04-10-2011, 05:13 PM
That GE job is an RCA, and I believe, R-78. The tube sheets do indeed, say that you can squeeze 20 watts out of those jugs. In the 1932-1934 era, RCA was sweet on Class B. The set that our young pal has here was sold as RCA R-78, the GE J-125 (the first two digits are the tube count). It also appeared as RCA's RAE-84. I have both of the RCA versions.
In order to have a class B power amplifier, they needed a hard power supply. A class B power amplifier draws little or no current at no signal, but increasing current at high signal levels. Thus, the hard power supply. RCA dod it by using the type 82 rectifier. It looks just like an 80, but with mercury in it. One of the things you learned (well, I did) in senior year communications courses, was that when using a mercury vapor rectifier, the tube MUST be brought up to temperature first, before hitting it with the high voltage. RCA knew better, but put a mercury tube in that set, and had it come on, just the same as a high vacuum tube, like the 80. The 5Z3 hadn't been born that year; I don't believe.
I was working on one of those chassis once, and didn't want to put the 82 (2.5 Volt filament) in it until I was finished. Just for kicks, I put an 83 (5 Volt filament) in the socket. As soon as the 83 got warm, the mercury fired, and the set worked. This is unorthodox as the blazes, but it just shows to go you that, once the mercury ionizes, the rectifier is in business. Ihe voltage drop in a mercury rectifier is about 15 volts, and it won't change, short of destroying the tube. The 82 is shielded in this set because mercury rectifiers generate horrendous interference. In some cases, suppression of that hash isn't easy.
So, in the RAE-84 I have, which I bought from Fred Rice (Philaddelphia), he told me the gruesome story of that set. Someone had put a 1/4" bolt in the fuse holder. A drop of mercury had caused a short in the 82, as it warmed up. A filament-plate arc, and the power transformer tugged at it. That kept up until the transformer was literally ashes, the 82 had a big dimple in the glass, and the house fuse blew. So, whatever you do, fuse that hunker as the service notes say to!!!! They were not kidding. I still have what's left of the transformer. Fred still has the tube.
In order to have a class B power amplifier, they needed a hard power supply. A class B power amplifier draws little or no current at no signal, but increasing current at high signal levels. Thus, the hard power supply. RCA dod it by using the type 82 rectifier. It looks just like an 80, but with mercury in it. One of the things you learned (well, I did) in senior year communications courses, was that when using a mercury vapor rectifier, the tube MUST be brought up to temperature first, before hitting it with the high voltage. RCA knew better, but put a mercury tube in that set, and had it come on, just the same as a high vacuum tube, like the 80. The 5Z3 hadn't been born that year; I don't believe.
I was working on one of those chassis once, and didn't want to put the 82 (2.5 Volt filament) in it until I was finished. Just for kicks, I put an 83 (5 Volt filament) in the socket. As soon as the 83 got warm, the mercury fired, and the set worked. This is unorthodox as the blazes, but it just shows to go you that, once the mercury ionizes, the rectifier is in business. Ihe voltage drop in a mercury rectifier is about 15 volts, and it won't change, short of destroying the tube. The 82 is shielded in this set because mercury rectifiers generate horrendous interference. In some cases, suppression of that hash isn't easy.
So, in the RAE-84 I have, which I bought from Fred Rice (Philaddelphia), he told me the gruesome story of that set. Someone had put a 1/4" bolt in the fuse holder. A drop of mercury had caused a short in the 82, as it warmed up. A filament-plate arc, and the power transformer tugged at it. That kept up until the transformer was literally ashes, the 82 had a big dimple in the glass, and the house fuse blew. So, whatever you do, fuse that hunker as the service notes say to!!!! They were not kidding. I still have what's left of the transformer. Fred still has the tube.
