Choosing an Output Tube

Choice of an output tube dictates the power supply requirements and the type of output transformer which can be driven.  I've changed my idea of the perfect output tube 3 times now.  My first choice was an EL84 pentode because that's what I had used for my guitar amp that I built in 2003.  My first prototype of a hi-fi using an EL84 were somewhat disappointing though.  It at least made sound, but it included some upper mid distortion and wasn't very loud.

I decided a 6V6 might be a better choice: higher output and easier to find NOS (New Old Stock) tubes around.  That was a bit louder, but still not enough to "crank".  The more I read, the more I started to think the sound I was after was really a large bottle directly heated Triode instead of a Pentode like the EL84 and 6V6.

Now I was on a quest to find the perfect Triode.  I've always loved the 300B, created by Western Electric in 1937 to amplify telephone signals.  It can belt out 7 Watts and was used in most every movie theater before the takeover of the transistor in the 70's.  The problem with the 300B is that it has a 5volt heater supply (my power transformers have 6.3v windings) and even new production 300B's cost well over $100.  That led me to consider the 2A3 and better yet, it's sister tube, the 6B4G which has a 6.3volt heater, Octal base, and still puts out 3.5 watts.  It's specs fell within my main design goals.  I scored a "matched" pair of NOS (New Old Stock) Sylvania 6B4G on ebay for $67.

 Those will be the output tubes for my first version.  For my second round, I might design in a 5volt heater supply so I can run a pair of 300B's.