[ARC5] FETs and ---odes

[Neil]

> Yes. It may ACT like a pentode, but it still has only three 
> leads....like a tri-----ode...so, when replacing a multigrid-tube 
> with one, which lead goes where?

Very simple.
Gate connects to grid, Drain connects to plate, Source
connects to cathode. Ignore totally any other grids such
as screen and suppressor. Their wiring simply becomes
irrelevant.

You need to provide a circa 12vdc supply decoupled
through say, a 100 ohm resistor for each stage, and each
cathode bias resistor will need changing to about 250
ohms. I used 270 ohms with MPF102's. Be prepared to
tweak the trimmers in the IFT cans to ensure realignment.

As I said, I did this very successfully when replacing
the RF and IF stage 12SK7's back in the 80's. The set
I modified was hardly pristine and had been hacked by
others before I re-hacked it.

It should be possible to replace the 12K8 with a couple
of Fets also, but I never bothered going that far.

73 de Neil ZL1ANM