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I. Topology

As I get things documented, I will post in this thread the circuit and some descriptions of the reasoning around the power supply, driver, and output stages. I'll revise those to incorporate things that come up, and eventually package it as a PDF file. One thing I've already added to the requirements is "Easily upgraded with better components". The purpose of describing my reasoning is to assist those who want to modify the design for their own purposes.

I. Topology

The chosen topology is a variant of the "monkey on a stick" which was quite popular on the internet in the nineties. The great advantage of this approach is that it always provides a negative DC bias to the power tube, even if the driver tube dies or is removed. This solves the sparking and cathode stripping issues, especially at startup.

I have chosen a power supply voltage of 450 volts, as a compromise between the high voltage requirements of direct-coupled designs, versus safety issues and the available voltage ratings of electrolytic capacitors. To get away with this voltage, the plate to cathode voltage of both driver and output tube must be relatively low. Therefor the 2A3 operates at the typical spec sheed value of 250 volts, 60mA, 2500 ohm load. The driver must have a low mu and low plate impedance; I have chosen the widely available 6J5GT.

Voltage budget:
output transformer DC drop 20v
2A3 plate to cathode 250v
2A3 bias 45v
driver plate to cathode 130v
driver bias 5v
Total 450v

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