I have a Audio  Research that is real particular about what 6922-6DJ8 you put in the power supply that's in the Reference Phono.......This amp uses a 5AR4, 6L6 and a 6DJ8.......When servicing the amp I changed most of the tubes to new German 6922's and a new RCA 6L6 ans a new Mullard 5AR4.....After powering it up the mute light brightened up after about 35 sec.   In about 3-5 minutes the mute circuit came on and the amp was muted.......After some time it came back on and off......I changed the 6922 in the power supply and the condition changed very a small amount but not fixed.......After thinking about for a while I pulled the second 6922 out and put a Amperex 6DJ8............Now it works fine, so what would you think of that ??          autospec
Can't say for sure, but it sounds like the amp didn't "like" one of those 6922's and went into some kind of protection mode.