Update:
I've been told by Mesa that indeed the A-A-B-B power tubes layout is correct for ALL versions of the 50/50.
Also, that the Driver tube the one closest to the input jack. (No, didn't say Jacks) So it seems obvious that the Driver is indeed V3 if we say V1 is the 12AX7 located to the left, V2 in the middle, V3 at the right side.
Stands to reason the Phase Inverter for each channel is located closest to that channel's power tubes, but the Mesa rep didn't answer my question regarding that.
One thing that Mesa rep told me that I find interesting is that "There is NO need to use balanced tubes in any of the 3 12AX7 positions."
Now that is one statement I'm quite ambivalent about:
-1- Of course Mesa doesn't sell Balanced (nor Matched) preamp tubes, so it's obvious that they won't encourage people to get some that are...
-2- True, in Combo/Head applications the use of a Balanced Phase Inverter is highly debatable for reasons I won't get into here, but mainly because the Improvement, wether in tone or feel, is highly controversial and if not impossible to notice, there are reasons why a balanced PI in a combo could adversely affect tone (marginally).
-3- BUT... In a Stereo Power Amp context, that's a different story. Tubes manufactured today are VERY inconsistent: the specs variance from one tube to another can be as high as 40-50% !
So just sticking any 12AX7 (including Mesa's, which, while tested for more consistent specs, still possess a non-negligible variance) in the Driver stage (each side of the 12AX7 drives a channel's PI) and in the Phase Inverter position for each channel WILL result in a noticeable difference in volume/drive between both channels!
But then again if you plug stereo into a 4x12", you're never going to notice this output difference since the speakers are so close to one another. And if you play live, and in 99% of cases you only get to mic one speaker, then again none of this matters... Same thing with recording mono (only one speaker mic'ed).
But at that point... What's the point of having a STEREO power amp? With stereo effects processors...? A good rack rig is a huge investment... so better use it for what it's meant to be, or put another way, to use it to it's full potential!
Finally. Fitting your 50/50 (or any stereo pa) with a Matched & Balanced Driver (each side of the 12AX7 will send the same signal level to each channel) and with 2 identical Matched & Balanced Phase Inverters will assure you that both channels/sides of your power amp will put out the same level of volume. So when playing live or recording in stereo, using 2 cabs and putting enough distance between them to hear the effects in stereo ( you can create awesome sounds with stereo effects!), you'll want to have a good volume balance... (Especially for recording)
So. This was my two cents on using any 12AX7's or some that are Matched & Balanced in a Stereo Power Amp.
Rock on!
