6550C vs KT88

KP, they were Svet 6550c's. Even more puzzling is that Rogue uses and recommends the EH's; so, your experience is unusual. Did you bias the amps for both tubes? I question that because you said the Svet's "ran much cooler."

The Rogues auto-bias.

KP

I have both the Svetlana 6550, and the kt88. In my system the 6550 has better bass, but the kt88 absolutely kicks but in the midrange. Of all the kt88's ( new tubes, not nos ) The Svetlana has the smoothest mids and highs. I have also had discussions with Leonard @ ARC. I think as far as he is concerned the only output tube that exists is the 6550. Oddly enough though they do sell kt88's on their website, for an ungodly price I might add. I would also recommend that you have the kt88's properly biased by an experienced tech.

Having said all that ( and reading the other posts ) the sound you get will depend on the rest of your system, and room. I've got a pair of Chinese kt88's that sounded lifeless in 1 system, but when I put them in my second system they sound great!

BTW, if your interested ( or anyone else reading this ) I have 2 matched quads of Svetlana kt88's Basicly new ) I'll sell you for .50 on the dollar.

Thank you all for your feedback.
Last night, I replaced the 6550 with my new quad matched KT88 in my Vt100 MKII. I was using PS Audio P600 power generator as the power source. As soon the VT100 is switched on, I saw the wattage meter on P600 keep going up pass 400 watts. Normally it should settle around 350 watts. Before I had a chance to check for bias, one of the resistor on the left channel closed to location V2 just broke off (the insulation part only). The smell was pretty bad. I have no idea what just happened, only the left channel (both right and left are biased very closely) had this problem. Is it my new KT88?? or just a bad resistor which could not handle the higher voltage??

So sorry to hear this. I know quite a few people, myself included, who run them without any problems. In fact a sales manager at ARC told me they had considered putting svetlana kt88's in as oem equipment.

Did you turn down the bias setting for the new tubes? As your old tubes aged, you would have been turning this up. I know the tubes have to be very closesly matched too.

From what I have read, usually these amps are designed to blow up a resistor when in trouble. You should be able to get it repaired without any permanent damage. I would give Leonard at Audio Research a call and ask him what to do. He's very helpful and will even ship you parts if you want to have a repair done locally. If you can get a local tube guru to do the work (and a checkup), it shouldn't be too expensive.

Good luck with the repair.
John.