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Spectra 30 T, bad socket.
Spectra 30 T, bad socket.
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Old 9th August 2017, 01:32 AM   #11
CapnDenny1 is offline CapnDenny1  United States
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I GOT YOU SUCKR!

Normal voltages on pin 5 of each tube. This is the bias voltage that sets the idle current through each tube.
Click the image to open in full size.IMG_1614 by Dennis Kelley, on Flickr

After about 10 minutes the one on the right started looking a little unsteady? And then it really went crazy.
Click the image to open in full size.IMG_1615 by Dennis Kelley, on Flickr

Not its now +23 volts! But it gets better, or worse I guess?
Click the image to open in full size.IMG_1616 by Dennis Kelley, on Flickr
OL or overload, means greater than +60V.
Click the image to open in full size.IMG_1620 by Dennis Kelley, on Flickr

So lets see the control grid wants -33 and the more positive the more current? I wonder what happens if you give it +60V or more? Ruined tube, ruined tube socket, blown output transformer.

So barring a short in the wires, it must be the coupling cap. The other side of those caps have maybe 250V on them, but it should block it entirely, but it doesnt. It did this and then just went back to -33V as if nothing happened? "What replate?" Its kind of unusual that it is intermittent, but I have seen caps do that too.

Last edited by CapnDenny1; 9th August 2017 at 01:35 AM.
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Old 9th August 2017, 04:00 PM   #12
danFrank is offline danFrank  United States
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Spectra 30 T, bad socket.
Yes, it's the coupling cap. Can you post a picture of the cap, I'd like to see what type/brand it is. Good job on the troubleshooting. I was wrong! I've never seen a leaky film cap, I have seen open film caps though. Hmm...

Last edited by danFrank; 9th August 2017 at 04:05 PM.
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Old 9th August 2017, 04:51 PM   #13
CapnDenny1 is offline CapnDenny1  United States
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I think it is a metallized polyester

http://www.illinoiscapacitor.com/pdf...R%20series.pdf

It might be polyproplyene, but it looks the same.
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