DEM, You are mistaken about an "auto-biasing circuitry " in the ARC VT100 and VT100MKII. The tubes need to be matched quads, because if they are too different, it will take out one or more resistors, reduce the sound quality, or reduce the tube life by burdening the other tube in the push-pull circuit. In regards to the bias figures quoted, it should be 130ma for stock 6550Cs and around 80 to 85ma per channel for the KT90s across the bias test points (resistor). In regards to replacing the driver tubes (6922 and equivalant) -- make sure the tubes are a matched octet and you obtain and perform an alignment procedure. If this is not done, you will have reduced fidelity, increased heat, possibly blown components, and reduced tube life. The procedure takes about 2 hours and requires at least one 3 1/2 digit DVM (2 is preferable), a schematic diagram, tube electronic circuit knowledge, as well as the removal of the top and side covers. In addition, the installation of KT88, 90 & 91 in VT100s could blow the screen grid resistor(s) upon power up. This can also happen with a 6550C replacement, but less likely. I know this is off track on this thread, but important information to know when considering tube swapping in the VT-100. I do agree with post regardin the KT-88s can be unreliable - I have also had bad luck with those... The KT-90s are better reliability, but they are not my first preference. The Tungsol 6550s seem to perform the best IMHO if you can find / afford them. BTW, I have NEVER had a problem with the 6550Cs that came directly from Audio Research.
