[Dacious]

Charlie, the Bassman is a fixed bias amp in that the bias point refers to the voltage on the grids are held at, to coax electrons off the cathode. Too little, the tubes shut down or won't even start amplifying. Too many electrons attracted to the grids and they go incandescent, or pulll too much current onto the plate, and the tubes cook. There are two dissipation limits, grid and plate. And both need to be respected for your tubes to live.

But be able to vary this, you either have to solder in a new grid bias resistor after guessing (not good) or calculating plate volts/current draw, or have a variable resistor (potentiometer) installed in the bias circuit, like Blackface amps used to have. The 6V6s will amost certainly pull a different amount of current to 6L6s, as their plate-plate impedance as a pushpull pair is approx 50% higher than the 6L6s.

What this means, is that if you just plug 6V6s in without rebiasing, they may work - or they may light up like roman candles and have a short, spectacular life. The JJs reortedly handle the plate volts, but if you make them dissipate 40 watts instead of 15-20 like they are designed for, they may go pyro on you - and take out your output tranny and maybe some other stuff, too!

The biasrite and other similar instruments can tell you how much current they're pulling - but they can't correct it, it needs an operator (you?) to do that.