If I might add one caveat here from an M-125 owner:
Should you change output tubes at some point (I swap between KT-120s and KT-88s) be sure, before you swap out the tubes, please turn the bias voltage all the way down. Different tubes require different bias settings, and you can fry KT-88s at the KT-120 setting pretty quickly. And yeah, you can can go nuts chasing voltages around. Just get it close (I leave it a bit under) the recommended setting after warm-up and you'll be fine.
Just to be clear here, the voltages remain the same (in my case, 1.2) but the pots will need to be re-set to deliver that same level of juice to different tube configurations. So with the old tubes in there, dial the volts down all the way (in my case about 0.
, change the tubes, warm things back up, and then slowly bring the voltage up.
A bit more about line voltage, if I might: I learned, after blowing a few rectifiers, that in the U.S.A. line voltage is, under current regulations, allowed to fluctuate 10% plus or minus, of 120V. So what's coming out of the wall can vary from 108 to 132 volts. Solid state gear seems able to handle this fluctuation but tubes cannot. Get a box (I recommend the Thurman, but it's spendy) that will clamp the voltage going into your amp and pre-amp to about 123V. I tried using a Variac to do the same thing but after two of the things blew up, went the expensive route and am not disappointed. Plus, the Thurman will take the noise of of your line voltage and you will hear an improvement.
Last edited by deepee99 on Wed Mar 20, 2013 11:00 am; edited 1 time in total (Reason for editing : Clarity)