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Positive Feedback ISSUE 69
september/october 2013


KT99 Tubes
by Scott Dorsey


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.