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Did some reading, and I see a significnat variation as to what each of us is running our KT88s at.

If I remeber correctly after full warmup... I bais my MKIII at .60v 600mv per tube. Ive run them at 700mv per tube and didnt notice any imporvment.

So what do most of you run your bias at and why?

Justin

120ma per pair.

The answer really depends on what your goals are. Today, most people are concerned about tube life, so they tend to bias their tubes somewhat cooler, with KT88s running as cool as 55 ma or so. No doubt this will help with tube life, but will also leave the unit incapable of being able to produce the distortion performance originally published for a MK III amplifier. The original bias point had each tube drawing 70 ma. This produced excellent performance, and tube life was still good given that the amplifier was originally supplied with authentic Genalex output tubes.

60-65 ma is a good compromise, particularly so if each output tube can be individually biased. The original amplifier did not incorporate this feature, and relied on matched tubes to create AC and DC balance in the output stage. Having this feature with otherwise matched tubes allows for an even closer DC match than most matched tubes can provide, so the tubes can be run slightly cooler than the original specification to achieve the same level of performance.

Running the output tubes cooler will produce the greatest performance loss under full power output conditions, although distortion performance is affected at all power output levels when the tubes are operated at lower quiescent levels. Personally, I have run my supply of GE 6550s at 70 ma each for years, and typically achieve ~ 2500 hours of usable life on average. This is based on a life end point as being defined by a tube that is no longer able to produce at least 64% of it's original power output when new.

Dave

I run mine at 68mA. I was aiming for 70mA, but had them pretty well balanced so I just stopped where I was.

Speaking of bias circuits, what range of voltages do you recommend I set the divider network to allow?

I think the assembly manual indicates a 4.7K and an 18K resistor in the bias voltage divider network. This should give you a range of roughly -67V to -42V, depending on your line voltage, etc. Actual bias voltages aren't really important - it's the idle current that matters.

I would recommend you substitute a 1K resistor instead of the 4.7K. I've seen enough people who have had difficulty getting the idle current under 70mA, and making this substitution seems to help them out. For what it's worth, I stayed with the original 4.7K part but my bias adjust pot is pretty much turned all the way counterclockwise. If I had needed it, I wouldn't have had much room to turn the idle current down any lower. If you want, you can also sub out the 18K part for a 21K just to keep the total load of the bias network approximately the same, but I don't really think this is necessary.

You should consider adding a 100K resistor from the top of the pot to the middle leg for safety purposes. It won't affect the bias during normal operation but it will save your tubes and likely your output transformers in the unfortunate case where the wiper lifts.