The Power Supply

Finally here's the schematic! Click on the picture to see it full sized:

I used a chord with a three prong plug, but included the "Phase Light" just in case I need to use it with an ungrounded outlet. It didn't worked as I expected: with the 3rd pin connected to nothing, some "life" is is inducted in the earth conductor, perhaps because of capacitive coupling from the "hot" wire, and makes the reading of the "phase light" erratic.

There's a tube rectifier socket. Both sections of the tube are in parallel, and are fed from a solid state rectifier. The preamp is fed from before the tube rectifier, "decoupled" with a solid state diode (to avoid power amp sagging the preamp - sag is restricted to the power section unless it lasts for a long time). Gus Smaley should patent this configuration. Since both sections of the tube rectifier are in parallel, they can handle double the maximum current a single section would. Average current is the same, however, because both sections will be "ON" in both semicicles of the AC waveform. I built a solid state rectifier in an octal plug so that I can change rectifier type (it's not switchable, but it's changeable). There's a LED in this plug so it glows in the dark just like the tube unity. The voltages marked in the schematic with yellow text were measured with the solid state plug. The voltages in blue were measured with a GZ-34.

The drop down resistors were chosen from what I had at hand, and as it worked nice, it's what I used.

The bias supply shows independent bias pots for each tube. The range is wide, so that I can bias a large variety of tubes in a lot of operating regions. I'm using a pair of 20 turn trimpots accessible from outside the chassis. The major drawback with such a wide range is that you can fry something if you exchange tubes with too different characteristics. It's not idiot proof and it's not something you would do in an amp that will be used by someone else, but it's MY amp and ONLY I will change tubes and bias it. The bias current at each tube in mA is equal to the mV readings at the two test points on the back of the amp.

The filament winding is 12.6 VAC with a center tap, not 6.3 VAC like most, so you will notice some little differences from the usual. I used it because it's the transformer I had at hand, taken from a PA tube amp. It is rectified and filtered (giving approximately 10 VDC) to feed the relays and a small (switchable) fan mounted at the back of the amp. Here's the schematic:

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