[TCML] 3-500Z tube coil project

[Jason Johnson <jasonmsusolar@xxxxxxxxx>]

So I have recently acquired a 3-500Z, supposed to be a working tube pulled
from service.

I have been collecting parts to build a VTTC with the tube, but have not
fully decided on all of the parameters and was hoping to gain some insight
from others more familiar with tube coils (like any of the folks with great
tube coil websites and/or youtube vids...).

So far I'm planning:
- 3.5" x 18" secondary, wound with 25 gauge magnet wire;
- 2.75" x 6" toroid;
- 6.625" outside diameter primary coil form;
- 22 turn primary, wound with 10 gauge magnet wire and tapped every 1/3 turn
or so starting at turn 16;
- 24 turn grid coil (or at least starting around there and tweaking as
necessary...);
- 1.42 nF 20 kV doorknob (high quality N4700 type) in parallel with a 7-500
pF 15 kV vacuum variable for the tank cap;
- 2200 VAC MOT level shifted with 2.4 uF;
- Standard filament transformer, fans, bypass caps, etc.

I haven't decided on grid leak components yet, my thoughts on that are kind
of as follows...

According to the spec sheets, the 3-500Z is very similar in overall ratings
to the popular 833A tubes with one exception - the amplification factor is
130 instead of 35...making this a fairly high mu power triode in comparison.
So I'm thinking that the grid leak capacitor should start out at about 1 nF
(on par with some 833 coils) and the grid leak resistor should be about 3k
(lower than what is typical on the 833 coils).

Is my thinking correct here, or is this off base for some reason? (I'm
planning on using a rheostat in the grid leak circuit as well, since I will
likely not get the best value on the first try anyway...). Also, I'm
wondering about the tank cap size. I've got more capacitors and could try a
range of values larger or smaller than this, but I was thinking 1.7 nF or
would be a good place to start with this particular tube.

Finally, how far do I want to be able to adjust the coupling? If I close
wind the 10 gauge wire, I don't think that I can achieve a k value of more
than 0.22 or so. Does this matter? Coupling in a VTTC can often be higher
than this from what I have gathered (maybe 0.31 or so). Of course it is
simple enough to space wind a bit and increase k, but how far is necessary
I'm not sure...

The main reason that I ask the above questions is the fact that although I
have seen the 3-500Z listed as a potentially good tube for a VTTC, I have
not come across any actual examples of this tube being used to drive a Tesla
coil.

Thoughts, comments?

- Jason
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