...and I 'm glad that Mesa does not make amps that can use earlier 6l6 types... I'd never be able to afford my vintage tube collection if today's mass produced amps for 6l6gc were made to voltage specs for earliest 6l6. ...maybe the tweed settings on some models might not kill 'em.
Once I stuck a 6l6 metal tube in my Frank-en-Champ and it was amazing, had the singing violin sustain thang going on, very sweet and nice, I suspect that the oldest metal 6l6's can be a real tonal treasure trove in a lower b+ voltage boutique amp.
Had a little problem, I lightly touched that tube while power was on and got the piss shocked out of me. I had the socket wired to take el34 as well as 6l6, and 6l6 metal tubes ground the metal envelope to pin 1, no need to ground a glass envelope, pin 1 and 8 need to be connected to each other within the socket to run el34 or 6550. I got a nice li'l zap at somewhere around 376 volts DC.
That amp now runs a really hot home fried turret board, and I'm now getting over 420 volts in the b+ in it. Glad I had my stupidity episode before the latest rebuild. And yes, that amp sounds amazing running a Siemens el34 through it, as well as a couple of vintage 5881 Tung Sols that have a slightly lower current draw than the rest of the batch, no redplating. Not too many of my 6v6's look happy in that amp, they red stripe, but I have an old Brimar 6v6 that works perfectly in that little tube hell pit.
There are some secret weapon 6l6-ish in power types for the few that don't follow the herd, want their own signature tones and have strong tech skills or convenient/great tech availability. 6bg6ga, 807, and el38 come to mind immediately. All require either rewired sockets or custom adaptors, and are most definitely worth the effort in the tones these tubes provide. Within an adaptor socket, the 807 had all of the 6v6 upper mids and top end with what felt like significantly more volume, like a 6v6 pumped up on steroids, with lots of glass in the crunch. The 6bg6ga in an adaptor socket with a plate cap sound just godly, an adventure back in time for some amazing tones, the Sylvania ones are very similar in tones to their now uber-spendy/onobtanium 6l6gc that Mesa used to use... hugely fat, musical, yet nimble and articulate.
Power tubes with plate caps tend to be rated for higher plate voltages in vintage designs. Another, the el38 Mullard is very similar to the el37, the el38 sounds smoother, in a gorgeous way, fatter than el34, more like kt66 and is available for about one fifth to one eighth the price, run them in an el34 socket with a wire going to the plate cap from pin 3. Watch out for excessive screen voltage with this tube in 6l6-ish circuits.
These alternate tubes would work best in a situation where a Mesa amp was old enough to be out of warranty and not the only amp a player owns...with a bias mod installed to boot. Similar in power range to 6l6/el34, much better builds, much better and yet affordable tones.