Re: Help w/ Big Tube Coil

From: 	FutureT-at-aol-dot-com[SMTP:FutureT-at-aol-dot-com]
Sent: 	Monday, July 14, 1997 5:54 AM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: Help w/ Big Tube Coil

In a message dated 97-07-13 15:27:24 EDT, you write:
< The Specs:
> Tubes:			CDR 250TH, PlateMax: 4kV, 350mA, Filament: 5Vac, 10.5A
> Filament Transformer:	5Vac, 30A
>Plate Power Supply:	2800Vdc, 680mA
> The Questions:
> 1. Is a feedback coil the best (easiest?) way to pulse the grid of a CW TC?
> 2. What primary/secondary size and arrangement should be best for this
power >level?
> 3. Why do the secondaries of most tube coils appear to break the classic
W:H >rules (i.e. too tall & skinny).
> 4. Can/Should a toroid be used?
> 5. Any additional comments, suggestions?...
>We're going to post start to finish construction details on our web site.
 >Jeff W. Parisse
> Art Director, Digital Design Laboratories
 >www.ddlabs-dot-com  1-800-796-1138


Nice find on the tubes.  I like to make the positioning of the feedback
coil adjustable, by winding it on a form that slides over the lower
part of the primary form, and can easily be slid up or down to 
increase/decrease the grid-coil to primary coupling.

I don't think it makes a whole lot of difference what primary--secondary
arrangement is used.  I've used flattish inverted cone primaries, and 
cylindrical primaries with similar results.   I'm not sure why tall 
secondaries are used, but I know that short, wide ones work just 
as well.  In  one design, I used a secondary 
that was 6" dia x 6" high (but may have been 9" high--didn't check
my notes), and it produced a 35" spark.  I used #28 Formvar ins.
wire.  I posted the complete specs on the coil some time ago.  
Actually at the highest power level, I obtained arc-over from the primary
to the top of the secondary, so voltage breakdown is a possible concern
using short secondaries.

For a more conventional design for your power level, you could use 
a 4" to 6" dia secondary 18" to 9" high, I like to use the flattish
primaries with the wider secondaries, and the cylindrical primaries
with the narrower secondaries...but it's just a personal preference. 
If you use a generally larger pri-sec, you can always increase the 
power at a later date by adding more or larger tubes and transformers.  

There are many controversies concerning tube coil construction--talk
to ten people, you'll get ten different answers.  But these are some
things I've noted in my work.

Small toroids can be used to assist in tuning the coil, but a sharp 
point should be placed on top of the toroid to allow the spark to emit
easily.  Large toroids cause RF losses due to dielectric heating of the 
air near the toroid.  I prefer not to use toroids on tube coils, but 
occasionally I do.  Small toroids without sharp points produce a
thick, raspy, short, noisy discharge, and can cause high voltage
kickback into the vacuum tubes, but you shouldn't have a problem 
in your set-up because of the low plate transformer voltage.

It can take some "playing" with the adjustments to get best results; 
coupling, grid feedback, grid-leak resistance, power input, tuning, etc.
A certain set of adjustments may produce a certain spark length, 
but the tube plate may become much too red,  sometimes a slight 
change in one (or more) of the adjustments can maintain the spark
length, but allow the tube to run cool.

Enjoy the project, and I'll be looking forward to your results.

John Freau