KT88s to replace 6550s -- biasing question

I think you should contact ARC.
Ask for Chris Ossana in their service department.

Hello, yes, you can replace them at the 65ma or whatever ARC recommends, however the amp(s) will not sound as good. They will sound different, but in most cases 88's make ARC amps sound a little warmer, but the amps will lack the bass punch and detail. That has been my experience with most ARC amps of various vintages. Give Leonard a call at ARC and he will probably try to steer you away from 88's, and at minimum he will give you the pros and cons. He is the Yoda on all ARC products. Mr. Ossana does most of the actual repairs and upgrades these days. I am sure he can tell you as well.