A question about Cathodyne/split load/concertina PI's

[aletheian]

I am trying to design an output stage with a minimal tube count. I have settled on the concertina splitter since it uses only one tube to so it's dirty work.

My problem is: I am having trouble visualizing I can inject NFB? Preceeding the splitter is a tone network that is fed from a cathode follower, and then a volume control, so I don't want to inject it into that stage since it will alter the foreward gain in the loop as the tone and volume controls are altered.

So I assume that I have to inject it into the concertina itself. I have drawn up a few variations. The .1uF capacitor I suppose is the culprirt in question.

[aletheian]

A variation.

[aletheian]

Here is the third option

[aletheian]

Here is the final thought

[Miles Prower]

Be much easier to just use a LTP. The LTP has both inverting and non-inverting inputs, so there's a natural summing node for NFB. Since types like the 12A*7 series, the 6BQ7, etc. are all dual tridoes, the tube count doesn't change.

FWIW: avoid the 12AU7A. That one has horrible linearity and more distortion than you want.

[aletheian]

Unfortunately, I am using the other half of the 12au7 for the preceeding stage cathode follower, that's why I went with concertina instead of LTP. I thought about subbing in a 12ax7 or 12at7 for the 12au7, but I'll deal with that later. It is the NFB that has me stumped.

Thanx

[SY]

Negative feedback works by trading off gain for distortion/bandwidth/impedance. The problem is that your circuit has no gain to start with...

It's difficult-to-impossible to feedback to a split-load; generally, the split-load is driven by a gain stage and the feedback is brought back to that stage.

[Miles Prower]

If you have to go with the cathodyne, then your best bet would probably be something like what I attached. With the cathodyne, you really don't have a second input for the NFB summing node. So try something like the inverting connection that you'd use with an op-amp. In this case: Avcl= Rf/Rs (or close to it).

[SY]

BTW, when you revise the circuit, make sure that the lower output is taken from the top of the load resistor, not from the load plus cathode resistor.

[aletheian]

Originally posted by SY
BTW, when you revise the circuit, make sure that the lower output is taken from the top of the load resistor, not from the load plus cathode resistor.

I have seen them noth ways, with the signal coming off the junction of the loadresistor/cathode resistor, and with the signal coming off the junctuin of the cathode trsistor/cathode.

Is there a benefit in doing it the other way? Possibly an impedance issue that I am missing?