6550C vs KT88

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.