A new tube for use in superheterodyne frequency conversion systems

The major disadvantage of existing methods of frequency mixing is found in high-frequency operation with comparatively low intermediate frequencies, where serious coupling exists between oscillator and signal circuits in spite of, electrostatic screening. Suppressor modulation of a radio-frequency pentode largely overcomes many of the defects but the oscillator voltage required is large and the low plate resistance limits the gain. For these reasons, a new tube has been developed (designated the 6L7) wherein the above disadvantages are largely overcome. The new tube contains five grids: the first grid is a remote cutoff signal grid, the second and fourth are screens, the third is used as the modulator grid controlled by a separate oscillator tube, and the fifth grid is a suppressor. The ideal characteristics of such a tube are derived. The actual characteristics of the tube developed are shown and a brief discussion of the results obtained is given. A discussion of the flow of electron current to a negative grid due to an unusual transit-time effect at high frequencies is given and it is shown that operation with sufficient grid bias reduces the phenomenon. The characteristics of this tube also make it particularly suitable for use as a radio-frequency amplifier in receivers where the available automatic volume control or detector voltage is low; in this case the automatic volume control voltage is applied to both No. 1 and No. 3 grids.