The Power Section

The power section is pretty standard. The only deviations from the norm are the pentode/triode switch (although it's becoming default in modern amps) and the independent user adjustable bias for each power tube.

The screen grid resistors can be lower if you will only use 6L6's, but the chosen value enables me to use other tubes as well.

TP1 and TP2 are located at the back of the amp. There's another test point connected to ground. So you just stick a multimeter at them and turn the -bias1 and -bias2 trimpots (also accessible at the back of the amp) until the readings please you. It's a nice idea to begin with the bias pots at the least current position, to avoid damaging something (the bias for a 6V6 in class A would fry a 6L6). Since I wanted to be able to use a large variety of tubes, the fact that I can bias the amp without opening it is a necessity. More details on the bias supply in the Power Supply page.

It's not shown on the schematic, but pins 1 and 8 of each tube's socket are connected, so that I can use EL34's.

The reverse connected strings of diodes are there to handle high voltage transients at the plates (for example when you forget to plug the cabinet).

I have not drawn the impedance selector because... I'm lazy. But having a lot of impedance taps allows me to keep things matched with a lot of different tube types. Or to mismatch it if I want to.

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