Well... these old amps often have more issues than meets the eye.
After changing the caps and giving it a general check-over I fired it up for the first time and it had a few problems...
a) The tremolo did not work.
b) WAY too much hum and noise.
c) One of the new output tubes red-plated (grew very hot - too hot).
Ended up having to do a complete rebuild on the tremolo circuit - it works as it should now apart from a small "flutter" that I'll chase down tomorrow. I'd never learned how to read a vacuum tube Transfer Characteristic graph... I have now. Needed to understand whether the tube was OK so I could eliminate it as a culprit (it's non-standard so i couldn't just swap it).
And a complete rebuild of the output stage (the circuitry around the 12AU7 phase-splitter and EL84 power tubes). The old components (resistors and caps) had drifted and deteriorated so much that the bias point for the tubes was completely incorrect. Replaced everything including the tube sockets - the sockets had been damaged by the excessive heat of the original tubes failing.
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BEFORE
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AFTER. Red line is around the output stage (and part of the reverb circuit). The new tremolo circuit is above it (in the middle) with the two blue caps.
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