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Re: VTTC ponderings. (fwd)



---------- Forwarded message ----------
Date: Tue, 07 Aug 2007 09:59:40 -0400
From: Shad Henderson <shenderson@xxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: VTTC ponderings. (fwd)

Hi All, Scott,

One reason separate filament transformers are used is that it allows the
user to bring up the filament power without energizing the plate.
Applying plate power before the filament is up to temperature could
result in cathode stripping, greatly reducing the power the tube can
process.  Larger tubes can take a few minutes to heat up, and once
heated, prefer to remain either on or off.  Tubes don't like heat
cycling the filament too much.  Especially large transmitting tubes that
use low-voltage high current (like the GU10A, 7.2V 70-80A).  

Voltage doublers are easy, cheap, and provide just what a VTTC likes,
pulsed power. A second MOT in series would do it, but I'm always leery
of a core shorting to the primary.  Nasty.  Two mots in anti-parallel
provides nice DC, but generally you want either raw AC, or pulsed DC for
a VTTC.  Raw DC, or smoothed DC provides a bushy brush discharge,
whereas raw AC or voltage doubled AC (actually it's pulsed DC) gives
longer, swordlike sparks.  Since most people prefer the longer sparks,
that counts out using DC for power.  

Staccato does exactly what you mention below.  It holds off the firing
for a short while, then lets the tube run until the next zero crossing,
and turns it off.  This lets you run more input power, yet not exceed
the plate dissipation rating.  

Shad H. 



On Mon, 2007-08-06 at 20:54 -0600, Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Mon, 06 Aug 2007 16:41:08 -0400
> From: Scott Bogard <teslas-intern@xxxxxxxxxxx>
> To: tesla@xxxxxxxxxx
> Subject: VTTC ponderings.
> 
> Hey everybody.
>      I am contemplating building a vacuum tube Tesla coil, I think I have a 
> decent handle about how one works (I understand the schematics), but I have 
> a few questions that my research has not addressed (granted, I did not take 
> the time to read all 4000 matches in the archives, but I have read several). 
>   Firstly, why does everybody use a separate transformer for their filament, 
> most MOTs have a filament winding for the magnetron (the number of turns 
> could easily be adjusted if they did not match your tubes voltage specs).  
> Secondly, most schematics call for a voltage doubler, why not just use a 
> second MOT, methinks this would make things easier and simpler (but heavier, 
> but most are strong enough to lift 2 MOTs).  Also, many systems run on half 
> wave output, why not use full wave to get twice the "on" time.  Thirdly, the 
> VTTC is famous for its sword arcs, they are neat, but I still like the more 
> natural looking branched arcs.  The gently flowing RF envelope of a VTTC is 
> not like a disruptive coils, which starts rather abruptly (when the gap 
> fires), could one put a string of sidacs in somewhere to cause the VTTC to 
> switch on at half or 3/4 full power to cause the RF envelope to look more 
> like a disruptive coils, and get more standard branched looking arcs, or is 
> there more to the swords than the RF envelope.  Thanks a lot guys.
> Scott Bogard.
> 
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