Champ 5F1 B+ voltages

[amptramp]

the datasheet for the 6V6GT lists the maximum DC plate voltage for a class A amp as 315v.

That's a design-centre value, not an absolute maximum. Guitar amps routinely operate outside the recommended 'hi-fi' enviroment.

6V6 specs were not even about "high fidelity". (That would be triodes.) 6V6 was better-grade home radio. Aunt Matilda is not going to change a bad tube herself. Or even move a large console herself. Conservative design makes less customer friction. OTOH a working musician has to move the amp every night, and can learn to change tubes. A loud design is worth a somewhat shorter life.

A loud design is worth a heftier output tube.  And for that matter, some margin in transformers as well.  With the high price and limited availability of tubes now, the working musician has to keep spares because he can't just go to the local TV repair store or drug store tube tester to get more when he is on the road.

[ThermionicScott]

OK I've built it. There were further complications with the transformers. I realised if I used the voltage doubler with a 230v transformer, I would end up with around 650 volts (not taking into account real-world losses). So, to avoid having to wastefully drop big voltages, I bought another transformer with an 18v secondary. That way, with back-to-back 12v and 18v transformers, I would get much closer to the desired HT voltage. I thought I would still need a dropping resistor but in the event, I didn't. I have ended up with around 340 volts which is fine. I got it biased today to 10.9 watts, pretty much bang on the recommended 90% for the 12V6GT.

And...I have already found a use for the smaller transformers, the Rick-Tone Trem-o-Drive. It certainly beats online gambling!

340 volts is what is shown on the 5F1 layout, so I'd say you nailed it!  (Layout also shows a bit over 12 watts dissipation.)