Thank you Watson
It's still a Class AB amp just biased hot, the power tubes will go into cutoff even at 50mA (1v reading). The only way to make a true Class A amp with EL34's is to run the plate voltage well below the 470v, more like 300v and bias current at 80mA, that should keep the tubes in conduction the full cycle. The plate voltage is just far too high to run true Class A.
It's optimal for the tech to give the lowest bias setting that doesn't show crossover distortion but from experience with these amps I'm confident you can easily go down to 30mA (.6v) and still have great sound, that's still 60% dissipation which is about average for Class AB amps.
Keep in mind that I am talking plate current and when you are taking a cathode reading it is the sum of plate current and screen current. CJ doesn't have any screen resistors which is odd, I'd add 100 ohm resistors directly on pin 4 but I believe these have the power tubes directly onto a circuit board so your tech needs to either cut a trace and install the resistors or if the wire from the output transformer is close enough to pin 4 just install it there. If not you can still easily measure screen current via measuring the DC resistance between the output transformer center tap and the screen taps. Once you write down this resistance place one lead on the center tap and the other at pin 4 and measure the voltage drop across this resistance, use ohms law to figure out screen current and deduct it from the total cathode current. Or you can just estimate roughly 3-5mA for the idle screen currents. So if you are measuring .8 or 40mA cathode current you are really at 37mA plate current. This still puts you at 72% dissipation. Not bad and if you like the sound then just leave it alone, it's better than running near 50mA like with the LED which would put you at about 100% max dissipation of an EL34, it gives you really not much of an advantage because you are still going to be running into cutoff, the Class B part of operation. I just haven't found any advantages to running Class AB amps like this really hot. To get a true Class A circuit you'll need to run much higher current, 80-85mA and 250-300v on the plate.
I have found with high voltage EL34 amps, 450-500v and fixed bias that you can go down as far as 50% dissipation without any crossover distortion but it also depends on load and feedback etc...
Instead of a variac, talk to your tech about a bucking transformer, they are a lot less expensive and you can fit it in the chassis. All you need is a filament transformer 6.3v, make sure phase is correct for voltage reduction because if not you will add the 6v to your current mains voltage and be way too high. Your tech should know what to do and how to wire it up. It's what I typically do for vintage amps and modern mains voltages. If there is no room for a bucking transformer I typically run a zener string on the full wave rectifier center tap to reduce B+, in your case with this amp which has a full wave bridge rectifier you would place the zener string between D1 and D3 to ground. The bucking transformer is better because it lowers all the secondary voltages down which is good because too high heater voltage will diminish tube life. If I can't use a bucking transformer and I use the zener string trick and I feel the heater voltage is too high I just add a very small amount of resistance to the heater circuit to drop it down closer to 6.3. I have seen vintage amps with 7v on the heaters which is too high in my book. This amp runs around 4 amps on the 6.3v tap, getting voltage down say .5v only requires a resistor value of .125 ohms. Use a precision wirewound current sense resistor rated for like 5 watts.
Something like this:
Order today, ships today. PNP4WVFR-73-0R2 – 200 mOhms ±1% 4W Through Hole Resistor Axial Flame Retardant Coating, Safety Wirewound from YAGEO. Pricing and Availability on millions of electronic components from Digi-Key Electronics.