> Ummmm, positive feedback from the output cathode to the input grid?
Where do you see that?
Assuming V1a grid and cathode resistors return to the same place (supposedly "ground"), there is no overall feedback at all. There is local feedback in each stage due to cathode resistors.
The circuit as drawn is very solid and stable. I know two headphone amps done this way (with bigger final stages), one designed by a member here.
If the 22K were returned to the top of the 550 ohm, yes, that would be positive feedback. You find that in the Dynaco preamp. It kicks-up gain so two tubes can cover the RIAA bass-boost. You find it in the Julie (and B-B) tube op-amps. It is nominally stable in these cases because an overall NFB loop controls it. (The Dyna can get a bit over-excited below 1Hz....)
If you bleed v1b cathode back to V1a grid, you have negative feedback and also a low input impedance. Variations with pots and caps make great tone controls.
Where do you see that?
Assuming V1a grid and cathode resistors return to the same place (supposedly "ground"), there is no overall feedback at all. There is local feedback in each stage due to cathode resistors.
The circuit as drawn is very solid and stable. I know two headphone amps done this way (with bigger final stages), one designed by a member here.
If the 22K were returned to the top of the 550 ohm, yes, that would be positive feedback. You find that in the Dynaco preamp. It kicks-up gain so two tubes can cover the RIAA bass-boost. You find it in the Julie (and B-B) tube op-amps. It is nominally stable in these cases because an overall NFB loop controls it. (The Dyna can get a bit over-excited below 1Hz....)
If you bleed v1b cathode back to V1a grid, you have negative feedback and also a low input impedance. Variations with pots and caps make great tone controls.
