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In Reply to: RE: Baby Ongaku 2A3 posted by PaulF70 on October 10, 2014 at 20:03:15
I have linked below the original article, which includes the circuit diagram.
A 2A3 with a 1000 ohm cathode resistor is reasonable well matched to the 2500 ohm output transformer. A standard 2A3 will then draw about 55mA (55 volts at the top of that resistor, and at 275v plate to cathode will dissipate the rated maximum plate power of 15 watts. That puts the plate at 330 volts, and allowing 20 volts for the resistance of the PT primary, gives the design 350v power supply voltage. Any higher voltage will increase the power dissipated by a little more than the voltage ratio squared. If you actually have 375v at the plate (i.e. 395 or so at the high voltage power supply) that would suggest about 20 watts of plate dissipation. That will shorten the life significantly of old-stock tubes, but many modern tubes (such as the Sovtek types) are able to dissipate the power without serious problems. The JJ 2A3-40 is an exceptional example, it's essentially a 300B with 2.5v filaments.
Follow Ups:
Thanks a bunch Paul. It sounds like 375V should have the amps putting out under 4W and should be no stress to modern tubes.
You could reduce the 0.68uF cap to a 0.47uF cap if you wanted to trim the voltage down a bit.
There are also rectifier tubes that would give you a hair more voltage drop.
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