Discoveries on ‘Rolling’ the KT88, KT90, and KT120: A Possible Reason Why the KT90 and KT120 are Loved by Half of the Population of Tube Amplifier Users, and Absolutely Hated by the Other Half
Tube rolling is no doubt highly addictive. Take yours truly, for instance. I designed a tube amplifier around the new Genalex Gold Lion KT88 tube, and after coming up with something that passes our sound tester’s ear as perfect, with good bass, great highs, and an excellently detailed midrange, I promptly go and say, “Now what happens if….” (Famous last words, by the way.)
So I bought a set of Tungsol KT120s, and it was while testing them that I made a discovery which would, in all likelihood, explain the rather surprisingly contradictory reviews made by real people using these in real amplifiers.
Upon plug-in and warm-up, the results were highly disappointing. While the tubes did have good bass, that was about all that was good. The midrange was…well, dead. The highs were pathetic. I suppose it might have passed as an ‘all-right’ solid-state amplifier, but when you have heard better—like way better, from the same amplifier, no less—it is rather disappointing to say the least!
What happened? How can two tubes be so different? I was expecting the Genalex to sound better, quite honestly, but I was not prepared for the KT120 to sound so appalling!
It was after walking away from the amplifier, its disappointing tubes, and the entire project for a while, that an idea came, as often happens.
I had read about high frequency oscillations in the output stage, and had seen this in previous designs. This is where the tube oscillates with itself at such a high frequency it falls well outside of the audible range. I had seen it make an amplifier sound lifeless. The more I thought about it, the more the symptoms seemed to fit. It wouldn’t be surprising; after all, the KT120s have longer plates than the KT88, which would mean more capacitance inside, which is what would cause oscillations to happen in all likelihood.
Upon returning to the project, I checked for oscillations on our old (tube-based) ’scope. I saw nothing. Still, the old ’scope only goes to 14 Mhz, and oscillations as high as 100 Mhz are possible…. I pulled out the soldering gun and a pair of 1K resistors and soldered them in series with the grid. These resistors, called ‘grid stoppers’ in this case, go between the grid and everything else — including the bias voltage. Their job: to provide a loss at the high frequencies where the oscillations occur. The change was astounding! The amplifier had life, and lots of it! Detail and all the rest — all by adding a 1K resistor!
My curiosity was now aroused, and I purchased a pair of EH KT90s. I had heard that many people found the sound disappointing, while a few others loved it. Suspecting the oscillation problem to be the cause of these contradictory reviews, I tried them out. They sounded good, but there was an inexplicable ‘rattle’ (for lack of a better term) in the sound at high volume peaks. Increasing the grid stoppers to 2.2K solved the problem, leading me to suspect that the rattle was caused by oscillations which only occurred at signal peaks.
To conclude, if the tube sounds dead, particularly when ‘rolling’ from one type to the next, suspect high frequency oscillations. The tube is unlikely to be so badly made that it produces a totally lifeless sound (although certain ones can be pretty bad, admittedly — buy quality tubes.) If you are solder-itchy, a ‘grid stopper’ might just work. Increase the value of resistor incrementally until the problem stops. (Also, in the case of these ‘kinkless tetrode’ tubes, make sure the screen resistor is the right size. Under-sizing this can also cause oscillations — been there, done that.)
By keeping this in mind, even more tube (hence, sound) options are available at your fingertips!