The KT88 is one of the few classic audio output valves. The figures given are for single ended use, but the valve is almost invariably used in push pull pairs.
With an anode to anode load of 4,000 Ohms and 550 Volts on the anode a pair run in ultra linear AB1 will deliver 100 Watts at 3.5% distortion. With 425 Volts on the anodes and a load of 6,000 Ohms the pair will generate 50 Watts at 2% distortion. If used connected as triodes in class A and with 425 Volts on the anodes and a load a - a of 6,000 Ohms an output of 27 Watts at 1.3% distortion is obtained.
Modern designs use these devices in two pairs for parallel push pull and with heavy negative feedback, high power with very low distortion is produced.
The KT88 was produced by M-OV for 28 years and in its final years was marketed under the Gold Lion Brand. It was phased out in 1984 in favour of production of integrated circuit packages.
The GEC paper label.
There are two getter holders, one each side.
The original GEC box the valve came in. Within the box the valve was wrapped in corrugated card.
The classic envelope is 50 mm in diameter and, excluding the IO base pins, is 102 mm tall.
References: Data-sheet, 1005 & 1040 Type KT88 was first introduced in 1956. See also 1956 adverts.