In the late 1980s, David Manley of VTL travelled to the EI factory in Jugoslavia to interest them in making audio tubes for him. He was looking for a higher powered and more rugged beam power tube that would drop into existing 6550/KT88 amps. What EI came up with was kind of a weird design based around a sweep tube structure and a very different envelope configuration, but it was a good one that had a lot of good points.
He started importing these tubes around 1990, and was sending out test samples of the new tubes out to the press.
I received two matched pairs in late 1991, one tube of which failed very quickly due to a bad internal spotweld. VTL very quickly sent a second replacement pair, giving me a complete set for my Harman-Kardon Citation II.
Now, the Citation II runs the output tubes very, very hot. Chinese "KT88" tubes fail very rapidly. NOS ones last a while but eventually fail with plate deformation after a while.
I wrote about my experiences in the Jan/Feb 1992 issue of Positive Feedback, back in the days when it was actually on paper. (In fact, I think that was the last issue without a glossy cover). In the July/Aug 1992 and May/June 1993 issues I wrote some follow-up information about my experiences with the tubes. They sounded good, and they kept sounding good.
Now, here it is July of 2013, and a second tube has failed, again due to a spot weld failure which might be considered a manufacturing defect.
Let me say this another way: after 22 years of more or less daily use in an amplifier known to be hard on tubes, there was a second tube failure.
22 Years Later
Putting a new set of "KT99" tubes from the EI factory in, the change in sound was not great, or at least not striking. Maybe it's a little more detailed, but it's not so much more detailed that I am worried about it.
Plotting curves on the original tubes (which is a little interesting since I don't have access to a tube curve tracer at work like I did 22 years ago), they are a little bit off from the originals that I made when the tubes were new. The curves diverge more than they did originally, and the gain is a little lower on all of the three I tested. But hell, for 22 years of use, that's saying a lot.
In this modern RoHS era, it's rare for anything to last 22 years. Hell, even distribution formats don't last that long any more. But the Manley KT90 did.
I very seldom write for PF these days, but every once in a while I just have something that I need to say. And 22 years ago, I told David Manley that I'd write about these tubes when I heard any chance in the sound, and now I am doing it. Sadly, the tubes outlasted him.
But I put in a new set from the EI factory and we'll see if they last as long. Expect a follow-up here in 2035.