HP 421A    DC Tube Voltmeter

I bought this Voltmeter from a member of a electronic forum. As I opened it, I was kindly surprised. It looked like it came fresh from production. I was even more surprised, when I found no capacitor or something I had to replace. The values of the voltages I measured, matched those in the schematic.  That is high quality!!!

The tubes working in there are: a 6X4 as rectifier, 0B2 and 0A2 as regulators, 12AX7 and the penthode of the 6AU8 as amplifier and the triode of the 6AU8 as cathode follower/ output stage.

The only thing I repaired were the measurement cables.

I googled the relevant datasheets and linked them here, because I don't own them.


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