CFB and A2 operation

[hemgjord]

I am currently fooling around with an 813 triode mode SE amp and I am thinking about ways to lower output impedance. As I want to keep GFB as low as possible, none that is, the obvious method is of course to apply cathode feedback.
Then I started thinking, since I will venture into A2 above approx 20W, the cathode winding will carry both the main signal current AND the gridcurrent. To my understanding this will introduce a quite nonlinear feedback at the cathode.

Anyone has any thoughts or experiences regarding CFB in A2 stages?

Besides, the needed drive voltage for an 813 in triode makes this quite unpractical anyway but I am interested in if the concept as such is a big no-no or not.

If anyone has any other favourite way to lower output impedance I am grateful for advice. (I am already running 9K opt so I don't really want to increase impedance ratio of the OPT)

Regards
/Olof

[Miles Prower]

Originally posted by hemgjord
If anyone has any other favourite way to lower output impedance I am grateful for advice. (I am already running 9K opt so I don't really want to increase impedance ratio of the OPT)

Use parallel feedback from the plate to the control grid, or even better, back to the driver stage in such a manner so as to preserve the proper phase for negative feedback. This accomplishes the same thing as cathode feedback (or an Ultralinear connection for that matter).

[hemgjord]

Thanks Miles

I'll look into that. It needs some consideration though since Va is 900V+ and swings 1600Vpp at full power

Regards
/Olof