Like many users of single-ended valve amplification, I love the glorious tone produced by my gear. Great though it is, there are certain areas where SET amplifiers have traditionally not been strong, usually in the areas of bass control, treble extension and dynamic impact. A lot of this is speaker and room dependent of course, but speaking broadly this is the accepted wisdom.
A few years ago the New Sensor corporation acquired the Tung-Sol name and started producing new tubes. They also surprised a lot of people by inventing a brand new tube, The KT-120 reviewed here. It's a KT-88 on steroids basically, able to handle a lot more power. This holds the promise of starting to overcome some of the problems previously spoken about when used in a single ended configuration.
But what about using it in current amps, designed for KT-88s and 6550s? I had already tried using some KT-90s (an earlier beefed up KT-88) in my Audion Sterling Plus and had been reasonably impressed with the increase in dynamics but not overly impressed with the tone. The KT-120 is much more beefed up than the KT-90 is and so in theory should provide a better experience. When Anthony Matthews of Tube Distinctions joined in the chorus of people on the net singing the praises of these tubes in older circuits, I knew I had to give them a try.
A word of warning here: While the KT-120 is compatible with any KT-88 or 6550 amplifier, they do draw a little more current for the heater circuit. Any modern amplifier is unlikely to have a problem with this but older amps may run into difficulty, especially if its a push-pull design using 4 or even 8 tubes. Check with your manufacturer before purchase!
After receiving the all-clear to use them in my amp from Audion, I put an order in with Hot Rox here in the UK. As cheap as anyone else, they provide free tube matching and got them to me in 2 days, very well packed. 5 stars for them! Taking them out of their boxes you notice immediately how much larger they are than regular KT-88s. They look like serious kit...! I removed my Svetlana Winged 'C' 6550s, plugged them in and let them warm up for an hour. Then I settled down for some proper listening.
What is immediately obvious over the 6550s is the scale of the music. The soundstage has grown noticeably, and sounds that were locked near to the speakers previously now jumped out into the room. Bass was no more extended than before but certainly had more texture and felt more in control. Midrange presence and tone were maybe a bit better than before but what really stood out was the clear, extended treble. I hadn't realised that I had gotten used to the rolled off sound of the 6550s and was pleasantly surprised at the sweet, shimmering sounds now being played. Also of note was the way these valves separated the strands of the music out more than before, allowing you to hear details previously covered up.
I expect them to improve as more hours are accumulated on them but even if they don't I would be very happy with my purchase. They have made a bigger difference than I expected and I don't hesitate in recommending you give them a try.
A couple of things to remember: though they are designed to produce more power, they are unlikely to give a lot more than your current amp gives out presently. This is because your amp is configured to work within the constraints of the output tubes it was designed for and though you will probably get a few more watts because these tubes are more efficient, you will not suddenly get the 60 watts a side these beasts are capable of. The flipside of this is that they will never be run ragged by your amp and should have a very extended life. Also, the differences I heard are in comparison to my original power tubes. Whatever differences you hear will depend on the tubes you are currently running!