Opinions on what's wrong? (Video attached)

[Kamakiri]

Got this GE 516F radio playing again with great sound, but it developed this very odd intermittent sound, almost sounds like something is arcing underneath?

I've attached a video below of the unit playing. Any thoughts are much appreciated

Click the picture to watch.......

[Nick_the_'Nole]

The Riders info (Vol. 22, NostalgiaAir has it) does mention a wiring change in later production units to prevent arcing in the 35W4... maybe look into that?

[Kamakiri]

You know I did see that and I thought of that as I posted this. I'd assume that the arcing would be visible, or that there would be some visible changes in the tube when operating. Time to pull the chassis again, me thinks.

[old_coot88]

Sounds like a microphonic tube causing an acoustic feedback loop. Possibly a 12AV7(12AT7). Try tapping on the tube and around that general area.

[Kamakiri]

Tried that, and it was unaffected. Although it's intermittent, I just got it to stop at least temporarily while it was doing so by wiggling around the 50C5 and holding it in place at an odd angle. The socket seems fairly tight, but not as tight as I'd like it. I had to tighten up some of the tube sockets when I had the chassis out.....wonder if I might have missed a loose pin.

[Kamakiri]

Nope, that didn't do it.

[Nick_the_'Nole]

Now, the 50C5 does have a tendency to develop H-K leaks/shorts... have you tried a different tube?

I had a similar issue, actually in a similar GE radio, come to think of it, and it took me awhile to track it down because the tube tested perfectly fine. I had a 120 cycle hum, yours is more like 240, but then again you also have an AC motor in the mix (the clock), which could be putting spikes out on the line.

[Kamakiri]

Well sonofagun! Nick was dead on. The 50C5 that was in there tested absolutely fine, but I pulled a used 50C5 out of a cigar box, and tried it. Bingo, noise is gone. Wow.

Thanks Nick!

[Nick_the_'Nole]

If you put a multimeter to the pins of the bad tube, you'll probably find a few megohms between pins 3/4 (heater) and pin 1 (cathode). I guess it's too high a resistance to show up on a short test, but it doesn't take much of a leak to be very audible.