When signal is present, there is always *some* signal that flows into the B+ supply. Due to the low pass RC filters of the power supply, all of this signal is shunted to ground through the filter capacitors. Since the filter caps were not attached to ground, the signal was able to make it to the screen grid (g2) of the 6V6 tubes. This is how the audio signal made it to the speakers when the PI tube was pulled. (I show the signal path in green on the schematic below when the filter caps in blue are not connected to ground.)I'm still not sure I understand.
In fact, I'm sure I don't.
The Normal Channel and Vibrato Channel are not in phase in a Deluxe Reverb. Without the PI tube, it seems the global negative feedback made for a positive feedback with the Vibrato Channel when the signal went through the screen grid (g2), hence the oscillation on just that channel.if I remove V6, I can still gear a signal through the normal channel, quiet but no oscillation. But the vibrato channel still oscillates if I turn up the volume at all.