Re: [TCML] valve coil problems

[Carl Noggle <cn@xxxxx>]

Hi, Carlos,

The coil I did was in 1956 when I was in high school, and it was very 
small.  I can't remember the tube type, it was an 8xx.  I got a huge 1 
1/2 inch corona out of it.  I could get a similar arc one third of the 
way up the secondary, but nothing at two thirds.  I guess we gotta start 
somewhere.  My next coil was with NSTs and got a three foot arc.

I would hope you could make a driver with a half-bridge of FETs.  Do you 
have characteristic curves for the control grid?  The driver will have 
to take the grid several hundred volts negative for cutoff, and positive 
enough to saturate the plate current.  This will require lots of current 
flow from the grid, but from the sketchy data I could find on the net, 
the grid dissipation is only 200 watts, so it should not be too 
difficult to build.

Is there a full data sheet available?

---Carl


> Hi Carl,
> Thanks, I will try it out.
>
> I am definately interested to try the driver idea you mentioned though.
> Can you suggest a source of info to take some of the R&D out of it?
>
> If I remember correctly you did a similar valve coil recently...
> Do you think it could be scaled up, given that I would need about 500
> watts of RF power to drive the grid into Class C.
>
> Thanks, Carlos
>
> On Tue, 01 Nov 2011 12:58:37 -0700, Carl Noggle<cn@xxxxx>  wrote:
> > Hi, Carlos,
> >
> > If you have a scope, you could shock-excite the secondary and measure
> > its resonant frequency.  Take it away form the primary, then put a
> > one-turn coil around the bottom (bottom of the secondary is grounded)
> > and hook the one-turn coil up to the scope.  then flick a 9V battery
> > across the leads to the one-turn coil.  With persistence you should be
> > able to get a damped oscillation and measure the frequency.
> >
> > As a fix, try resonating both the primary and the grid coil at the
> > operating frequency.  Or you could make a series-resonant circuit to put
> > across the grid coil--the L should be 2 or 3 times that of the grid
> > coil, with capacitor to resonate at the third harmonic
> >
> > Or for that matter, you could try running it at the higher frequency.
> >
> > I still suggest driving the grid with a low-impedance square wave.  Then
> > you can run in class C.  Also, make sure that your screen grid supply is
> > well-bypassed.
> >
> > Good luck--sounds like a cool coil.
> >
> > ---Carl
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