Triode fixed bias vs cathode bias diferences

[catalin gramada]

Hello. I have the weird ideea to bias my drivers (ecc99) in the grid with negative voltage provided by same negative supply for power tubes. Should I expect any diferences in respect with cathode bias with more common bypassed cathode resistor please ? Thanks.

[Helmholtz]

I'll start thinking when I'm seeing a circuit.

Generally I like auto-bias because
1) it stabilizes operating conditions against tube variations (including aging),
2) it provides excellent power supply ripple rejection.

A fixed bias voltage would need to be extremely well filtered.

[catalin gramada]

Thanks.Dirty sketch of actual circuit. I may remove cathodes resistors and insert the bottom of grid resistors directly into -8v negative supply instead of ground please. With cathodes tied directly to ground, of course.

[Helmholtz]

Ok, so a paraphase without global NFB.

With fixed bias it might be necessary to individually adjust bias for each triode.

[catalin gramada]

And one more question regard the proper wiring of negative supply circuit. May I use both versions without the risk to get into a ground loop please ? Thanks.

[Helmholtz]

And one more question regard the proper wiring of negative supply circuit. May I use both versions without the risk to get into a ground loop please ? Thanks.

While I don't see a ground loop, the problem with the upper wiring is that any noise between the 2 grounding points will be amplified like a signal.
So I prefer the lower one.

[catalin gramada]

Ok, so a paraphase without global NFB.

With fixed bias it might be necessary to individually adjust bias for each triode.

I just wondering why should the bias be individual adjustable ? Even the triodes are not balanced the second one is tailored for gain 1 by feedback resistor.

[Helmholtz]

As fixed bias is not self-adjusting, applying the same bias voltage to both triodes might result in somewhat different tube currents.
Result would be lowered headroom.