GEC KT66
A beam tetrode with a 30-watt anode dissipation ceiling and an 8,000-hour service life. It powered a generation of British amplifiers.
Overview
The KT66 is a beam tetrode with an absolute maximum anode dissipation rating of 30W. It was built for the output stage of an audio-frequency amplifier or as a series valve in a stabilized power supply. It has a 6.3V indirectly heated cathode drawing approximately 1.3A, and it sits in an International Octal (B8-0) base with a dome-top tubular bulb. That bulb is not small: maximum overall length is 135 mm, maximum seated length 121 mm, and maximum diameter 53 mm. This is a full-size octal power valve, not a miniature and not a compactron.
GEC documentation describes the KT66 as a commercial version of the CV1075, the CV number being the military/industrial designation. The design maximum anode voltage is 500V, with an absolute maximum of 550V. The same figures apply to the screen grid. Control grid voltage maxes at 200V negative, cathode current at 200 mA, and screen dissipation at 25W design / 30W absolute. The combined anode-plus-screen dissipation is rated at 25W design maximum and 30W absolute. GEC puts an asterisk on the triode or ultra-linear figure, meaning the 30W absolute ceiling applies specifically under those operating conditions.
Capacitances, measured on a cold unscreened valve, are 14.5 pF for grid 1 to all other elements, 10.0 pF for anode to all others, and 1.1 pF from anode to grid 1. Those are small numbers. Low inter-electrode capacitance is one reason the KT66 behaves predictably at the high end of the audio band and why it was considered a stable choice for push-pull output stages.
The valve's reputation rests on its flexibility. GEC's own circuit supplement from June 1956 lays out three operating modes - triode, pentode, and ultra-linear - and gives comparison data for all three. In ultra-linear operation, the screen grid of each KT66 returns to a tapping point on the output transformer primary, with those taps including 20% of the total turns of each half primary counting from the centre. The screen voltage is not held constant as in pentode mode, and it does not track the instantaneous anode voltage as in triode mode. It sits somewhere between the two. According to GEC's own documentation, the high power output of pentode operation remains available while distortion stays closer to triode levels. GEC noted at the time that considerable publicity had been given to this arrangement in both the British and American press, and that the claims made were generally substantiated.
The operating tables give the practical side. In ultra-linear Class AB1 push-pull with 40% taps and fixed bias, a pair of KT66s will deliver 50W into an 8kΩ anode-to-anode load, with total distortion of 3% and intermodulation distortion of 15% (measured using two input signals at 50 and 6000 c/s at a 4:1 amplitude ratio). Quiescent anode-plus-screen current is 2×35 mA, rising to 2×80 mA at maximum signal. The required grid-to-grid input for full output is 127V peak, and output impedance is 10kΩ. GEC's circuit supplement specifies a negative bias range of ±25% of the nominal value for each valve. These are fixed-bias designs, so the bias supply is not optional.
Drop the anode voltage and the numbers change predictably. At 350V DC with 25+25 mA quiescent current, a pair yields 20W. At 425V with 30+30 mA quiescent, it yields 30W. The anode-to-anode load stays at 8kΩ in both cases, and GEC notes that no circuit modifications are needed because the bias supply tracks the anode voltage. That scalability is part of why the KT66 turned up in so many different amplifier designs. It could be run conservatively for long life or pushed harder for more output, and the same output transformer ratio worked across a useful range of supply voltages.
The 50W ultra-linear design in GEC's supplement shows what the valve could do when everything around it was done properly. The circuit used a pair of B65 double triodes ahead of the KT66s - one as a self-balancing phase inverter, the other as a push-pull voltage amplifier. Full output arrived with about 150 mV rms input. Independent cathode bias resistors of 560Ω were specified, and GEC's footnote is emphatic: it is essential to use two separate cathode bias resistors, not a shared one. Small capacitors of 2000 pF across part of the output transformer primary addressed stability, as did screen grid and control grid resistors. The output transformer was a Partridge unit, reference C1732, with two equal secondaries for 4Ω or 15Ω output, leakage inductance of 15 mH, primary inductance of 80H, and a ratio of 22:1. GEC was clear that an output transformer of high quality was required to obtain the full advantages of the ultra-linear circuit.
The anode supply in that 50W design used an inductance filter with 80 µF of effective capacitance, split across two series-connected electrolytics to handle 900V working voltage. GEC's reasoning is plain: the large current variation with input signal demanded that capacitance. A lower value would have needed an additional filter section to tame ripple, and it still would not have supplied the peak current a transient demanded. With the specified transformer, output fell 3 dB at 18-20 kc/s into a 15Ω load.
Protection against bias failure was handled with a relay circuit. A triode - potentially half a B65 substituted for the L63 in the first stage - sat in series with a relay across the 500V supply, its grid tied to the bias supply. Under normal conditions the relay short-circuited a resistor in the KT66 cathode circuit. If bias failed, the relay energised, the contacts opened, and the cathode resistor let the amplifier keep running at roughly half maximum output.
Life expectancy was documented with unusual specificity. At absolute maximum ratings, the average life is at least 8,000 hours. Reduce the absolute rating to 21W anode-plus-screen dissipation and that figure rises to at least 10,000 hours. The conditions are strict: GEC specifies a static environment, no more than 12 power cycles in any 24-hour period, and adherence to British Standard Code of Practice CP1005. End of life is defined not simply as failure but as the point where characteristics drift past specified limits.
The screen current figures differ across the three operating modes. In pentode operation at 400V anode and screen, screen current reaches 12 mA per valve at maximum output. Sustained output at that level would exceed the screen dissipation rating, though GEC notes this is acceptable for normal speech and music. Ultra-linear operation cuts that to 7 mA at maximum output, and triode operation drops it further to about 4 mA per valve. So ultra-linear also reduces the stress on the screen grid during demanding passages, on top of lowering distortion.
At full output in the reference design, 475V rms appears across the output transformer primary. The anodes swing 400±340V, meaning they travel between 60V and 740V. The screen grids share the same DC potential but see an AC voltage one-fifth of the anode swing, operating between 335V and 465V. These are not gentle conditions, which is why the KT66's 30W dissipation rating was treated as a real ceiling.
The KT66 is a British beam tetrode that arrived when ultra-linear operation was being actively promoted in the technical press. AudioXpress (January 2001, p.87) noted this in its review of the GEC audio tube data compilation covering the KT66. The valve proved adaptable enough to serve in triode, pentode, and ultra-linear designs alike. It is not a rare valve. The documentation GEC produced for it - the data sheets, the June 1956 circuit supplement, the detailed operating conditions - is unusually thorough for a device of its era.
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