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I answered this a few days ago, but Yuku ate it.

KT90: an extinct tube if New Sensor isn't making them now. Originally conceived by Manley for high end studio amps, it's essentially a cross between the 6KG6 sweep tube's voltage handling and an EL34-like sound, with a taller, thinner bottle relative to most 6550/KT88's. They were made by Ei in the old Philips plant and the last few runs of them are famous for being dangerously faulty. New Sensor started making them right as Ei's production got bad, but they are hard to find. Given New Sensor's track record with big audio beam tetrodes, their KT90 is probably an excellent tube. I have quite few old Ei KT90's, but have never encountered New Sensor's version.

KT99: Can be one of three different tubes, two of which were made in China and the other in Yugoslavia in the 80's. The Chinese tubes are either just relabelled KT88's or another tube that resembles a fatter KT90, but without the KT90's voltage handling, which is a copy of the Yugslavian KT99. It was supposedly a 'modernized' version of the Gold Lion KT88 for McIntosh. The sound is supposedly nice, but they do not hold up well under operational voltage. They are very rare, but from what I have gathered, do not live up to the hype and most modern KT88's are superior.

KT100: This can either be the aforementioned "KT99" or the far more common version, a relabelled Chinese KT88, which is an often unfairly maligned tube. They sound nice and balanced, and handle voltage well, but have a slightly muted high end and midrange. I have dozens.

Any other questions?

BTW, what are you thinking of using those tubes for? There are lots of other KT88/6550 types out there right now that are much easier to find. I also forgot that JJ made (makes?) a KT90 called the "EL509II." It's not on the Eurotubes website anymore. For more info on it and other big beam tetrodes, check out this thread: http://ampworkshop.yuku.com/topic/1976?page=1

I know that it had a couple of revisions later, and I have heard of a few people building it, but that's as much as I know. The big tube bass amp market has cooled dramatically in the past couple of years.

The layout is here: https://taweber.powweb.com/store/ab200_layout.jpg

It looks like he got rid of the EL84 driver and went with an EL34 and backed off the supply filters. On paper, it looks like a cross between an SVT and a 400PS, but set up more conservatively than both. With a great set of tubes and a little hot-rodding, that thing could be evil. It's even listed in the schematic: https://taweber.powweb.com/store/ab200_schem.jpg as an option.

jbmojo wrote:

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What about the KT100's? Still available?

The last ones you used to be able to find them on eBay fairly regularly were just relabelled "6550-98's," new production Chinese 6550's made using Richardson Electronics jigs sold to them out of the old GE Ken-Rad plant. I wouldn't take too much stock in the "KT100" being necessary to boost the plate voltage. Any decent 6550/KT88 ought to do fine with 700 plate volts.

The JJ KT88's kick ass, but bias up a little hot for full power for my taste; I don't trust them to handle an amp with a powerful drive stage, like an SVT, unless you back them off a bit. KT90's (good ones anyway) are tough as nails, but mostly mids; I have them in my cathode biased Wizard Bass 500, which redplated everything else, JAN GE6550A's included. I'm really partial to the coke bottle shaped New Sensor tube that used to be sold as the "Sovtek KT88" but is now called the "Tungsol 6550;" they're tough, cheap, AND sound good. About the only modern 6550 I DON'T like is the "Flying C" /old "Svetlana" 6550C with the square plate holes.

What kind of bass/cab/music? Sound City's are pretty "middle of the road" tonewise (the active EQ notwithstanding). If you want more snarl, then go with KT90's, otherwise the JJ's or New Sensor'Tungsols" should be fine.

I'd say stick with the JJ's. They have a nice bite that the Tungsol 6550's lack, which can get a little 'compressed' sounding and are better for "smooth" music requiring more clean punch than growl. I'm an SVT guy first and formemost, but my amp (and cab) stable is huge. I play loud rock ala Motorhead and I love old SVT cabs, but when they're too tall, I use an Acme Series 1 4x10 for all my amps or an Ernie Ball HD212 if I'm using an amp with an 8 ohm output. It's funny because the HD212 is very heavy, 103 lbs, but there are stages that are too short ceilinged for the 8x10 stack, and no room to set the head beside the cab.