UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

G6Tanuki · 23rd Jul 2021, 8:31 pm

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!

23rd Jul 2021, 8:31 pm	#10
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	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!