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.

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.