Plate voltage is another thing to consider when contemplating using 6v6's in a Mesa amp designed for 6l6's or el34's...I would not run 6v6 at all unless a lower b+ voltage "Tweed" setting is available on that Mesa amp. If the plate voltage is too high (not uncommon to see 475 volts b+ in a Mesa) there is not going to be a successful biasing to get both sides of the output tube pair up to where they need to be current wise if the voltage is already too high, they might die comparatively quickly compared to 6l6 while also not sounding their best. Stick a 6v6 in a socket biased for el34 and you might see some redplating in five minutes or less, risky to the amp besides the 6v6's...
Off the top of my head I think that 6v6gt are only rated for 300-ish volts b+. Now I know that the hifi weenies are more conservative in plate voltage numbers than the guitar amp guys, but the guitar dudes have been pushing way over tube limits for a long time.
However, the very latest thang in handmade low production boutique amps is to run a lower b+ voltage, has been happening only within the last couple years unless you are Chris Siegmund, his Midnight Blues (old school Marshall jtm45 type done modern boutique and better) is and has been available with three different main transformer options for plate voltage/wattage since at least 2005...I get to run all the old glass I want in mine.
I have recently purchased a Weber MASS attenuator, and the purchase of that one item gave me a whole new stable of amps. I use that now to tune for the room, and it is interesting to get some real power tube distortion in a Mesa, besides the preamp distortion which is pretty much the norm, especially the higher powered models. Spendy but if I had gotten it a few years earlier I would not own so many amps... :wink:
Having that attenuator has opened more doors for varied tones from the same amp by far than changing tubes, and I have hundreds of vintage tubes. Been there, done that, and got the t-shirt...
Hope this helps. Peace.
