I just finished a Spitfire build using an old Hammond organ amp chassis and transformers. I left the heater string wired up but that’s about all that was stock. I followed the info on the Watts Tube Audio site, but I changed it over to use 6CM6 output tubes. They are a close cousin to the infamous and expensive 6973 tube used by Supro. I can sub a 6973 and 6CZ5 directly. This thing has massive balls and a great overdrive tone. I’m trying to get a decent clip of it. I’m a big fan of the 6CM6. I used it in a little single-ended amp and it had the same qualities there. Kinda like a KT66 in a 9-pin bottle. It’s a fairly easy sub for an EL84 as long as the plate voltage isn’t too high (these are running at 350VDC). Here it is. I need to clean it up and make it pretty.