vintageHIWATTconvention.com

Hi all,

Silly question - Has anyone here tried the Russian TungSol KT120 tube in a Hiwatt DR405 or have heard any reports? According to the marketing blurb:

New Sensor now announces the Tung-Sol KT120, which has a plate dissipation of 60 watts, making it the most powerful tube in the 6550/KT88/KT90 family. A pair of these tubes in push-pull configuration can deliver power levels of 150 or more watts. When used at the parameters found in existing 6550/KT88/KT90 circuits, the Tung-Sol KT120 is impervious to overload, delivering peak power with extreme reliability and long tube life. However, taking advantage of the higher current handling capacity of these tubes, a very unique and super powerful and stable amp can be designed using the Tung-Sol KT-120.

Sounds pretty neat in print. My bass playing friend and fellow list member, synthi, has been kinda kicking himself for decking out his 405 with a set of JJ KT88's. They sound okay, yes, but a fella needs to get the full 400W out of such a beastie.

I use a set of TungSol EL34B's in my DIY VHR DR103 and they're very rock solid. Any opinions on these KT120's would be much appreciated before paying for a new set of bottles.

Many thanks,
Emil

Assuming they will actually fit physically (haven't checked measurements yet) my main concern is heater current... The KT120s data sheet states not less than 1.7A and not more than 1.95A vs a nominal 1.6A for a KT88 (though I have heard of KT88s drawing towards 1.8A sometimes anyway). Worst case scenario is an extra 2.1A, which doesn't sound promising.

It's an intriguing idea though

I heard very good critiques about those. 20% more power comparing to KT88.

But watch out, DR405 uses pretty thin heater wires, so if you put those tubes in, you should definitely check heater wire temperatures.

I have not heard any feedback on those myself, but they sound pretty beefy.

I second the heater current questions brought up above. The heater current rating was always approximate, but +/- 15%?

--mark

I haven't tried them.
call me crazy, but I never felt like there was not enough power available with good old JJ's

Grr, had a nice long post written when my browser refreshed itself and wiped it…

So here goes take 2:

Mark, I think I might email Audio Research Corp to see if they have any real-world measurements of the heater current draw (since they use them in their hifi amps now - 250W RMS @ 0.5%THD from a sextet!) and whether they've had trouble retro-fitting the KT120s to older amps. Might not get an answer but can't hurt to ask the question.

Given the pristine condition of the amp, re-wiring with beefier wire to use these bottles is entirely 100% out of the question. In fact anything that doesn't involve plugging them in and having them "just work" rules the whole crazy idea out. I'm definitely of the mind to do the proper engineering homework first. If anyone knows the type/gauge of the stock heater wires, that would be handy (save me opening the beast up again, which is might difficult to do alone). Will definitely be checking on how the wires run temp-wise with the JJ KT88 and check where the voltage is sitting now. Given the massive size of the transformer, I don't expect an extra amp or two will be bother it that much.

As far as driving them is concerned, considering the PI/driver setup in the DR405, I don't imagine there'd be much issue in getting enough current into grids. Plus I expect they'd end up biased more conservatively and thereby stay further away from the cut-off region. (might be another good question for the ARC folks - though I do note that they are using another PAIR OF THE KT120s to drive the sextet of output devices in the Reference 250 amp!)

Doc, I certainly wouldn't call you crazy! Tone-wise they are fine, but just seem to lack the low-end kick/transient response I was expecting. I do wish I'd stuck with my plan to get the winged-Cs, but they were out of stock at the time. I really expected the DR405 to knock my Marshall VBA400 (still with original factory svetlana 6550c) out of the park despite the Marshall's extra pair of output tubes, but alas it didn't… at least not in that particular regard. Both are incredible amps though, and I love the fact that they are quite different sounding. Sometimes I really wish my chops could live up to the expectations of owning such beasts!

Anyway, perhaps the power-hungry madness will subside and I will put this crazy plan to bed… but somehow I fear not!

p.s. topic for another thread another day (in another forum): Anyone got any experience with the Karlsson K15 speaker enclosure? Thinking of getting a couple built... John McVie's tone in the Peter Green era kills me!

I would be really careful about that wire gauge. Stock Hiwatt is 22awg, I used 20awg and it heats up to 45°C.

Thermal pictures:

viewtopic.php?f=5&t=1311&start=30#p11564

Thanks Ribba, I guess it's starting to look like a not so great idea.