Tube Biasing ?

My RM-10 manual refers to the tube plates glowing red when over-biased. A $29 Radio Shack meter set for volts is all you need to get an accurate reading and I've seen several references to monthly checking although brand new tubes will change almost hourly for a few days.

Already broken in tubes stray about 5 mV after 3 weeks or so. I use a somewhat arbitrary number (275 mV) based on an absolute upper limit of 300 mV for the RM-10. After break-in it's more something to do then anything to be concerned about. When new, however, I have had them stray from 275 to over 300 and under 225 after just a few hours. Always wait at least 20 minutes after turn-on so the tubes are warm when you measure.BTW, this applies only to the output EL-84, the inputs, 12ax7 are self-biasing.I imagine the changes are somewhat a function of the design so you need to consult your manufacturer.

There's another thread here on bias vs. sound with some posts from people who I feel are more experienced than I. You all should be warned that I don't hear results from some of the tweaks that are mentioned. Whether it's my ancient ears or my system just isn't resolving enough, who knows? But, Q.E.D., if I hear it, it must be there. Lowering the bias a gross amount, by 1/2, which I tried, gives a "warmer" sound. After a few minutes it becomes clear that the "warmth" is rolled off highs and a general loss of detail and attack transients. I value my RM-10 for the accuracy, detail, and solid bass it conveys while retaining some of the mid-range euphony of tubes. That's the way Rodger designed it and the upper range bias keeps it there.