Re: Vacume tube TC
David: Connect the 4-500Z grids together for a triode configuration;
that should cure your concerns about the extra grid. Sounds like the
addition has thrown your coil out of resonance. Fool around with the
taps; adjust the distance between the grid coil and the "tickler" plate
coil, that kind of stuff. 3-500Z's are expensive these days; stick with
what you've got.
I am thinking of constructing a copy of a coil a friend and I built for a
high school science fair many years ago. We used one 304TL, but on the
swap net they're available for around 50 bucks a pair. If I did it, I'd
put two in parallel. Filament transformer needed for each tube: 10 V at
12 A, or 5 V at 25 A. I've got a pair in a pushpull KW rf deck I'm
resurrecting: the things'll heat up the room. They're rated at 3000 VDC
but could take another thou, I'm sure, run 'em at half an amp and watch
the plates heat up!!