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Core poweramp phase inverter pulling signal out of the ground
So I've got two amps on my bench now, one is a sunn inspired amp using old Fender iron and 4x6L6, the other is a two channel marshall inspired amp using an ecc99 for the output, a p1x preamp with a 6sn7 and a modified tone stack for the clean, and a pretty much stock jcm800-style dirty channel. They have both exhibited the same phenomenon, and one thing that is common between the two is that they both use the same phase inverter, the one from the Core 20watt push-pull poweramp.

Basically, the amp is pulling signal out of the preamp ground and amplifying it. It almost sounds like signal that is getting bypassed through the caps is bleeding back out of the ground and into the phase inverter. How loud the signal is depends on the position of the master volume knob, it gets loudest around the middle, goes silent at 0, and dims again a max.

I removed the r27 100k resistor between the volume control and the bypass cap, because that resistor basically made the bleed signal much, much louder. Grounding the preamp-side of the bypass cap (c13) kills the bleed signal, but it also kills the audio signal too.

Basically, as it stands, the Sunn-inspired amp sounds OK mostly but the bleed signal is audible in the background and adds an unpleasant edge to the sound.

The marshall type amp is basically unusable. It has a basic relay channel switching circuit, and if I turn up the gain of the dirty channel, the bleed signal is loud enough to cut into the clean signal and make it sound awful. I already tried grounding out the output of the dirty channel.

My thought is that there is some kind of strange impedance node issue with the 20w phase inverter which is somehow causing it to pull signal in somehow, but I'm at a loss as to how or why it would happen, or how to solve it. 

Both amps have very different grounding schemes and overall layouts, but the symptom is the same, so the one major similarity is the PI.
Just as a quick update, the signal that is bleeding into the phase inverter seems like it is probably coming out of the cathode followers in both amps.

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