the ultimate guide to tube-rolling:
6550 / KT88 / KT120 / KT150
A range of power tubes for all tastes
The 6550 is a beam tetrode that's similar and often interchangeable with the KT88 and KT90. It was introduced by Tung-Sol in 1964.
A beam tetrode that is similar to the EL34. Introduced by GEC in 1956 specifically for audio amplification, and made by Shuguang, JJ Electornic, Svetlana, and New Sensor Corporation.
KT120 / KT150
Both tubes were developed for higher power from audio amps. The KT120 is a beam pentode, that, until the KT150 was the most powerful audio tube you could buy. The KT150 is an octal beam tetrode, and the egg-shaped glass helps dissipate heat and reduces microphonics.