So all of this worked out as it should. I pulled the rectifier out and measured pins 8 and 2 were as they should be (600+. My DMM maxes out at 600v) and so were pins 4 and 6 (5v). So I did the next step, Put the rectifier tube back in and pulled out all the other tubes got like 420v @ pin 8Hold on... I suggest that you first find out if it's the rectifier tube that's flakey or that the problem is somewhere else.
1. Pull out the rectifier tube and start the amp up
2. measure the voltage between pin 8 and 2 (should be +- 5V AC)
3. measure the voltage between pin 4 and 6 (should be +- 640V AC)
If this is all ok switch off the amp. Put the rectifier tube back in and pull the power tubes and preamp tubes.
1. measure the voltage between ground and pin 8 on the rectifier (should be around 400V DC)
2. if it's again around 355V DC then your rectifier tube is the suspect, if the voltage is rather up to spec then the problem is probably in the bias circuit or the power stage.
My guess would be the bias setting. OTOH, I think my Princetons have about 370V DC on the plates and the tubes draw about 22mA.
I made up a voltage chart. My B+ side is still way low and my control grid voltages @ V5P5 and V6P6 are -214.9 and -216.9 and I believe should only be -34.
I'm spending a significant amount of time checking a rechecking. Looking very closely for the problem to present itself.