"What historical fact? You seem to be implying that SET amps are obsolete when you referred to them as being primitive."

The historical fact IS that the SET amplifier is the first and oldest amplifier design. There is no dispute of that. Tetrodes were designed to overcome the problems of triodes, pentodes were designed to overcome the problems of tetrodes, and beam power pentodes were a later development yet. The 6L6 was invented by RCA in 1936. I don't know where you get your history or electronic theory from but if I were you I'd study a textbook, not advertising copy. Here's a good one I used myself; Analysis and Design of Electronic Circuits by P. M. Chirlian Published by McGraw Hill. Too complex for you? Try RCA Receiving Tube Manual. I have the 1964 edition in front of me right now. The front of the book has an elementry primer on how vacuum tubes and tube electronic circuits work. The bulk of the book has data for all of the popular tubes RCA manufactured then on the market (virtually everything you'd ever see in a consumer product.) The back has schematics for different projects. You can build a class A 1 watt amplifier using a 35W4 rectifier tube and a 50EH5 pentode power output tube which has only about a dozen parts including the tubes and output transformer (page 593.) It's the first amplifier I ever built. Want a little more power and flexibility in a triode amplifier? Try building the 8 watt unit using a 6L6GC output tube, a 6EU7 and a 6AV6 for preamp tubes and a 5Y3GT full wave rectifier which only needs about 45 parts, mostly resistors and capacitors and of course 2 transformers (page 592.) These were considered primitive designs for beginners 40 years ago. For low output SET amplifiers, all that changed between the 1930s and 1960s was better parts, not the design philosophy or the circuit topology. The design of low powered vacuum tube audio amplifiers hasn't interested many people since. Except for this niche market, nobody really cares today either. There are NO new tubes, NO new circuit designs, NO new anything except for higher and higher prices. They are lucky there are people eager to pay them. I never would have thought so.