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Re: [TCML] valve coil problems



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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