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The situation is demonstrated graphically in Fig 2 where it can be seen that over a narrow band of operation where screen load impedance is about 18.5 per cent of plate load impedance, the new arrangement provides the high power output of tetrodes with low internal impedance such as is normally obtained from triodes, while distortion figures are equal or better than the extremes of operation. We have achieved a new tube type without designing a new tube. This tube is neither triode nor tetrode, but its improved linearity over either of those types justifies the designation "ultra linear."
The Complete Amplifier Circuit
In applying the ultra-linear output arrangement to complete amplifier circuits, it was found that the simple version of (B), Fig. 1 could be used to advantage. By feeding DC to the screens through a properly placed tap on the primary of the output transformer, the operating conditions are preserved, and the close coupling between screen and plate is advantageous when feedback is carried around the stage. The disadvantage of this simpler arrangement is that screen and plate must operate at the same DC potential. In the particular arrangement used the screen and plate are operated at the same
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potential (350 volts plate to cathode) without exceeding dissipation requirements, either quiescent or at maximum output. This new output coupling arrangement reduces screen dissipation at high levels and is a safe mode of operation with respect to tube life.
A circuit arrangement has been designed to take full advantage of the ultra linear output stage. This circuit, Fig. 3, takes into account the necessity for complete stability under feedback conditions so as to eliminate tendencies toward transient instability under any type of load, including the varying impedance of loudspeaker systems. This complete circuit offers linearity of operation of a very high order. It is based around a special output transformer, the Acrosound TO-300, which is 6600 ohms primary impedance and has taps at the optimum point indicated in Fig. 2. A special seven-section symmetrical winding arrangement placed on a substantial grain-oriented lamination of unique shape permits a ratio of primary inductance to leakage reactance in excess of 15,000 to 1. The response of the transformer alone is within ± 1 db from 10 to 100,000 cps with extremely low phase shift and no resonances within this band.
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