Re: vac tube TC secondary design question

[FutureT-at-aol-dot-com (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

In a message dated 6/6/00 4:53:57 PM Pacific Daylight Time, tesla-at-pupman-dot-com 
writes:

> Original Poster: "Simon Winder" <swinder-at-microsoft-dot-com> 
>  
>  Hi coilers,
>  
>  I want to design a good secondary for my tube coil.
>  
>  My primary is 9" diameter and 5" high. The LC of the primary circuit
>  resonates at 650kHz. I cannot really change this much - in particular I 
cant
>  make it much lower.

Simon,

I used a primary of that size one time.  It was 21 turns of #12 pvc
insulated wire on a 9" form.  I used a 4.2" by 23" piece of PVC pipe
for the secondary form wound with 1500 turns of #28 magnet wire.
>  
>  I believe the problem is to design a secondary that resonates at this
>  frequency, has the highest number of turns and has the largest diameter
>  possible so that the coupling is high. I was thinking of between 5" and 6"
>  diameter, but this is difficult without building numerous prototypes.

It won't make much difference what the coupling is.  I've varied the
coupling from .2 to .3, and didn't really see much difference.  In a tube
coil, it is possible that there may be some benefit from using thicker
wire (as you are planning), due to the need for a higher Q.
>  
>  Has anyone wound a secondary that resonates at around 650kHz? What were the
>  dimensions?

I wound one of about that frequency, but it was much smaller because
it used thin wire (#28).  You can use a coil program such as John
Couture's free program to calc a suitable secondary size for that freq.
>  
>  Is there a good way of estimating the resonant frequency of the secondary
>  without building it?

Use a TC computer program is the easiest way.

>  I tried using the formula
>  f (MHz)= (1175 * (h/d)^0.2)/(d*N)
>  but it always overestimates the frequency when I build and test coils.
>  
>  Do you think it is more important to have a higher number of turns, or a
>  higher coupling factor for a tube coil?

The coupling should be rather tight for a tube coil, and helical
primaries such as you have will give a rather tight coupling.  If you
don't use enough secondary turns, you won't be able to use enough
primary turns, and the tank Z may be too low, so the tubes may
get too hot.  Although those graphite plate tubes can take a lot
of abuse.  One of these days I'll get around to building my 833C
based tube coil.

  My old 833A based tube coil (36" sparks) is at:

     http://members.aol-dot-com/FutureT/TT-42index.html

Have fun, and safety first,
John Freau

>  
>  I was thinking of using 16" length of #18 on 5" diameter, approx 380 turns.
>  Does this seem about right?
>  
>  Simon
>  
>  BTW: My tube coil now uses a pair of 572Bs, a 4kv supply, one huge 1nF 15KV
>  doorknob primary C, and a 16 turn copper tube primary L.