Tube Preamp: How many tubes does it take to .................

It also depends on if you are using dual triodes or single triodes, and if you are using half wave rectifier tubes or full wave. If you are using half wave rectifiers you can use 4 tubes for a full wave bridge, you can't do a bridge with full wave rectifiers.

I'll throw in another possibility for 'extra' tubes in a preamp that can improve fidelity, cathode followers after again tube. They supply a nicer impedance match to the next gain stage. If every gain stage in a preamp used them you end up doubling the number of tubes.

Another possibility is paralleling tubes to lower an amplification stage output impedance(like a cathode follower above). I haven't seen it used that much but I recall Conrad Johnson doing it on some of their top of the line products.

endless number of circuits to get from input to output, be it line stage, phono...and, holy smoke, how many ways to do a power section? feedback? no feedback. cool thread. 

 

Power tubes can even be used in a preamp, one manufacturer I know of uses power tubes in some of his preamps and DACs.