0 - 350 volt power supply/capacitor tester

[petervk2mlg]

I recently acquired an old item which turned out to be a variable 0 - 350DC volt power supply and capacitor tester. Home brew, no doubt.
The unit uses 2 x 807 valves as a full wave rectifier.
The glassware includes a little 6H6GT twin diode.
I have drawn up the circuit and include it here.
The capacitor tester section comprises a neon with resistor, and also in parallel a momentary switch with series resistor to charge up the capacitor under test. I'm thinking the neon is there to test paper capacitors and the switch to charge up electrolytics and leakage to be observed on the milliameter.
I'd be interested to receive comments about the circuit.

[Radio Wrangler]

That's a neat circuit and makes very effective use of the valves.

The small rectifier makes a high voltage, low current DC supply for the pot and the pot sets the grid voltage of the 807s. The 807s take turns at acting as a cathode follower or being cut-off with negative anode voltage. It isn't terribly well regulated, but it's a very suitable 'power-assisted pot' for the purpose, and 807s have always been dirt cheap. (long may they continue to lack pace and rhythm and so be available for radio use)

David

[petervk2mlg]

Thanks for the response, David

the one thing I'm concerned about is the high voltage across the 6H6GT
negative 320V on pin 3 and positive 350V on pin 8 (approximate)

P

[Radio Wrangler]

Hmm, I see what you mean. RCA only rated the thing at 100V AC per plate.

I'm probably more worried about the heater-cathode voltage as there is approaching 700V between cathodes. Doing it with valves would be rather tedious, arranging suitably rated devices... probably have to be two separate devices, and well insulated heater windings. This rectifier is one case where semiconductors are going to be hard to beat.

David

[turretslug]

Some TV makers even used the 807 as a line output valve in early post-war designs, which says a lot about its voltage capability. I can only assume that the audiophiles take fright at the anode cap- they like valves to be exposed in their glowing glory. The 211 was quickly hoovered up, a high voltage capable power triode, yet with base pin anode connection. Mind you, guitarists,with their enclosed PA cabinets and love of the 6L6. might take to the 807.

I, too, wince slightly at the thought of exposing the 6H6 to that sort of voltage stress, though.

[petervk2mlg]

Would it be ok to replace the 6H6GT with 2 x 1N4007 diodes?

[Radio Wrangler]

Probably a wise move.

David