For years I've been mucking around with valve amplifiers, much to the amusement of my family and friends. My first attempt was a version of what's called the Murray amplifier; a 20 watt per channel design by Australian Cyril Murray in the 1960's, which used 6CM5 television line-output valves. I used PL509 valves for a bit more power. Cyril Murray went on to build amplifiers for the Sydney Opera House and his work was very well respected.
Later I made a 60 Watt 'circlotron' design using four triode connected KT120's in each channel. The circlotron is a fully balanced cross-coupled power supply design which has the advantage of a very low output impedance; enough valves and enough negative feedback and it will drive a loudspeaker directly without an output transformer. I used a fairly low turns ratio output transformer mounted at the loudspeakers - some voltage isolation for my expensive VAF-i66 speakers. Using an output transformer also requires less output valves, less or no negative-feedback, and no need for expensive speaker cables. See the photo below.
I still use the circlotron amplifier when I need some power, but it's a real room warmer, so for everyday listening I built the RedRoo 5 Watt Class-A design. It's a great little amp, and I actually prefer the sound to anything else I have for casual listening.