So, I've got this Mesa 50/50, right, and I got to wondering. There are 3 preamp tubes in this unit, but it's a power amp.
first, why would there be preamp tubes in a power amp? My first guess is that one is a Phase Inverter, and the other two are for the L and R Presence Controls, but I've really got no idea.
Secondly, What effect do these tubes have on my tone? I've got a huge pile of preamp tubes lying around, EHX, Ruby,, Chinese, Tung-Sol, JJ, a VOS GE, etc, and I haven't messed with any of the tubes in the mesa, only in the E530. I'm gonna be opening up both of my 50/50s tomorrow, probably, so I may as well ask now because that way I can make swaps easily.
Looks like they are pre-amp tubes in the purest sense of the word - they balance out the levels and tweak them as necessary before hammering the power tubes.
Unlike in a dedicated guitar preamp, I doubt they affect your tone much - certainly not as much as the power tubes will in this setup, since you don't have any controls or gain.