6550C vs KT88

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!

KP

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.