[Phlogiston]

I don't have a "big box" 20. I do have the smaller one - 20C, I think. It is the only 3-dialer, on display, that has a dedicated power supply. I demo it for people, but, the more common use is to test 201, 201A, 200 and 200A tubes ( and their variations ). This helps me escape the "can't test a gas detector " syndrome. The power supply is fully variable so that detector voltages (as with all B+, A+ and C-) can be adjusted to find the sweet spot for best operation. Since the detector position is designed for a 1 amp tube 201s can work there - usually poorly.

Russ

P.S. Speaking of power supplies - I added plate chokes to all of the B+ outputs on this power supply eliminating any residual inverter hum:

https://www.russoldradios.com/blog/more-...ered-radio

This is not the supply running the 20C. It is a "bruit-force" unit, capable of handling tubes that fail.

- and - If you think about it, many of AKs "metal box" radios are so very similar to the 20©. Like the 40 - very common, it is basically the same circuit as the 20 with an AC supply built in and 226 tubes which are just a 201 with a heavy (important) 1.5 volt filament and improved output tubes and speaker.