Just had another look at that schematic. I mis-read the cathode voltage for the EL34s (pin 8 to ground). It should be +0.3VDC (not 3vdc). That'll be a function of the Output Balance control (R2) and the Bias control (R3). The grid voltage should be -35VDC. On these old amps the coupling capacitor between the Driver and the Output tubes (in this case C6 & C7 - 0.15mfd) often become leaky leading to damage to the output transformer. They should be replaced first thing (this is unless it hasn't already, the perf board in the photo makes me suspicious - I think this amp has been gotten to and is not original). Output tubes should be a matched pair. Careful with the voltages, 425VDC won't let you let go.