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Something I'm just starting to play with is low-voltage tubes.
There is a whole family of tubes that were designed to work in car radios, using only a +12V B+ supply!
Some tubes that I'm playing with right now include the 12FM6 and 12AE6A, both of which are triode/dual diode tubes that were designed to be the detector and first audio stage in a car radio. Typically, the triode was used to drive a power transistor (anybody else remember the 2N301?) which drove the speaker. These tubes are readily available and dirt cheap... I paid $1.10 each for 12AE6's at Antique Electronic Supply. Then there's the 6GM8/ECC86 dual triode, which has been used in some audiophile equipment already.
Jeff Duntemann has a page with lots of 12V tube info here!
Anyway, so far I've had really nice results using a 12AE6A triode, running on a B+ of 36V, with a plate load of a 1N5291 0.56mA constant-current diode! Gives a gain of 10, fairly low output impedance, and runs right to the rails before clipping... but still has nice single-ended-triode distortion characteristics, on the order of 0.2% at 1V RMS output.
The Hybrid headphone amp uses this type of tube.