The Super Tube SE is what happens when the Simple 45 gets put on steroids. Tube purists who believe that all silicon is evil shouldn’t go here. The experiments are just beginning but the progress will be charted on this page.
I have just started experimenting with a circuit topology which (for lack of a better name) I will call Super Tube. The idea is to combine the superior voltage amplifying characteristics of a triode vacuum tube with the current boosting characteristics of a transistor in the emitter (or source) follower configuration. I have been experimenting with this configuration for a while with good results. The circuit below is the current design. It is way too complicated, but it is my attempt to get around the need for a 600 ohm SE output transformer that can work with 500mA of DC current. This amp puts out about 8 watts. The output stage is running at 380 volts 180mA. That is all that the Knight Kit power supply can give me. I guess I will have to make room for the big power supply after all. The circuit is here for those who want to try it. I am not happy with the sound yet though.
When working with bipolar transistors you have to pay close attention to a parameter called safe operating area (SOA). Most high voltage transistors can handle only a few milliamps of current at the rated voltage. There is usually a graph detailing this in the spec sheet. Violate this spec and your transistor may work for a long while but it will fail at the most inopportune moment. Loud rock music turned up till the bass just clips will usually find the faults in your design. Hooking up the guitar preamp, setting the volume to 11 and cutting loose is another severe stress test. I don’t believe an amp design is done until it passes these tests. Mosfets do not have the SOA limitation, but so far I have obtained better sound with bipolar devices.
There has been no interest in this topic from you so I have not taken this design any farther. Some day I will take this design through to a serious amp.