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high voltage) capacitors and diodes (up to 400v) are both much more readily available and cheaper than the higher voltage parts (greater than 500v, let us say). If a 120 VAC isolation transformer were connected to a full-wave voltage doubler, the output voltage would be usable for this amplifier and the voltage rating of the power supply capacitors could safely be as low as 200 VDC, as each would only see half of the total power supply voltage. Furthermore, output transformer distortion decreases with higher current.
Of course, this 5 tube amplifier could be made even simpler by using only one cathode resistor and its bypass capacitor for all the output tubes. While this is certainly sound in terms of circuit operation, it will require tightly matched tubes, whereas the individual cathode resistors are much more forgiving of mismatching. A further consideration is that a single cathode resistor can be dangerous if only one tube is in place while the amplifier is powered up, as the value for the resistor will be far too low to bias a single tube correctly and might result in excess heating of the tube. The work-around to this problem is to wire all five heaters in series and place that series across a 32 volt power supply. Now if a tube is removed from its socket, the others will not function, like the old light bulbs on the Christmas tree.
//JRB
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