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Originally Posted by Kevin Kuehn
If the 71 filaments aren't open they should still light. Did you measure the 71's filament voltage, both at the 71 sockets and wherever it exits the power supply?
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The filiment pins on the tubes had continuity on them, which is the strange part.
Also I measured the filiment voltage on both 71A tube sockets and they both had 5 VAC on them, the filiment winding for the output tubes on the power transformer has continuity, between the transformer and the tube sockets, so its not an open winding either.
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Edit - Also, neutralizing a Neutrodyne has nothing to do with eliminating hum in the audio. It's purpose is to neutralize capacitance in the RF tubes.
Hum is an entirely different problem.
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I had just read in a Rider's tube substitution buidebook that I have that the 71A tubes need to be replaced as a matched set or else they will hum which these 71A tubes were NOT a matched pair, one was the original RCA Radiotron branded 71A tube and the other was a Tung-Sol branded tube, which I think might of been where the hum was coming from the fact that the tubes weren't matched.
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Did you replace the power supply filter caps like you planned?
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Yes I replaced the filter caps like I said I would, I replaced them both with 4 MFD 450 VDC Electrolytics wired in the exact same spots as the originals were.