Re: My First Toob Coil

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

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

In a message dated 7/8/01 8:27:08 PM Eastern Daylight Time, tesla-at-pupman-dot-com 
writes:

>  the schematics James sent me calls for Mica caps, but I'm sadly lacking 
> those,
>  so I'll be stringing up MMC caps to fill this duty.  I have enough strings 
> to
>  get my to the necessary capacitancies.  Though the voltage ratings will be 
> far
>  (10-15x) overkill, I don't know about the current ratings.  I'm not worried
>  about toasting caps though, I'm more concerned about the tubes.  
>  Now for my questions
>    How critical is the value of the tank cap and grid leakage cap?  

Shad,

MMC's work fine for tube coils.  The grid leak cap is not too critical,
The tank cap is more critical, but again there'a a range that will work
well.

> The grid
>  leakage R determines the "breakrate" of the toob coil.  

The only way that grid leak resistance can contribute to the
breakrate is if you use such a large grid cap and resistor, that
the time constant becomes very long and oscillator "blocking"
occurs, which causes the coil to run intermittently in what
I call the "sputter mode", which is a simple form of staccato
action.  I do not recommend the sputter mode, since it's
kind of uncontrolled in it's behaviour.  Normally,  with normal
sized components, the grid resistance controls the tube bias, and
indirectly the conduction time or angle of the oscillator.

>  I'm assuming you'd want
>  it to run the tube as hard as you can for the least plate redness while
>  charging the tank cap up as to full.

It's not so much charging the tank cap up fully as in a spark gap
coil, rather you want more power in the system for minimum plate
redness.  

>    What's the difference between staccato, pulsed, and raw AC (feeding the
>  tubes)?

Staccato and pulsed are basically the same.  I use an SCR in the
tube cathode to disable operation for a selectable number of AC
60Hz cycles.  The SCR is controlled by a timer circuit.  Coils running
on raw AC will give longer sparks than on DC, and will be more
"efficient".  Raw AC operation is a kind of pulsed operation, since
the coil runs only on the positive 60Hz half cycles.  Raw  AC
operation is highly recommended if you want long sparks.  You
can of course use staccato with Raw AC.  It's difficult to use
staccato with DC because it's harder to turn off a DC powered
oscillator.

>    do I have to ground *everything* around the coil that I don't want to get
>  shocked by?

I never worry about those kinds of things,

John Freau
--
>   
>    I think that's got it for now.  Opinions, comments, and criticism welcome
>                                                                             
>          Shad
>