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Jon_G4MDC · 26th Feb 2020, 5:31 pm

I was suspicious of that volume modulator in the RCA circuit. I wondered if they had somehow wrongly drawn the filament connections, thinking too much power would be needed. I was suspecting the valve would be heated by other means and the Volume Oscillator was only changing the bias by g-k rectification.

Looking again, and that the UX120 was a 3.3V Valve for battery portables maybe it was powering the heater after all.

Hand capacitance to the volume antenna would increase primary current in the transformer meaning output at the secondary and that would heat up the valve.
Thermal lag must have limited how fast the Volume would swell.

Did you need a radio licence to run one of these?

26th Feb 2020, 5:31 pm	#14
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Professor Theremin advert. I was suspicious of that volume modulator in the RCA circuit. I wondered if they had somehow wrongly drawn the filament connections, thinking too much power would be needed. I was suspecting the valve would be heated by other means and the Volume Oscillator was only changing the bias by g-k rectification. Looking again, and that the UX120 was a 3.3V Valve for battery portables maybe it was powering the heater after all. Hand capacitance to the volume antenna would increase primary current in the transformer meaning output at the secondary and that would heat up the valve. Thermal lag must have limited how fast the Volume would swell. Did you need a radio licence to run one of these? Last edited by Jon_G4MDC; 26th Feb 2020 at 5:40 pm.