DAY 18 POWER PLAY

The last couple of days have been spent trying to wire and test the JMD power supplies. It has not been without some unexpected difficulty.

Yesterday my goals were to wire the on/off switch and transformer leads to appropriate tie points, test the voltages, and wire the filament string to light up the tubes.

Everything went reasonably well until testing the filament strings. The 6AU6 IF amp refused to glow. I triple-checked the wiring against the Handbook, measured, scratched my head, had a beer, came back to it but no joy. You really do feel below average when you can’t manage to wire a working heater string.

Faced with a return to first principles, i.e. the RCA Receiving Tube Manual, turns out the Handbook article pin designations for the 6AU6 heater were wrong (you can see the schematic on the home page, and most radio guys would see the issue right away I expect, but not moi, Mr. Below Average). Rewiring the heater to pins 3 and 4 fixed that problem, and I started seeing that orange glow we’re all high on.

The bias circuit runs off the heater string so that was next – easily built on a single tie strip bolted to the back wall of the receiver, this fired right up and gave the requisite -19volts DC.

This left the high voltage supply. We’re hoping for about 160 volts under load. The transformer was the best available, and I wired up a full wave capacitor input circuit which gave me – wait for it – 325 volts no load, and 180 volts at about 120 milliamp load. Part of this was a nominal 10 percent high line voltage that I can fix with a variac (no unused 5 volt heater winding on the transformer to use to buck it) but clearly that was still a bit excessive. More head scratching, handbook review (no more beer when working on B+ circuits) and I quit for a while.

Finally decided to wire the supply as a choke input to try to drop the voltage a bit. At 120 milliamps, the B+ dropped to 148 volts. As I won’t be drawing that much current, I can still expect a proportional drop which should put me nicely in the 160 to 170 volt range spec’d in the Handbook. Life is good.

Day 18 project status (lockdown day 11, isolation day 17) pictured below. Blue wire is bias/AVC, green is filament, red will be B+ and black is AC primary. Starting to run out of colors….

Next week I’ll try to build and test the two stage audio amp. Still at 27 bucks out of pocket not counting happy hour requirements and no smoke other than solder smoke :))))))

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