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!!

Stephen, KJ6LH