Re: [TCML] vttc primary capacitance

[Steve Ward <steve.ward@xxxxxxxxx>]

Earl,

It appears you are operating at a pretty low frequency compared to other
VTTCs, and this means you can afford to use a larger tank capacitance and
still maintain a "high" tank impedance.  I dont see why 4nF would be too
much, typically one just watches for plate redness or other signs that they
might be stressing the tube too far, and often the solution is to increase
tank impedance (smaller tank cap, more primary turns)

There are some things that could be improved in your design, but mainly the
primary to secondary coupling looks very sub-optimal, and this alone can
cause excessive primary currents and additional tube heating.  Basically, in
a CW machine, tight coupling is desirable because there is no real need to
store energy in the primary, you want to transfer it to the secondary as
fast as possible.  In a CW situation, this means that the ratio of primary
current to secondary current will be smaller with higher coupling (this is
good!).  If the coupling was really loose, you'd run a bunch of primary
current and only get a little bit of secondary excitation.  Again, this has
to do with the physics of energy transfer, and because its a CW system,
there is no benefit to energy storage like there is in a SGTC, where low
coupling is acceptable because it all eventually can get transferred to the
secondary.

I'd be interested in hearing any counter-examples, but in general i think
most experimenters find that the highest coupling you can manage without
flashover, typically works the best.  Of course, there could be subtle
tuning issues that might suggest a slightly lower coupling worked better for
someone, but id have to guess that if they were to finely re-tune the tank,
that higher k would almost always be better, probably until you get to
around k=0.5.

Steve

On Mon, Jul 26, 2010 at 5:11 PM, earl rhodes <earl_1975@xxxxxxxxxxxxx>wrote:

>
> hi guys
> im building a large vttc using a gu5b tube rated a minumum of 3.5kw
> i was wondering what is the max capacitance somebody could use for the tank
> circuit
> at the moment i am using  4nf or there abouts anyway thing is i cant find
> any other examples using this much capacitor
> i dont want to wreck my tube there are videos of my progress on my youtube
> channel
> thanks
> earl
> aka teslaspigeon
> http://www.youtube.com/user/teslaspigeon
>
>
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