put the treble pot at max. When I open the gain pot ,even just a little, I get loud noise when put the treble at max. The noise didn't come progressive, just switch suddenly into noise mode.

[catalin gramada]

I just thinking, maybe circuits are not isolated enough due to PS scheme, or grounding, and interfere at a point...

[Helmholtz]

Please note my posts #4 and #9.

How does the noise sound? Like hiss or power supply ripple or 50Hz hum or a combination?

[catalin gramada]

In this moment the circuit looks like this:



I suprimed the volume pot after tone stack and tied the screen grids of 12au7 directly to the wiper of treble pot. It act the same