6550 vs KT88 vs KT90

Thanks trlja

I've learned a great deal from this thread and I have been having a great time tube rolling. My Audio Valve Challenger amps actually allow me to use ANY combination of 6550, KT88, KT90, or EL34 tubes so I have been mixing and matching to fine tune the sound (and generally driving myself crazy). I have not tried KT90s partly because of the expense but also because I am told they will only work optimally in an amp with appropriately designed transformers. Any thoughts?

I'd suggest you ask the manufacturer of the amps directly. The new Jadis have transformers optimized for the KT90s, but I believe that VTL amps can use both 6550s and KT90s, so I don't know if you have to have a transformer specifically designed for that tube. The manufacturer will be able to tell you what he/she thinks is the best tube for your amps.

I second the vote for the Svetlana KT 88 vs 6550 (haven't tried KT 90's). Just retubed my Aronov monoblocks with KT 88's from Upscale Audio and the improvements in the midrange and the top end are dramatic. If the bottom end is less defined, I sure don't miss it. Then again, my monitors only go down to about 45 Hz, and rest is handled by a powered sub.

There is so much good information on this thread that I made a hard copy of it-- thanks everyone. Craig.