simul class/class a switch?

[billyg121]

i love the mark 4.although it is a constant tweek job for me to get tone bliss.the other day i tried the 6l6/el34 combination.then i tried just class a el34 and rhythm 2 came to life.i absolutly love the tone.r3 is cool as well but i think i like it better with 6l6s in their.sooooo...is it practical to switch between class a and simul class on the fly?what kind of switch would i connect to the input in the back to do this?i feel like i am adding another channel to the amp.did metallica use el34s?the sound is very close.

 

[AdmiralB]

When you select "Class A" you also have an impedance mismatch - the amp wants to see twice the load marked on the jacks, so when you are in that mode the 8-ohm jack becomes 16, and the 4-ohm becomes 8.

That mismatch can change the tone and feel significantly.