Re: Radford STA12
My guess is the same as yours, it's for shielding. It is not there on later amps.
Output power is dependant on the +HT volts?
Mullard 5-20 it is around 430 volts at the anodes.
MA / STA15 has about 355 volts at he anodes, and
STA12 has about 310 volts on the anodes.
The Radfords are under run with respect to the Mullard design and the output power is suspect is quoted at a specific distortion level. Plus they do give their output valves an easier life.
They both do more in actual life if you 'turn them up'!
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