[TCML] A New Kind of Valve Coil
pslawinski at gmail.com
Thu Jun 23 07:25:17 MDT 2011
> 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
> 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).
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