I was using those russian made 12AX7 mullard RI's for a long while in V1 of my 2204 style home built amp. Then i switched back to the old 80's chinese i've always used. Now months later i tried the mullards again in V1 and it acted like what happens when a grid resistor is missing....hit the strings and you get sound but it's nasty, then it fades to nothing and no more sound till you unplug the cable then plug it back in or turn the amp off them on again, etc. Thought it was a bad tube but i have 3 and all did this and all worked fine last time i used them. I also found it does this with a EH tube, but those 2 tubes aside, all others i tried worked fine. Today i found if i remove the 68k grid stopper at the input it works with those tubes ! of course i suspected the resistor so i tried a couple others and same thing, Ended up changing the tube socket, the shielded cable from the jack to the grid stopper, and even the jack and 1 meg resistor. WTF could this be?!!! This is just too bizarre.