rectifier tubes 
 

I have had a lot of success using the 6X4. (See: Ampage: home brew
 amps - The Junk Yard Dog.) The 8 pin equivalent is the 6X5. Both
 have indirectly heated cathodes, which means you don't really need
 a standy switch to apply power after they warm up. They take 11 to 14 seconds of warm up to fully conduct, which adds longevity to filter caps, etc. Directly heated cathode rectifiers (eg 5U4) heat up & conduct much faster.
 
6X4's and 6X5's are not popular and can be gotten for very low prices.
 These two will operate small amps up to & including PP 6V6's. I got 
24 new 7 pin sockets in an electronics store for 10 cents each. Octal 
sockets were $3.50 or so.
 
Using a 6.3 volt rectifier simplifies power supply design. A single
 filament XFMR can power all your tubes. Sonically, they sound good.
 
For anything bigger than a pair of 6V6's you need at least a 6CA4.
 After that, you need to go to one of the big boys like a 5U4, 5AR4, etc.
 
I hope this helps.
 
Gary