RCA 1X591 hum problem - Printable Version
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+--- Thread: RCA 1X591 hum problem (/showthread.php?tid=21444)
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RE: RCA 1X591 hum problem - MrFixr55 - 07-25-2020
Interesting problem. great advice from the experts. While a little risky to modify the wiring, I usually try to create a "Mecca" type of ground. The 12SQ7 / 12AV6 Detector / First Audio Amp heater and cathode are referenced to "ground" (actually neutral or the common "B-"), and usually by capacitor but sometimes directly, to the chassis in these radios. I will usually find a way to connect the negative of the filter caps, (if they are referenced to B-), the cathode resistor of the Power Amp (50L6, 50C5, etc.)and the heater, cathode of the 12SQ all to the same point, extending the "ground" to the rest of the circuit. This is reminiscent of how "Hi-Fi" manufacturers built their amplifiers. I have a 1947 "Philharmonic" AA5 Radio built in this manner.
Remember that even on a circuit as simple as the audio circuit of an "AA5" radio, lead dress is important. Another consideration is to keep the coupling capacitors away from the heater wiring. Original caps were marked "outside foil". These should be referenced to B- for bypass caps or the plate circuit of the stage being coupled to the next stage for interstage coupling caps. Newer caps seem to be not made with "outside foil" so care should be taken in their installation and "dress".
Like Ron Ramirez, "Phil's old radios" and others, I am a big proponent of rewiring the radio using a polarized cordset and wiring the power switch to the "Hot" side instead of the "Neutral" side of the power line for increased safety. However, this act may also increase hum slightly, and may be the reason why the manufacturers of series string radios switched the neutral instead.
Best Regards,
John