Oscillations problem help

[catalin gramada]

Hello. I have a simple gain stage which present oscillations issue at beginning and the end of the volume pot track. The input have not a grid leak resistor but the circuit is closed by input jack. The oscillations are just at the beginning and the end of the volume pot like a low freq hiss/buzz. I tried some experiments to find the cause as follow: 1.with the cathodes bypassed the oscillations stop at the beginning of track but still are at the end of the track. 2. With LED biased cathodes I get no osscilation. 3. also if I plug guitar at the input the oscillations are gone. I wonder how I can get rid of this oscillations with unbypassed cathodes and input shunted to the ground please ? I trying to figure what can be the reason please? Thanks.

[Chuck H]

If it doesn't have the problem with an instrument plugged in I think the the only possibility is a ground scheme/layout problem or a jack switch wiring problem. I suspect a ground or layout scheme problem since you are using bus wire for grounding. The schematic provided is insufficient for any diagnosis of this sort of problem. The actual issue may not even be with V1 or the volume pot circuit area at all. It could be a layout and/or grounding matter that has to do with what is in proximity to or grounded with what.

[catalin gramada]

If it doesn't have the problem with an instrument plugged in I think the the only possibility is a ground scheme/layout problem or a jack switch wiring problem. I suspect a ground or layout scheme problem since you are using bus wire for grounding. The schematic provided is insufficient for any diagnosis of this sort of problem. The actual issue may not even be with V1 or the volume pot circuit area at all. It could be a layout and/or grounding matter that has to do with what is in proximity to or grounded with what.

Thanks. Actually the the posted pic follow identical the layout. Meant one grounding point colect all signal returns for those decoupled stage. It is a bus but the housing point are well defined and all signal returns corect related to in one point...meant not distributed along the bus...The only new thing I tried was to derive a second bus, also from the point related to each filter cap so any shield wire to run directly to the node and not linked to signal wires returns. Do you suspect any coupling interferences ?
Anyhow I can disconect the second section coupling cap to the rest of circuit and scope if the oscillations stops here in the same conditions. ..

[Chuck H]

All ground bus systems have resistance. All power supplies have resistance. Wire has resistance. At the highest gain levels (think guitar amp first stage) these small resistances can make a difference. Even a shared space on a bus wire has to share resistance with anything else on the bus wire. Same with chassis grounding actually, but the resistance of bus wire is just a bit greater than most chassis.

You might try grounding to the input jack sleeve along the signal chain to see at what point the oscillation stops. This could be a good indicator of what the input stage is reacting with via the ground scheme or proximity..

[Dave H]

Hello. I have a simple gain stage which present oscillations issue at beginning and the end of the volume pot track.

Try a grid stopper (100k?) on the second triode's grid.

[catalin gramada]

Try a grid stopper (100k?) on the second triode's grid.

Oh yes! Stucked 100k direct to the socket and complete stable now. Thanks mate.

[J M Fahey]

Try a grid stopper (100k?) on the second triode's grid.

THAT.
12A*7 triodes have *significant* Miller capacitance (grid-plate capacitance times stage gain) Cg/a at grid inputs so a resistor in series with this "hidden in plain sight" capacitor kills highest frequencies.

Shown in datasheets and ignored by most since they are a few pF (or uuF as they used to call them) and "look" insignificant.
But multiply them by 50X to 100X and drive them through a few kOhm and now the cutoff frequency approaches Audio bands.

This is for 12AX7.
1.7pF looks impossibly small and irrelevant.
Multiply by 100X (max gain possible), now it becomes 170pF
Drive 170pF through 100k, now frequency cutoff is ***9kHz!!!!****

[catalin gramada]

THAT.
12A*7 triodes have *significant* Miller capacitance (grid-plate capacitance times stage gain) Cg/a at grid inputs so a resistor in series with this "hidden in plain sight" capacitor kills highest frequencies.

Shown in datasheets and ignored by most since they are a few pF (or uuF as they used to call them) and "look" insignificant.
But multiply them by 50X to 100X and drive them through a few kOhm and now the cutoff frequency approaches Audio bands.

This is for 12AX7.
1.7pF looks impossibly small and irrelevant.
Multiply by 100X (max gain possible), now it becomes 170pF
Drive 170pF through 100k, now frequency cutoff is ***9kHz!!!!****

Thanks. Just wondering how changing the cathode impedance, bypassing the resistor with a large 100u cap or biasing with red LED changed the condition of osscilation.? As I mentioned bypassed resistor cured the osscilations at the beginning of the pot track but it still oscilate at the end of the track. Changing the cathodes resistors with a red led and get rid of oscillations at both ends of track. The tube used was ecc81 - 12at7 equivalent. Thanks

[catalin gramada]

I try to understand : bypassing the cathode it turns the phase at the frequency which create the osscilation conditions whilst the grid stopper just damping it with the price to create a new pole which roll off in respect with miller capacitance please ?

[Helmholtz]

I try to understand : bypassing the cathode it turns the phase at the frequency which create the osscilation conditions ?

There might be no simple answer to your question.

Any spontaneous oscillation requires a feedback path.
Is there an NFB loop that is not shown in your schematic?

What is the frequency of the oscillation?

whilst the grid stopper just damping it with the price to create a new pole which roll off in respect with miller capacitance

The most obvious effect of a grid stopper is that it forms a LP filter together with the effective grid to ground capacitance (increased by the Miller effect)) and thus rolls back HF gain.

The other effect of a grid stopper is that it damps possible RF resonances between parasitic wiring inductances and capacitances.
Such parasitic resonances can couple between the wirings of the tube electrodes (causing an invisible feedback loop) and thus turn the tube into an RF oscillator.

The cathode bypass capacitor in theory only causes a phase shift at low frequencies, but it might have some inductance, which the LED doesn't have.

[catalin gramada]

Thanks. Replacing the tube with some similar type in the same circuit may show if the osscilation is a fact of the external circumstances (eg. Layout, wiring etc) or a matter of internal characteristics in respect with operation conditions was set which prone to be in a edge of stability?( Meant a 12ax7 could be stable in same situation where a 12at7 was not)

[Helmholtz]

You did not answer my questions.

Oscillation caused by wiring effects should be in the high MHz range.

[catalin gramada]

Replaced rft ecc81 with a jj ecc81 , removed the grid stopper and powered on. No sign of oscillations, the tube work perfect stable without grid stopper...

[Helmholtz]

Replaced rft ecc81 with a jj 12at7 , removed the grid stopper and powered on. No sign of oscillations, the tube work perfect stable without grid stopper...

Interesting, but not conclusive.

I guess that minor differences in transconductance and interelectrode capacitances may determine if a tube oscillates in a given environment or not.

Don't forget that the ECC81/12AT7 was designed and optimized for RF(MHz) - not audio - application. It was typically used as RF amplifier or VHF oscillator/mixer.

[catalin gramada]

Interesting, but not conclusive.

I guess that minor differences in transconductance and interelectrode capacitances may determine if a tube oscillates in a given environment or not.

Don't forget that the ECC81/12AT7 was designed and optimized for RF(MHz) - not audio - application. It was typically used as RF amplifier or VHF oscillator/mixer.

I think so as well. Fortunately I have a box of new rft ecc81 somewhere and I will do a test with some to confirm . But more I want to do this test cause the nos tube I used seems a bit microphonic- think is right term...meant is ringing a bit when I knock it with a stick-and wanna be convinced I didn't take my conclusion on a "defective" tube who although work perfect well-