In the Tube FAQ, the following questions are answered:
However, elsewhere in the forum, it's pretty much stated by the Line 6 repair tech that you should NOT replace the tubes with anything but the EXACT same tube. So I shouldn't use a 12AU7 for the pre-amp or inverter as is indicated below?
Q: What are the common Preamp tubes?
A: The 12AX7 (ECC83 is the European term, or the military term of 7025) is the most common preamp tube for driving the Gain and EQ sections of the amplifier, as well as driving the signal into the power amps.
The following tubes have a lower power rating and can be used in the preamp section or other amplifier functions such as the reverb tank drivers, inverters, etc.
12AT7 (ECC81, 6201): 60% power of the 12AX7
12AU7 (ECC82, 5963): 19% power of the 12AX7
12AY7 (6072): 45% power of the 12AX7
Q: What are the common power amplifier tubes?
6L6 (5881/KT66): Commonly used in American amplifiers such as Fender, Mesa, Peavey. Typically has more low-end emphasis.
EL34 (6CA7/KT77): Commonly used in British amplifiers such as Marshall. Typically has more midrange emphasis.
EL84 (6BQ5): Commonly used in Vox , Gibson, Matchless, and Bad Cat amplifiers
6550 (KT88): Commonly used in VHT, Ampeg SVTs, and Leslie Speakers cabinets. Typically has more power output than the other tubes listed here.