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.

Carl ?

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.