Input Stage

The input stage consists of a single 6AU6 Pentode with a 300V on the plate. The goal was to get maximum gain so that global feedback could be added later.

Differential Stage

The differential stage uses two 6AU6 Pentodes that are triode strapped. Instead of a resistor, a constant current sink is used in teh tail to reduce distortion. Although a differential tube such as a 6SN7 would be better to use as a differential amplifier, 140 6AU6 tubes were more convenient.

Complete Amplifier

This is a schematic of the amplifier once it is completed. The differential amplifier splits the phase into two opposite halves. Each one of these halves is coupled to the grid of an 807 tube. For the output stage, an 807 transmitting beam tetrodes. These tubes were chosen due to availability, convenience, cheap cost, and low distortion. Essentially, the output stage is a push-pull circuit. B+ voltage is applied to the center tap of the output transformer. The screens are connected to the 40% taps of the output transformer, resulting in an ultra-linear circuit. The operating point of these tubes behaves inbetween a triode and a tetrode, while the circuit is biased to operate in class A/B push/pull. While it could have easily been biased in class A, it was desired to get enough power to be able to saturate the output transformers. According to the tube datasheet, it is possible to get a 4% distortion at 32W of power with this circuit before feedback is applied.

Modifications

Currently, the proper amount of global feedback to apply is being worked on. It is desired to appyly between 20 and 30 dB of global feedback, and then optimize it from there. My DOZ preamp can output 6 volts at maxiumum, so plenty of feedback can be applied and still get a useable range from the preamp. The tube constant sink will probably be replaced with a transistor, as it is far easier making a -20V supply than a -70V supply.