Re: [TCML] 3-500Z tube coil project

[bturner@xxxxxxxx]

>From what I heard, the grid winding can start out with the same number of
turns as the primary - main thing is enough grid voltage to bring the
triode to cutoff. Approximate the volts/turn on the primary, assume that
for the grid coil, knowing the maximum grid current for the tube will then
tell you what sort of grid resistance range you will need.

My understanding (and how I built my VTTC - using a pair of 805's) is to
use a fixed number of turns on the primary and add or remove tank
capacitance to bring it into resonance. Varying the primary turns can
alter the plate load too much.

Be sure to decouple the plate power transformer from RF - use a mica
transmitter cap across the transformer winding - 0.01uf should be enough
and also have a means to adjust the cathode voltage. The thoriated
tungsten cathode can be damaged if the voltage (thus current) is not
within spec. Too low and electron emission suffers ('sucking' electrons
from the tungsten), too high, and it can physically weaken the cathode.
Use a balanced-current method - cathode transformers normally have a
center-tap to use for the remainder of the cathode circuit.

 - b




> 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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> Tesla@xxxxxxxxxx
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>
>

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