Tube Biasing ?

I have spoke to Both VTL & Audio Research & they were both very helpful regarding this any any other tube issues . First try the manufacturer of the amp if possible, as they know the most about their amps design. A good service center like www.Sound-smith.com in NY or Steve Sank in NM bk11@thuntek.net are great for tune ups !I am sure you will get some good advice from this post also!

The signal to the speakers alternates from positive to negative with respect to ground. To increase efficiency in a push pull power amp, the positive half of the signal is amplified by one half of the tubes, and the negative half is amplified by the other half. This means that half the tubes are off about half the time. At the point where the signal is passing from positive to negative, or back, one half are in the process of turning off while the the other half are turning on. If the bias is too low, the turning off half will turn off before the turning on half can turn on, that is, they will both be off during the transition. This is called crossover distortion and it is not pleasant. In other words, both halves need to on slightly even with no signal applied.

If the bias is too high, the tubes will have too much current through them while idling, they will run hotter, and this will shorten their life.

In low level settings like pre amps, the bias is usually set by the manufacturer and is not user adjustable. These tube are run class A and amplify the signal through both the negative and positive half of the cycle. It would be difficult to change this bias without knowing what you are doing. If you have to ask, don't try it.

Bruce : Crossover distortion would be considered a symptom of an (possibly) under biased tube ? Have you experienced crossover distortion ? What would one listen for if this was occuring slightly ?
In the best of your estimation , is the only effects of a slightly over biased tube excess heat and shorter life , or is there another obvious sign to look or listen for as an indicator without putting a meter on the circuit .

Distortion is hard to describe. It would add higher frequency harmonics to the signal which could be described as perhaps adding a fuzzy quality to the sound. Not a gross distortion unless severely under biased. If you turned the bias all the way down and played someting at a low level you should be able to hear it. I don't think you could hear the effects of an overbiased tube unless you were really in tune with how the amplifier normally sounds. The only way to really tell is look at the signal with an oscilloscope and see if it is distorting. Since most people don't have a scope, the manufacturer usually supplies test points to measure the bias current so you can adjust it with a meter. Unless the amp has a built in visual indicator like the CJ amps, you will probably have to have a meter to properly set your bias. As always, consult your manual.

How often should bias be checked ?

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.

Kitch , Upon checking bias how much has the voltage strayed on your broke in tubes ?

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.

Kitch , Have you noticed any sonic difference after calibration ?

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.

KITCH , You sound like you've been around the block too , not being talked into the kings new wardrobe . Thank you all for the input with no loss of output and no distortion .