Tubes in Audio Research Equipment: 6550

I have had an ARC VT130 for almost 4 years now. It was wonderful with KT88 (slovakia tubes). The midrange bloom was awesome. But the reliability with these tubes was terrible. One would die every month or so and with the goofy ARC design, a 100 ohm power resistor would blow too. So I was busy with my soldering iron every time a tube died. I had enough of this and went with 6550 tubes. Some of the magical bloom was gone but I got more extension on the low end. There did not seem to be much difference with power rating or high freq. extension. I think ARC goes with the 6550 tubes mainly for reliability reasons. I have had the 6550s for 2 years now and not a single problem. I understand now there are KT88 tubes from Russia so maybe this would be the way to go.

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.

Does anyone have any experience with the Sovtek 6550's?

J_k. Thanks for your helpful feedback regarding the requirement for matched quads in the VT100 and VT100MKII, as it related to my post. In my post, I was relating cautionary information I was told so that anyone (not experienced with ARC gear) reading my post would not confuse this recent ARC design with earlier designs which had individual tube bias points. Thus, they may not know of the need for matched quartets. Also,I may be wrong on this, but I understood that the factory output bias setting for the VT100 6550C's is 65mA. However, for adjusting across the test points for each channel of the VT100, ARC recommends a reading of 130mVDC (0.13 Volt DC) for the 6550C's. Best Regards!

DEM -- You are also correct -- ARC does bias EACH 6550C at 65ma. The VT100MKI & II uses 6550C x2 in each side of the push - pull circuit, making the bias setting across the TP 130MV, equating to 130ma of current running through the resistor. I heard VT100MKIII has individual adjustments..