(01-09-2021, 08:08 AM)BusyBee Wrote: None of the rod pentodes are variable mu though. This is one reason why I am considering other than a pure rod pentode superhet set
I'm actually building an entirely rod-pentode superhet, and trying to accomplish AVC by varying screen and suppressor voltage. Both work, although reducing screen voltage reduces the grid base, so input overloads potentially occur more easily; and taking suppressor negative, while achieving the aim of sending electrons to the screen rather than the anode and thus reducing gain to anode, also reduces anode resistance thus compromising selectivity in the IFT.
However, all engineering is a compromise. I've got 3 valves 1j24b in the AVC module alone (it's amplified, delayed AVC).
One unforeseen event (which should have been foreseen!) is that taking screen and suppressor downward in voltage, from the same source, caused transitron action with screen current shooting up as all the cathode current went to screen; and the common source impedance then kept the suppressor negative. At least I know I could potentially make an oscillator... or even a single-valve bistable, given a negative supply rail...
I'm hoping to close the loop today!
