Series Regulated all-Tube Power Supply


The Kay model 397 power supply, a surplus store find. Full of some good parts, this actually works, and I’m considering how to use it in a  project as-is. It was originally used in a television-related test instrument, but was found all alone. I’ve replaced the power cord, built a new interconnect cable, and replaced some small capacitors underneath. This would have been part of a very expensive piece of equipment back in 1952 when it was built. I have a freshly drawn schematic and brief description for you:

Kay Power Supply_Bill

Here is the schematic of the Kay  Electric Company power supply. This circuit is typical of vacuum tube era regulated power supplies, with 6Y6 pass tubes, a 6SL7 “long-tail-pair” or current mirror voltage amplifier, 6J5 error amplifier, and VR150 150-volt reference tube. Note how this circuit is supplied by a floating 6.3VAC filament supply, while a separate transformer supplies 6.3VAC to the off-chassis circuitry. This second supply could be biased to eliminate hum in that circuit without exceeding heater-cathode voltage limits in this circuit. The output is adjustable via the potentiometer next to the 6J5, and with the components shown, the range of +250 to +300 is accurate. Changing the 82K resistors in series with that potentiometer would increase and/or alter the adjustment range. This power supply might be useful as a bench test instrument for trying amplifier circuit ideas, or to allow construction of finished items without each having an included power supply. What it does not include as-is is any sort of negative bias supply, although one could be added very easily.

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