The KT66 beam tetrode was introduced in 1937 primarily for radar use. But soon became highly acclaimed as an audio output valve.
The KT66 was essentially a 6L6 redesigned to have a larger cathode and a more robust wide section anode but more importantly, the electrodes were configured within the electrode cage in maximizing efficiency by positioning of the anode such that it was a specific multiple of the distance from the screen grid to cathode distance. A pair of beam deflection plates connected to the cathode were added to focus the electron beam into two discrete planes 180 degrees apart hence providing an elegant solution to the limitation of space charge secondary emission without using an extra suppressor grid.
More importantly, this ‘focussed’ four electrode design mimicked the characteristics of Philips’ power pentode designs but neatly sidestepped their patents.
One important performance benefit of the beam tetrode electrode configuration Is that the beam plates limit the negative resistance kink in the anode current vs anode voltage in the lower regions of the characteristic curves of a full tetrode which could serve to produce disturbing instability in Push Pull amplifiers using a full tetrode, having said that, the beam tetrode was not very forgiving of poor design which could reward the unwary designer with severe parasitic oscillations.
A further sonic benefit is that the beam tetrode electrode configuration also produces the lowest distortion of any output valve due to it’s inherant stability producing very low third-harmonic distortion and even lower intermodulation distortion, especially when used in ultralinear mode with appropriate levels of negative feedback.
The KT66 has been seen in many presentations over the years. Many prefer the early ‘fat boy’ baluster balloon devices which were used to great effect in the H2S radar during WW2. Although they have a sublime appearance and exquisite sonic performance, care must be taken as these early incarnations of the device are prone to heater-cathode shorts so must be chosen and tested accordingly – especially as they are very expensive and hard to obtain.
More familiar is the classic ‘coke bottle’ shape, earlier devices having an internal charcoal mist screed applied and more typically a black micanol base, with later devices of the same shape having a clear glass envelope allayed to both black and brown micanol bases which show the copper grid supports and distinctive carburised grey electrode cage to good effect.
I shall briefly discuss the Mullard EL37 here, an interesting device often touted as Mullard’s equivalent to the KT66 but the EL37 is actually a pentode device!!! It does however make an interesting alternative and many guitar amplifier experts are staunch advocates in the use of EL37 even though they are not as robust to overloading, have a shorter life than the KT66 – conversely, they do offer a nominal mutual conductance gm of 11.0 mA/V – almost twice that of the KT66 and as a pentode needs only limited signal to drive them, so a unity gain triode gain stage could be de rigeur - something you could never try with a KT66 which needs a good belt of power for healthy driving!!!!
The KT66 sees duty in so many classic hifi and guitar amplifiers it’s hard to know where to start the list, PYE HF91, HFS25, QUAD II, Williamson, the Marshall JTM45 but here’s a further list of exotica, old and new to lift your spirits:-