The British Marconi-Osram GEC Valve Company of England introduced in 1938 the legendary KT66, K.T standing for kinkless tetrode. This designation was given as when the performance of the valve is measured from it's output curve it was found to be linear - hence it's great sound.
The KT66s superlative sonic performance found audio fame in the Q.U.A.D II amplifier in the field of hi-fi and in 1965 Jim Marshall used the KT66 in the JTM 45 guitar amp to power those famous biting solo's to a crescendo in Eric Clapton's band!! Hence the KT66 joined the rock and roll valve legend in the first position where it has been ever since.
The performance of the MOV originals is all due to the heavy grade glass and rigid mica spacers with stiff grid support specified by MOV engineers to eliminate microphonics and provide longevity of performance. Many replicas of this valve have been made over the years, mainly in China but these replicas don't 'cut the mustard' in comparison to the original GEC MOV devices in terms of sound, longevity and visual aesthetics.
The problem is not just locating vintage devices but getting good ones which have been fully tested using an AVO VCM to determine stability, microphonics, cathode heater insulation hot & cold, internal shorts, anode current and mutual conductance. For this you need expertise and the requisite Valve Characteristic Meter - hard to find and jealously coveted these days - and so they should be as they cost the price of an average semi back in the 1950's
The device offered here today is AVO VCM163 tested and as good as you can get. Its a GREY glass, black low loss micanol based unit from wartime production. The base is tight to the envelope with no sign of re-fixing. The markings on the front are in etched cartouche and show KT66 Osram and the rear of the envelope carries a perfect Osram ellipsoid yellow and blue decal.
Tested using an Avo specification for 100% emission of anode current, Ia =63 mA; mutual conductance gm = 6.3 mA/V @ At Va = 250V Vg = -15V the results obtained were:-
Ia = 95 mA.
gm = 8.0 mA/V.