I have one with the low power switch, and I don't think it has any 12ax7's (I can't see any).
Do I need mesa tubes, or would a good 6l6GC from Eurotubes or any other work?
That is correct. Looking into the unit facing you the rightmost valve is the input pre-amp, then the phase splitter for the A-channel and finally the splitter for the B-channel. These valves can be which ever brand or make. As you know you can also use other dual triodes here.
Note: This is an area of electric shock hazard. Before servicing any amp make sure the transformer and capacitors within it are fully discharged by not switching the amp off in ordinary fashion off but pull the plug out of the socket when on at idle.
The power valves is a bit different though. Probably the warranty for your Fifty/Fifty is no longer valid but for any unit that is you're required by Mesa to only use their stock (the joke of it is Groove Tubes test their valves more thoroughly than Mesa can ever dream of). Bias matching and the bias # is crucial here. If I got it right Mesas are biased cold so on a scale 1 - 10 you want to go with a 6 or 7. An educated guess would be adding a zero gives you an approximate dissipation level.
I've been using jj tubes for my older 50/50, and have been happy with the sound- I've been recording a cd with the jj's and the guitar tones have been tasty...
It's a Mesa, remember - no biasing required. Do choose the bias rating of the valves carefully though. You probably want to go with 6 or 7 on a scale 1 - 10.
For the warranty to apply you are limited to Mesa branded valves only. However e.g. GrooveTubes have a better screening process and as long as the valve is within Mesa spec's it shouldn't be a problem. Just ask your dealer.
I go with 6s or slightly above medium. Thes are comparable with Mesa whites and Blues. Anything between 4 and 6 is within Mesa specs.
