VTL 6550 vs. KT90?

I own a pair of VTL225 with KT90 tubes. I talked to VTL, they recommend me to upgrade to 6550 tubes since they are more available and cheaper. They said they must change the amps to accept 6550. Does anyone know if KT90 and 6550 tubes are interchangeable? Does 6550 sound better than KT90?? Where can I get good KT90 tubes at reasonable price?

KT90 is a 6550's substitute.No problem they are interchangeable.Best KT90/6550 is KT90 Ei type 3 at reasonable price.You can buy them at www.upscaleaudio.com .Kevin Deal is really the best!

Thanks Marcorin. Did you compare the 6550 to the KT90 in a VTL amp as well?

No,i haven't compared them in a VTL amp, but in Jadis 80 mono, Sonic Frontiers Power 2 and CJ.Same result! Kevin at www.upscaleaudio can tell you the best tubes with a VTL amp.He retubes all the amp and he is very honest and friendly.Contact him!Ciao.Marco from Italy....

This is a really old thread, but has anyone ran KT90s in a VTL ST 150, instead of 6550? I have KT 90 225 Deluxe monos and really loved them.  Curious what the bias difference if possible would be in the ST150.  Cheers.