I'm making a PPP 6GB5 for about 5 months now. I got it working except for some instability, when I put in more than about 14db of global feedback, I'm getting a oscillation, ultrasonic waveform. Less feedback and it works fine. The tubes in it now are: 6N2P preamp, 6N1P phase splitter, and 6GB5.
I tried a 6CG7 in place of the 6N1P and I could get about 17db of feedback before the oscillation began.
I haven't drawn up a schematic on the computer yet, but this one is close, except, the outputs are PPP 6GB5 into a 100 watt 2200ohm output transformer with the 16ohm leg hooked into a 8ohm load. R67 and 69 are 187Kohm, might need to lower them. B+ is about 410, 6N1P B+ is about 330. It's the same on both channels. I have several tubes to match into pairs, but I haven't yet.