Re: PEN3520 Amplifier project
The cylindrical transformer as a phase-splitter/driver sort-of suggests that the amplifier was intended to operate in a mode where the push-pull valves could be driven into a hint of grid-current [Class-AB2 or Class-B] - the transformer secondary windings providing a low-DC-resistance so there would be less perturbation of the bias-conditions of the push-pull stage when grid-current-rectification occurred.
If you want it to be a 'hi-fi' amp rather than one optimised for maximum-loudness PA service then you'll be wanting to use a mode like Class-A or Class-AB1 in which case it would be possible to repurpose the AC/HL as a phase-splitter.
The PEN3520 pair, with 250V on the anodes and 40mA anode-current per valve should give 10 Watts or so of audio output.
Note that they're quite 'high slope' valves for their time - gm of around 8 - so you should include 'grid stopper' resistors to minimise the chance of the amplifier turning into a push-pull ultrasonic power-oscillator!
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