Tung-Sol KT-120 Power Tubes

[smegger68]

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!

[mr.me]

Nice review James. I am also using Audion Sterling monoblock amps but they have been modified to run 5881's. This was done before I purchased them but why they were modded I don't know. I have not tried any different tubes other than the New Sensor Tungsol 6L6G, large ST shapes, and they work fine. Would love to try the KT120 but don't honestly know whether they would work in my modified amps.

[smegger68]

In theory, it should at least be safe to try. The amp would need to have been modified to put less voltage through the 5881's as they are a lower output device. Otherwise the 2 tube types are pin compatible. Only problem I can see is that it may now not put enough voltage through something like a KT-120 to properly get it going and might sound a bit raspy. Someone else might have more first hand information!

[i_should_coco]

5881s are 6L6 types. They're somewhat different to the KT88/6550 family and bias up rather differently, so I would advise caution. You could cause damage.

[rabski]

Further word of caution.

KT120s don't draw more heater current than 6550/KT88/KT90 AFAIK, but they may be difficult to bias properly in some amps and to run in spec, they do need a bit more voltage than most KT88s.

Should be fine in an amp that can use EL34/6550/KT88/KT90, but it's worth checking first.

[crimsondonkey]

I bought these for my Audio Research Ref110 a while ago to replace the 6550's. ARC use these in their new power amps and ok'd their use in certain legacy products. Obviously its best to re-bias them when replacingl.

Your findings match mine very well, bit more air and body, firmer grip on the bottom end (oo-er!). as you say, because they're tougher I would expect them to last longer.

For £200 it represented a fantastic vfm upgrade, bit like getting a bigger amp from the same stable. Obviously you should check with the manufacturer before rolling them in, and ffs if you don't know what you're doing then don't do it, and ask some-one who does.

[leenorris78]

Thanks for the interesting review.

I just ordered a pair for the audion sterling.

Lee

[Cable Monkey]

Rolled 4 of these into my D70 mk II yesterday. Very promising results so far though I do need to make sure they fit with cover on. So far very impressed when compared to nearly new Tung Sol 6550's.

[smegger68]

Thanks for the interesting review.

I just ordered a pair for the audion sterling.

Lee

Let us know how you get on Lee :-)

[diceman]

I believe typically KT88 and 6550 run at around 1.65 amps for the heater and the KT120 1.7-1.95 amps. I have been advised not to fit them to my WAD WD88VA as the mains transformer heater winding is only rated to 7 amps total (4 tubes) and if the heaters run at 1.95 amps then it will be in excess of the transformer rating.

It is worth checking with your manufacturer before fitting.

[rabski]

I believe typically KT88 and 6550 run at around 1.65 amps for the heater and the KT120 1.7-1.95 amps. I have been advised not to fit them to my WAD WD88VA as the mains transformer heater winding is only rated to 7 amps total (4 tubes) and if the heaters run at 1.95 amps then it will be in excess of the transformer rating.

It is worth checking with your manufacturer before fitting.

My bad. I had a provisional datasheet for KT120, but the published ones now seem slightly different. KT88/6550/KT90 are indeed between 1.6 and 1.7A. EL34 and similar are lower. KT120 Tung-Sol datasheet states between 1.7 to 1.95A.

For a routine amp running four power tubes, that's 6.4A vs nearly 8A. Not a massive difference, but certainly enough to make it worth checking with the manufacturer as it's an area where more than one company is a touch marginal in terms of specs.

[BobC]

My bad. I.

[i_should_coco]

My bad. I had a provisional datasheet for KT120, but the published ones now seem slightly different. KT88/6550/KT90 are indeed between 1.6 and 1.7A. EL34 and similar are lower. KT120 Tung-Sol datasheet states between 1.7 to 1.95A.

For a routine amp running four power tubes, that's 6.4A vs nearly 8A. Not a massive difference, but certainly enough to make it worth checking with the manufacturer as it's an area where more than one company is a touch marginal in terms of specs.

Hmm... that's 25%, which is plenty to cause concern. Certainly not a drop-in without checking, as you say.

[crimsondonkey]

Yep, to be crystal clear - KT120's are NOT a drop in. You should always check with the manufacturer first.

[Cable Monkey]

I will weigh in and re-inforce the above, I have found a source that suggests this tube should not be put into a standard D70. *However* I am exploring options of a minimal intrusion method of boosting 6.3v heater current because they do work, bias nicely and sound fantastic to my ears. The transformers don't seem to get any hotter than in normal use and I am not getting any hum or buzz but obviously I don't want to risk a classic amplifier.

[leenorris78]

Let us know how you get on Lee :-)

I ordered these on saturday and they arrived, nicely packed and in 2 working days.

So, with great trepidation, I put the KT120's in and switched the stereo on. Great, no bang, no smoke.

I have been listening for about an hour or so and can totally agree about there being better bass impact. Its the equivalent of turning up the sub. But where bass can sometimes be a hum, there is more thud to the leading edges of the notes. The bass is also fuller.

So far, so good, though I read they take about 50 hours to truly come on song. At the moment, I feel there was a bit more intimacy with the Winged C KT88's but this may change.

Currently listening to Honeyroot - The sun will come. And it sounds pretty good!

Cheers James for the tip!

[rabski]

I will weigh in and re-inforce the above, I have found a source that suggests this tube should not be put into a standard D70. *However* I am exploring options of a minimal intrusion method of boosting 6.3v heater current because they do work, bias nicely and sound fantastic to my ears. The transformers don't seem to get any hotter than in normal use and I am not getting any hum or buzz but obviously I don't want to risk a classic amplifier.

Just needs a check on the specs. As long as the heater circuit can cope with the load, then there shouldn't be any issue once they're biased properly.

If the heater circuit can't cope with the load, do not try under any circumstances. Mains transformers aren't cheap...

[smegger68]

Mine are improving as I put time on them, soundstage has opened up even more and the tone has improved. Happy to keep clocking the hours :-)

[Cable Monkey]

Right, I have dug out a drawing supplied to me by ARC. 6.3v heater winding is exlusive to the power valves and rated at 6.9 amps so not enough for the KT120's but easy enough to lash up an auxiliary supply without butchering the D70. I need to source a 240v to 6.3v 10amp transformer.

[i_should_coco]

I'd use. 6V 80VA. As you'll not be drawing rated current, the voltage will rise up a little.