KT88 - 6550 -KT90 - KT120
The KT90, KT120 and KT150 are all tubes that were designed in the modern era. They are all loosely based on the KT88 but with much more power. You can always sub a KT150 for a KT120 or KT90 but never the other way around. The KT88 and 6550 are always interchangeable.