First and most important. There are four different functions that different tubes perform.
Preamp tube - Typically these are from the 12AX7 family, which includes the following:
ECC83- for the Marshall sort of sound
7025 - for the Fender 60’s and 70’s sound
5751 - for blues and less aggressive attack
Preamp tubes are the most critical, least expensive, most overlooked tubes in your amp. The preamp tube in the first gain stage (usually V1 and / or V2) sets the tone and initial gain structure of the amplifier
Each half of the preamp tube works separately in the circuit even though it the same tube. It is good to have both halfs matched.
Best buy is in this group (In decreasing order)
1) JJ/Tesla 12AX7S
2) GE 12AX7WA
3) Ei EC83/12AX7
http://www.guitaramplifierblueprinting.com/12ax7.html
Rectifier tube
The rectifier basically converts the AC line current coming into your amplifier to the DC voltages that are needed. The power limits of your amp's transformer limit the choices that can be made!
GZ34 = 5AR4 GZ32 = 5U4 = 5V4GA GZ31 = 5U4GB GZ30 = 5Y3GT
Driver tube
The driver tube can be consider the interface between the preamp section and the power amp section
Power tube
Power tubes drive the speaker transformer and must be impedence match to it and the speakers.
Biasing - These are the tubes that are biased. (Although several amp manufacturer preset the bias and market it as "Self biasing or Cathode Biased) all thee tubes can be self-baised, that is preset to run with a certain voltage.
Class A verses Class A/B1 - Again more marketing stuff, all these tubes can be setup to be Class A or Class A/B.
Several different styles and power outputs for each power tube grouping.
6V6 Group - Fender Princeton sound
EL-84 Group - Mesa Boogie Dual Caliper sound
6L6 Group - US Rock and Roll via Fender
5881 Group (Confused with a 6L6, because it was designed as a military replacement, but not as a copy)
GEC KT-66 - Beano sound, Marshall bluesbreaker sound, No longer made by GEC - currently cheap copies
EL-34 Group - Classic English Marshall sound
KT-88 Group - Big round sound
6550 Group - Thickest and most powerful tube currently being used for guitar amps - Classic US Marshall sound
