This is what I can infer from reading through this thread. The plate voltage is above 500 volts. The screen voltage is 513 or 480 volts. The grid was at around 80 volts negative, but this is now -52 volts.
Look at the triode curves for a 6L6GC. With over 450 volts on grid and plate of a 6L6GC and -52 volts of grid voltage at least 20 mA of plate current should be flowing. This is enough current for signal to be amplified in the tube.
The first step is to see if the tubes are actually conducting.
Measure the DC voltage on either side of the 470 ohm screen resistors. It should be slightly lower on the tube side of the resistor. If they are exactly the same, then no screen current is flowing.
Measure the DC voltage on all three wires on the OPT primary. If they are exactly the same, then no plate current is flowing.
If there is NO current flow under these conditions, verify the DC voltages right on the tube pins, including zero volts on the cathode, then the only logical conclusion is 4 dead tubes. Pop at least one good tube into the amp and see if current flows.
If there is SOME plate and screen current flow, see if the current increases considerably as the input signal is turned up. The B+ should drop as the drive is increased. If this is the case then there is likely a dead short inside the OPT, or across it's output. I have seen exactly one shorted OPT that wasn't obviously fried (burnt and smelled fried) in my 50+ year history of dealing with guitar amps, so it's a rare possibility. I have seen more than my share of bad jacks.
Most OPT's have different resistance readings between the two primary halves. The "80 watt" guitar amp OPT on my bench measures 38 ohms on one side, and 49 ohms on the other. A 2:1 mismatch may be excessive, but could also be normal.
True enough, but Guitar amps are biased much colder, probably to stand abuse better.
That amplifier and 99% of American amps based on 6L6 follow Fender designs, in facty that amp is nothing but a somewhat modded Fender, no matter who signs the schematic 🙂 , and Fender used -52V there, see Fender Twwin Reverb bias.
While EL34 based British amps , Marshall is an example, used -42V.
Hiwatt, also British, typically used unadjustable -38V and EL34 and tht value was shown by experience to be too hot, eating through tubes, at least modern production ones, kludges are needed to increase that voltage because you straight can´t get a higher one from available bias winding.
A modern 6L6 with 450-400V on Plate-screen won´t last much with mere -37V bias.
