I've always been fond of the 807 tube. They are a great sounding triode and are still very affordable. Many varieties are out there - English Cossar, Mullard and Haltron. THere are French versions and of course all the American manufactures like RCA, Sylvania, Raytheon. With all these options tube rolling is cheap and fun. the 807 can be ran in a SE layout but is most commonly seen in P/P amps.