time to discuss kt-88's

To stay on thread, I have really liked the Svetlana kt-88s but had some reliability problems running them in Audio Valve Challenger amps. Recently I came across some RAM-tested kt-88s that I now prefer.

Regarding kt-90s I find the assertion that they require very long burn-ins to be interesting. When I rolled them into various amps, usually replacing 6550s or kt-88s, I only let them burn in for 40-50 hours before determining that I did not like them. Who knew 400 was the ticket?

My current tube reference amp, the Jadis JA-100 uses 4 kt-90s per side and it sounded great right out of the box. It improved over the first 60-70 hours and then settled in. The improvement was noticeable but not dramatic. Jadis assures me that the tubes were new. They say that the kt-90 places special demands on the power transformer so they developed a proprietary hand wound unit specifically designed for the kt-90 tube. The amp self-biases and the tubes do not glow bright in the plates. I am not an expert in this area but I do trust Jadis to know a few things about tubes. Maybe it takes 400 hours for kt-90s to burn in a standard transformer?

One thing I don't see discussed much, except in this posting is the compatabity issue. Bias voltage, and plate voltage seem to be different for this tube, and they don't seem to be a drop in substitution for Kt88s, or 6550, if all paramaters are not met, resulting in plate glowing, short live, etc. Has Any one used them in dynaco mark IIIs, what luck, and what mods were made to accomodate them, from bias, to plate voltage? Check out the Citation page on the web to see comments about bias, and voltage, which makes me post this, as I need to retube, and am unsure of going with the EH Kt88, or the EI Kt90. Then of course there is the Kt 100 which we don't seem to hear much about.

Any suggestions?

Issue #19 of Vacuum Tube Valley did comprehensive listening test with current production and NOS KT-88 and 6550 tubes. You should check it out.