Tube coil with 833A

As some of you may remember, I was considering
buying the 833A triode. Now I have a "working" construction
with it giving 4" sparks, running with a 500W mot, 1.6 kV plate voltage.

I would like to know how long it normally takes until the plate gets
its bright orange color. I have a large sharp-edged hot spot in the center
of the both 
sides of the plate only five seconds after power-up. Is it normal?
I've also tried to use the thing as an induction-heater - no success.

Some data: negative grid bias -26v, filament transformer not center-tapped,
tuned-grid, feedback from anode to grid, plate coil 20 turns 1.4 mm dia.
copper wire, space wound, 4 mm spacing, grid coil 3.5 turns 1 mm dia.
copper wire wound above the plate coil, all wound onto 10 cm dia. plastic
grid tuning cap 1.5 nf (tested value) grid parasitics suppressor resistor
90 ohms.
Secondary: 700 turns of 0.4 mm copper wire on a 4 cm dia. plastic tube.
No grounding needed, discharge terminal... a sharp screw on top of a
bullet-shaped al-foil construction.

I would like to know about calculations for this type of coil.
formulas, voltages, experiences... all are welcome. Especially 
frequency calculations for the coils with known dimensions and
coil-cap oscillators.