Re: Tube coil questions (833A's etc)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

Robert,

833A's and other tubes can sometimes be found at hamfests
for $20 to $40 or so.  But sometimes you have to check many
hamfests for a year or more to find them.  Also, it's hard to be
sure the tubes are good.  Be sure to get the seller's name and
telephone number so you can get your $$$$ back if the tube is
a dud. 

The theory of operation of a tube coil is somewhat different
than a spark gap TC.  The tube does not simply replace the
spark gap. 
One coil is the primary, which is analogous to the primary of
a spark gap coil.  The other coil is the grid coil which picks up
a pulse from the primary, and sends it to the grid to keep the
oscillations going.  A tube coil is really a Power Oscillator 
coupled to a resonator.  An Oscillator is really an Amplifier 
with feedback.  An amplifier is a circuit which uses a small
amount of energy to control a large amount of energy.
We run the whole thing in Class C however, so that it
behaves more like a switch (for efficiency) than an linear amplifier. 

Tube coils are much less "efficient" than spark gap coils.  A spark
gap coil can give 42" sparks using 600 watts.  A tube coils needs
6000 watts to give 42" sparks.  I often run my coils in a pulsed or
"staccato" mode, to reduce the input power and give the sparks
an interesting look and sound. 

Other tubes which work well are;  810, 805, 813, 304TL, 4-250A,
HF-300, 4-1000A, 3-500Z, etc.

I have some schematics and photos at my website at:

  http://hometown.aol-dot-com/futuret/page3.html

Cheers,
John


> I've been hearing a lot of people talk about VTTC's lately and I am 
>  inerested in building one to get away from the conventional spark gap 
>  tesla coil (i've built a 4.5" one and am working on a 1.5" one), but i 
>  have a few questions. first, where can i get one or two 833A's and how 
>  much do they cost? i've seen them run for almost $100. is that really 
>  how much they cost or can i get them any cheaper? second, what is the 
>  purpose of having two coils that are coupled to the secondary? can 
>  anyone point me to some resources about these? thanks a lot,
>  Robert