Summary
The KT88 beam pentode is primarily utilised in the output stages of
audio amplifiers but can also be used as a series regulator in a stabilised
power supply. With an absolute maximum anode dissipation rating of 42
watts, a pair in Class AB1 produces a continuous power output of up
to 100 watts.
History
The KT88 was released in 1954 and was essentially the British version
of the American 6550 tube. With its standard eight-pin (octal) socket,
it was a larger variant of the KT66 and in the same general family as
the more familiar 6L6 and EL34. One of the largest tubes in its class
it has the ability to handle significantly higher plate voltages (up
to 800 V) than similar tubes.
Current KT88 production is limited to China, Slovakia and Russia with fairly wide variations in quality. New old stock (NOS) examples in good condition are extremely rare and therefore quite expensive. Given its limited availability, the KT88 is seldom used in current production amplifiers, whether for guitar or hi-fi reproduction.
Classic amps using KT88s
McIntosh MC75 and MC275
Harman Kardon Citation II
Leak TL50 Plus
Dynaco Mk III
Grommes 260A
Marshall mid 1960’s 200 watt guitar and PA amplifiers
Current prices
Vintage Genalex Gold Monarchs/Lions new old stock (NOS) fetch somewhere
in the vicinity of $1200-$1800 for matched quad sets, modern version
quads can be obtained for around $150 - $200.
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