6550C vs KT88

I am using Audio Research VT100 Mk2. Thinking about trying the Svetlana KT88 to replace my 6550s. What are the biggest differences between the two? will this cause any kind of electrical problem in my VT100??
Thank you,
ARC does not recommend KT88's but I'm not sure why. I've seen people try them with no problems. I had a VT100mkII and its an outstanding amp. I felt it was a little "dry" sounding for a tube amp so I suspect the KT88's might warm it up slightly over the 6550's. On a related note, my CJ amp is inherently warm and sweet with the 6550's and the KT88's didn't sound as good in the CJ amp.... It's all about personal preferences and this is what I found.
I just purchased a second hand ARC CA50. The power tubes in the amp were EL34 instead of 6550C which was, ofcourse, undisclosed. Being a tube newbie, I wasn't able to recognize the power tube isn't 6550C. And, yes, there were electrical problems. $700 dollars later, I learned to stay with 6550C in ARC amps.

I also talked to numerous ARC dealers and Leonard at ARC. They all say using anything but 6550C is to be used at your own risk. The tech that serviced my amp was even more adamant. He claims every tube, driver, input and power, must precisely match.
I also have an ARC CA50 Integrated Amp. I replaced the power tubes with Sovtek KT88's, and the stock 6922's with German RCA NOS Gold Pins.
I biased this configuration at the same settings as the stock. This is absolutely necessary!
I have been running this configuration for about 100 hours now ... and every thing is running so good! The midrange is more liquid and the bass is tighter.
After, listening to this configuration, I swapped the tubes back to the stock ones, rebiased, and listened. I had to go back to the KT88's and RCA Gold Pins ... it was quite evident! This combination makes this good ARC amp an excellent ARC amp ... I would like to hear an integrated that can beat this sound.
I would love to try the Gold Lion KT88's, but they are quite expensive. So, I will be happy for awhile.
I am driving Silverline SR17's in bi-wire mode, and using an Arcam FMJ CD23.
It is safe to substitute the kt88 for the 6550. You just have to make sure you have the tubes appropriately matched for a vt100 (or whatever arc you are using). I have called arc about this very question, and they said kt88's are ok.

I personally prefer the kt88's in my vt100, but I know several people who feel the opposite. The 6550 has a little more detail, a little better bass, and is more consistent from bass to trebble.

The kt88 punches out the midrange a little more & is a little warmer. If you lean more towards the conrad-johnson side of the house, the kt88 is for you.

One thing I would never do is put a set of NOS tubes in an arc amp. Apparently they are designed to have fairly specific tolerances on their power tubes. At least not without the gold stamp of a good tube dealer (like upscale audio)

Has anyone out there heard the electro-harmonix 6550 as compared to the svetlanas ? I understand that ARC is starting to use these.
I had the same experience as Abe Collins regarding KT88 in my Conrad Johnson amps (11a, then 12's): The 6550 seemed a better match given the inherent slight sweetness of the amps. OTOH, my friends Sonic Frontiers amp (model 2 I think) benefited substantiall from the KT88.

I do think that KT88's are not all the same and that one brand, I forget if it's Svetlana or Sovtek, make KT88's that are particularly good for swapping into 6550 applications.
Do Not substitue KT88's for 6550's in all ARC amps! It is my understanding that if you try this in a VT 130 SE you are going to cause a problem. Consult with ARC before you try it. A friend did this and the amp torched, and was latter told by ARC that it was indeed the KT88's that did it.
John l,
I repalced EH6550's with Svet6550c's (Rogue M120 Magnums) and found the Sevets to be better all-around. In particualr, thy sound more detailid, more powerful, and less fuzzy that the EH's. The amps also seem to be more powerful with the Svet's in.
What's more, they run much cooler, at least by 75% -maybe more- than the EH's which would really cook!

I TOTALLY DISAGREE with most of the posts. I have a VT-100 MkII. I do not like the original Svetlana 6550s. I put Electro Harmonix 6550s in it, and love the result. As opposed to Killerpiglet's experience in a Rogue amp, I found the EH6550 to have more punch, tighter and more powerful bass, more detail and extension than any other tube I used. I also borrowed some Svetlana KT88s, which were highly matched, and I found them to sound too distant and undetailed. At this point, I was used to the EH6550, I need to point out.

The EH6550 is almost identical in appearance to a KT88, with the "coke bottle" shape. I have had NO reliability problems whatsoever. I have, however, heard that Svetlana KT88s are not the most reliable tube, especially in ARC amps. If anyone is interested in the EH tubes, I have a source for them at a good price.
Carl ?
Kevziek, very interesting post. Did you try the Svet 6550c's or the 6550b's?
Of course, these things are so amp-dependent let alone system dependent; still it's a 180 over what I heard.

I'm loving the fact that changing the tubes can give me a very different amp, and I can always go back to the old amp by saving the "old" tubes.

Now, time to start thinking about that tube pre...

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.

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.
I used Chinese KT88's for years in my D250MKII with no problems. Only had one tube ever fail!
Thank you John, I took my unit back to the local dealer, they had talked to ARC as well, KT88 is ok to used in VT100 MKII, not MKIII. Instead of replacing the resistor myself, I trust ARC more in their QA and listening test. I decided to go with MKIII update, hope I had made the right choice. BTW, I'm testing my quad matched Svetlana KT88 in a friend's tube tester, just want to make sure they are OK, since my new MKIII can't use them, I would like to sell them (less than 5 minutes used). Let me know if anyone is interested. Thanks again