Just finished restoring this. It took three #75 tubes until I found a good one that didn't cause a muffled hissing after the set played for about 5 or 10 minutes. As soon as you'd just touch the top of the cap on the tube, the hissing would cease. It looks a lot more shiny than it really is. The flash made it look too glossy. I had just applied a coat of Liquid Gold spray to the cabinet, and the excess that didn't wipe off hadn't dried yet.
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Nice Job! Look lke an eastern set, most of those stations I can get over here. WCAU is WPHT now as of 10 or so years ago. I don't understand why stations change call signs after many many years. Kinda like us hams the FCC has allowed us the to have call signs which don't necessarily have to be in the call district we are in. It's a bit of annoyance you think the guy is in Calf. but he's next door.
Terry
Any stray capacitance between the 75 grid cap and the plate of the output tube ( 41, 42, etc.) can cause hi freq audio oscillation. Dress the grid lead away and/or try some shielding.
Beautiful radio. Great job!
I've got a 39-30 as well. I have seen some 39-30's with only 1 tube shield, on the 75 tube. For whatever reason, my radio has provisions for two tube shields, on the 75 and the 37. But, when I got it, it only came with only 1 actual tube shield installed, on the 75. Could that be part of your problem?
Tom
Bill/Tom: great advice. Funny thing about 75 tubes, the mutual conductance of the triode is only 750 uMhos, and it doesn't take long for it to degrade to the 500 range. In sets where the 75 is the only AF amp before the output tube this can be an issue.
Also: 42 tubes can be noisy after they warm up. Same goes for 6F6 tubes.
Mine only has one tube shield too over the 75 tube.
Quote:It took three #75 tubes until I found a good one that didn't cause a muffled hissing after the set played for about 5 or 10 minutes
That might have been a supersonic parasitic oscillation. A "grid stopper" resistor, around 68K, placed in series with the grid wire right at the top cap of this tube (its grid), should tame those unruly tubes you have. This resistor, in combination with the tube's stray capacitance, acts like a low pass filter to disable the oscillation.
Real nice looking set--great restoration !
Thanks, everyone. The set is working fine now. I think it had a lot to do with shielding around that 75 tube. No more muffled hissing.
I really like the rear lighted dial on these 1939's. And, using the "Rebuilding Philco Pilot Lamps" tip found in the tech section (
http://philcoradio.com/tech/lamps.htm ) it no longer flickers on and off!
Still love those wine flavored buttons!
Another thing to watch for on a 75 tube, or any other glass tube with a grid cap, is the solder joint on the grid cap giving out. Also the clip on the end of the grid lead can have problems, the solder joint going bad or the wire breaking internally.
Regards
Arran
Wow. Really great restoration!
