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Re: [TCML] A New Kind of Valve Coil



Phillip,
as you know I have been following your progress with your coil as well as Steve's work. I solved the grid coil issue a few years ago and there should be some posting in the archives. Instead of using the secondary as the major feedback source, I put a flat spiral pancake coil under the primary with the inner diameter about the size of the primary. It is also covered by a circular piece of phenolic so no one can see it. I have always used a high wattage adjustable grid leak resistor set for the sweet spot. This system is sound, never any flash over. I used 14 awg insulated wire because it was easier to make the pancake.

Now, there is no reason why you can't pick up the current from a primary lead with a current transformer, but then you need to get that to control a high voltage with proper timing for the grid. I am not really sure if this solves problems or adds to them. But, I guess you will find out. However, if you can, give the pancake grid coil a try.

John W. Gudenas, Ph.D.
Professor Emeritus of Computer Science



On Jun 23, 2011, at 8:25 AM, Phillip Slawinski wrote:

Scott,

1. If the good performance obtained by the elevated position of the feedback coil was just because it reduced the grid drive voltage, could you get the same effect with a low-mounted feedback coil shunted with a variable resistor (plus your series inductor) so you could adjust grid voltage "on the fly"?

Perhaps you could obtain similar results with the grid coil lower, I'm
not entirely sure about that.  I have had a variable grid leak
resistor for quite some time now which allows me to control the drive
to a degree.  I recall trying the grid coil lower in the past on
several occasions.  No adjustment of the grid leak R yielded the same
results as running the coil higher up on the form.  That said, I never
tried removing turns, so maybe the same performance can be had lower
on the form with a lower turn grid coil.  I can't say for sure without
trying it, something which I'm hesitant to do, since I'd need to
destroy my current grid coil to do this.  Also it's rather difficult
to make good measurements of the grid.  Little changes will cause the
scope to trigger at different times, and if that's not enough the
presence of the scope probe messes up the performance of the grid.  I
really wish I could say definitively why the way I have the grid
configured now works so well.

2. In a VTTC with a conventional feedback coil, do you think the most significant magnetic coupling is between the feedback coil and the primary, or between the feedback coil and the secondary?

I know for a fact that the coupling to the secondary is not
insignificant.  I measured it at ~.2K.  Primary to secondary coupling
is similar, about .25K.  I never measured coupling between primary and
grid coil, that's something I should do.  The whole grid feedback coil
system is pretty unstable really, and it doesn't take much of a change
to throw it out of whack.  Hence my reason for abandoning it as a
control method.

3. Do you have any idea of the value of the inductor that you placed in the grid circuit?

I sure do.  It's right around three microhenries.

4. Is this inductor in close proximity to the primary coil (and possibly affected by the primary's magnetic field), or is it located remotely, closer to the tube itself?

The inductor hangs off the grid terminal.  It's about four or five
turns on a ferrite slug.  I did try an air cored model, which quite a
few more turns, but that was too delicate (no supporting structure).

-Phillip Slawinski
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