About the NFB loop in a Princeton Reverb..

[Iplayloud]

I aquired a little amp that is basically a AA1164 PR clone, except without the reverb. (It's not like a simple BF Princeton because the reverb version has 3 stages before the PI, versus 2 on the Princeton)

There are 2 differences, it has no NFB loop, and the cathodes of the 2nd and 3rd stages share the same 1k/25uf.

I was hoping to install a switchable NFB loop to see how it would sound, but I would have to un-share the two cathodes because the NFB loop must be hooked up to the 3rd stage.

My question really is: is it possible that this shared cathode idea be some kind of negative feedback system? When cathodes are shared there is definitely some leakage going on, like in a JTM45 when you are connected in input 1, you can crank channel 2 and the is some signal going there too..?

[pdf64]

When cascaded stages share a cathode resistor, if it's not adequately bypassed there's positive feedback.
Vintage BF / SF Fenders whose V4 share the cathode may oscillate when the reverb is turned up, due to the bypass cap ageing and developing too much ESR.
Due to this lack of robustness, sharing cathodes of cascaded stages may be seen as poor design practice.
For stages in parallel that share a cathode resistor, the bypass cap will have some ESR and so signal from one stage will appear in the other, via the cathode.

If the bypass cap on the shared cathode of your amp is working properly, I don't think that the tone will change much if you split the cathodes.