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I'm not from Manley but here's my two bits:
I assume you're talking about an audio power amplifier, not a preamp. I guess things are similar in a preamp anyways.
My tube amps are a bit different - unbypassed cathode on the first stage, second stage is a split load phase inverter. Anyways, what I do to get the feedback is vary the resistance from the output terminal back to the cathode of the first stage. The cathode resistor is always small (maybe 2.2k) and the feedback resistor is always large (like maybe 150k) so the DC current of the first stage is no issue to run through the output transformer. But because of stability, I run the amplifier at a fixed gain and attenuate the input signal, I don't vary the feedback because of phase shifts through the output transformer.
One caution - since the amplifier is AC coupled through a transformer, you need to watch the response carefully with a scope. I missed this step and oh boy did it oscillate. Too much phase shift in the transformer coupled with too much negative feedback results in too much positive feedback at 150 kHz. Now I check the feedback stability by feeding in a small square wave and looking at the response across a dummy load and also across a real speaker.
Good luck!
-Dale
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