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Old 16th October 2011, 12:09 AM   #1
bichela is offline bichela  United States
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Thumbs down Use a tube rectifier for grid bias voltage, AA5 Radio tubes and usage.

Just kicking an idea around. if its dumb... sorry this whitehead isn't always the sharpest tool in the shed.

I got one rectifier tube that is to be for my 6bm8 push pull stereo amp that right now is doing nothing useful as I don't have the other parts yet. I want to try some low voltage expirements with it. it is a 5U4GB full wave rectifier tube. I also have a 35W4 from an All American Five tube table radio. Not sure of that ones condition anyway... Can you use a tube for the C- bias voltage of -24vdc? Would I still need a 20uf cap on the output of this tube. Right now, I want to see if it can rectify 24VAC to around 22VDC for testing with my DVM. Not really much more than that for now.

While we are on the All American five tubes... what modern use can a 50c5 amplifier be used for. 12AU7, 12AT7 tubes? I got several of these seven pin beauties and I want to do some low voltage expirements before I comitt myself to the big project with a high voltage P/S.

From my personal expierence with my Panasonic Electric RA-55 AM radio, in daily use in my kitchen, never needed a recap. No buzz or hum. .. its pretty impressive output for a 50c5. It is LOUD! Really Loud and you can hear it quite far away if I crank it up. Is it due to the SPL//m of the speaker or the speaker is very well matched for the Single Ended output transformer stage?

I know there was many early Power transformerless tube products. I have a DC to AC inverter that acts as a isolator. What kind of nice SET can I build with two 50c5 tubes and the other tubes of a AA5 set.
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Old 16th October 2011, 12:53 AM   #2
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I'd steer clear of the inverter. Those things don't produce true sine waves, which could be problematic. L_RD knows a Triad N-68X isolation trafo is inexpensive enough. Bridge rectify the O/P of a N-68X to get the B+.

The 50C5 draws 150 mA. of heater current. Construct a series heater string with a 50C5 on each end and a 12AX7 in the middle. That string draws 150 mA. at 112.6 V. I leave the Ohm's Law and wattage calculations for the pair of dropping resistors (1 added a each end of the string) to you.

Signal topology is simple enough. Cap. couple a 'X7 section voltage amplifier to a pentode mode 50C5 "final". Regulate g2 B+ with a 105 V. gas discharge tube. Add some GNFB and you're done. Edcor should have something suitable for the O/P trafos, at modest cost.
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