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A High Sensitivity Power Pentode Using the 'Shadow Grid' Technique

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Using an old idea, the Shadow Grid technique, and applying it to a new tube design, an audio power pentode, a tube with very desirable characteristics was obtained. Following the introduction of the 6FG5 Shadow Grid pentode, a logical extension of the low screen current advantages of this construction ws to incorporate it in a power pentode for audio applications. Because of the unique field configuration it is possible to obtain substantial plate current and power output with a high turns-per-inch control grid, leading to extremely high values of power sensitivity. With suitable shielding to reduce interelectrode capacitancees, this same design may also be adapted to perform vertical output and video amplifier functions in television applications. Characteristics and construction of an experimental pentode are given, with typical output and distortion figures. Featuring very low audio input drive voltages, this new design may in many cases permit a reduction in the number of amplification stages required.

JAES Volume 9 Issue 2 pp. 152-156; April 1961
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Scott Dorsey
Scott Dorsey

Comment posted June 16, 2020 @ 17:23:34 UTC (Comment permalink)

Why was this published in the JAES rather than in a general electronics journal?  This is a description by some GE engineers of an interesting manufacturing technique for vacuum tubes.  Because the electron beam from the cathode is physically shaped by the grids being made of physical wires rather than a uniform charge, problems occur when tubes have multiple grids whose wire spacing differs considerably, and this paper describes a solution for that to make a more linear small power pentode.

I am pretty sure that the tube described in this article became a production item as the General Electric 6GU5.  It was second-sourced by RCA but the RCA datasheet is very cursory.  Because the shadow grid isolates the control and screen grids  (by making the electron beam between them more uniform) a very high gain control grid can be used (high gain because the spacing between wires is very tight) and consequently the voltage gain of this tube is very high for a small beam power tube.

I can only think that the GE engineers placed it in the JAES hoping to get audio engineers interested in a new upcoming product.  Marketing placement of journal articles has been around since the Royal Academy opened up.

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