Rod Pentode TRF

[Mike Watterson]

Another reason for a potential divider on the G2 is that over a significant range the g2 is a mirror of Anode current. Not a problem where the g2 connects direct to a battery or HT rail, but otherwise strange things can happen, especially on an IF amp. The current variation drawn on G2 means the voltage can change dramatically with load current. On the 1j24b the gain is very low when g2 is below 20V and is at a maximum at about 45 to 55V depending on other voltages. As there is no variable mu / remote cut-off on normal rod pentodes (achieved on regular valves by increasing spacing of the spiral of g1 turns along the length) and amplified AGC to G2 was often used. It's also why the rod pentodes are reasonable mixers when g2 is fed direct from the anode of an oscillator that's biased to about 40V DC.

I'm not sure that the variability on later models is much different to regular valves, which have quite a variation in Ia vs vg for new ones, hence push pull using adjustable bias or large enough cathode resistors (negative feedback). Naturally direct filaments can't use a cathode resistor, though from about 1937 most battery sets used a series resistor from -HT to 0V to bias the output valve. Some 1950s German sets used the fact the LO grid goes negative and filtered this to get a more negative bias for a pair of DL96 in near class B push pull, but the quiescent current on battery push pull sets isn't well matched.

But the Rod Pentodes SEEM less well matched than regular battery pentodes due to the sensitivity of gain to g2 voltage.