[Amps] "Tubes 201" - How Vacuum Tubes Really Work

[Peter Chadwick]

Two things make grids emit. One is heat. You try to minimise that by flashing of gold etc, although if you boil that off, it's not there, so  you're left with the basic material. Unfortunately, in oxide coated cathodes, barium and strontium can boil off the cathode, and land on the grid. Then you have a nice low work function emitter.  The other is secondary emission, and for that, a carbon coating is useful. (all this from care and feeding and the RCA tube manual)The first tube that I know of with a gold flashed grid was the 6J4, gg recieving triode of around 1943/44. That has a gold flashed grid because the grid - cathode spacing was pulled down, and gold flashing helped. Often used in Wallman cascode with a triode connected 6AK5, although a lot of people used 1/2 a 6J6 for the gg stage (why only half? - the idea of the cascode is that the low load impedance on the first stage minimises the voltage swing and thus the Miller effect. So a lower impedance with the two halves of the 6J6 in parallel would seem to be indicated) However, the gold flashing wasn't considered necessary for the frame grid pentodes such as the EF183/6EH7 and EF184/6EJ7, and they ahd very close grid cathode spacings, giving a very high gm.

73
Peter G3RZP