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II. Power Supply

Notice I have already slightly altered the circuit; I'll post a revision after all the words are up and we see some feedback.

Power Supply

Direct coupled amplifiers need very high voltages because there is no coupling capacitor between stages. The highest voltage rating for widely available electrolytic capacitors is 450v; components that can produce and tolerate higher voltages are harder to find, so that is the voltage chosen. The fairly typical Hammond 273X power transformer is rated 350-0-350 volts at 110mADC, 5v at 2A, and 6.3v at 4A. It is also available with universal primary and 50Hz rating as the 373X. With a 5V4 rectifier, about 440 volts should be available. To keep the voltage loss low, a CLC filter is used; Hammond 156L is 5H at 135 ohms. Ripple should be small enough to produce less than 1mVrms hum, giving capacitors of about 50uF.

The first capacitor seems large, but at 60mA total current, the peak rectifier plate current is well within the maximum rating. A 47uF 450v rating would work in place of the two 100uF in series, but I've shown the more robust method. A film or motor run at 600v would be nice. The second capacitor is split, as a cathode bypass plus ultrapath capacitor; this is to keep the signal current loop short. The ultrapath capacitor is the first candidate for upgrading. A 5AR4 can substitute for the 5V4 easily; other rectifiers may need different filament current and supply voltages to get the right output voltage.

A separate 2.5v transformer such as the Hammond 166M2 is used for the 2A3 filament; Hammond does not offer a 2.5v transformer with 50Hz rating. There are a wide variety of other ways to power a 2A3 filament, however!

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  Kimber Kable  

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