The KT88 was first produced by Marconi – Osram Valve, a subsidiary of GEC in 1956 and continued for a 28-year production span. A classic audio output beam tetrode characterized by high power output allayed to low distortion made it ideal for audio amplification, however, it does find use with many guitar amplifiers where an end user prefers a tone showing less distortion to that obtained with the 6550 - which was designed as a servo amplifier valve.
The KT88 is one of the largest valves in its class and can handle significantly higher anode voltages than many of its contemporaries - up to 800 volts. A KT88 push-pull pair in class AB1 fixed bias is capable of 100 watts of output with 2.5% total harmonic distortion or up to about 50W at low distortion in hi-fi applications. The screen grid is sometimes tied to the anode so that it becomes effectively a triode with a lower maximum power output, but considered by some users to have superior sonic quality.
The transmitting tubes TT21 and TT22 have almost identical transfer characteristics to KT88 but a different pinout, and by virtue of their anode being connected to the top cap have a higher plate voltage rating (1.25 kilovolt) and a higher power output capability of 200 watts in class AB1 push pull.