time to discuss kt-88's

Kevziek -

I have also heard horror stories on early production runs on the Ei KT-90's. I can not comment as to the type I and the type II's as I have only had experience with type III's. Here is a link to show you the visible differences:

http://www.manleylabs.com/galleria/KT90.html

I have also owned a matched octet of the EI "fat bottle" EL34's many years ago (about 6 or 7) in Cary SLM-100's. I did not prefer the sound of the amps and only kept them a few months, but have never had problems personally with any of the Ei tubes yet.

However, even good tubes go bad, as I once had a Sovtek KT-88 blow and take out the left channel of my Sonic Frontiers SFS-80 in the past.

I talked to Mike Sanders today. I wanted to get his recommendations for kt-88's as well as discuss the viability of kt-90's in the silver 90's. (Quicksilver makes a currant model that is very close to the silver 90's, the mono 100's, and he offers them with kt-88's or kt-90's, but does not list kt-90's as replacements for the silver 90's).
As I mentioned before, there is some glowing of the plates, so I initially ran them at 80 or 90 ma to minimize this, as opposed to the 100 ma specified for the silver 90's. It made me wonder if the silver 90's could handle the kt-90. Mr. sanders said the silver 90's are fine with the kt-90 tube. I told him about the glowing plates and he said that is a tendency, but he wonders about it himself. He runs the kt-90's in the mono 100's at 80 ma, but some dealers run them as high as 120 ma.
I also discussed with him the slightly varying bias currents I was experiencing. When I first installed the kt-90's again, it took some 2-3 hours before the bias would settle, it would not jump quickly, but it would be at say 95 ma, and over the course of time, drop to say, 85 or 90. Now, some time later of break in, it takes about 20 to 30 minites and starts off with what seems like 4-6 ma higher than what I adjusted them to before settling back. Mr. Sanders attributes this to the power draw of my conditioner, or my house, but I wonder if it is related to the plates, or both.
Anyway, I can attest to the LONG break in of the kt-90. Perhaps this is due to the unusually robust nature of this tube that seems to be what I am hearing about them. I am also aware of tubes that are more robust than the standard that do glow in the plates, although for most tubes this would be bad, and evan if they didn't fail would wear quickly. I do not know if this is a tube with a tendency to glow in the plates when used within its specs, or how long it will last operated like this, but if I adjust the bias in the silver 90's to where they don't glow at all, I get audible crossover distortion, and I like them best presently at about 95-102 ma, so that is where I have been running them and everything I have said about them has been at this setting.
I thought this may be interesting to you guys as it relates to the reliability, and maybe the break-in as well.

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.