Tube coil related questions

[cwillis-at-guilford.edu (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hello fellow tube coilers,

I have an 833A coil that has given great performance with one small
problem:  the microwave-oven diode that I use in my "level-shifted (or
doubler)" supply gets toasted now and then.  I believe John Freau has
suggested, and probably correctly, that this is due to large RF kickback.
Anyway, I wanted to eliminate this issue, and was thinking about other
rectifier options. By getting all the weak spots out of my coil (all the
silicon) I will be able to do demos and carry it around without having to
be surprised by killed parts.

Perhaps a magnetron with the magnets removed could serve as a nice
high-voltage rectifier tube for the doubler.  The only disadvantage would
be the requirement for filament power- but this is supplied in MOT's from a
HV-insulated winding.  Any thoughts about this approach before I go ahead
and try it? 

One especially useful piece of info for me now would be the difference in
resistance between the "maggie" and a oven-type rectifier.  I think they
might be similar, but I have nothing to conveniently measure the rectifier
resistance.  Has anyone done this?    

And finally, a more general question.  Does someone have a good idea of how
to estimate the inductance of flat spiral coils that are stacked and linked
in series?  Of course it's not just going to be the sum of the individual
inductances but will include the mutual inductance as well, and that's the
part that I really don't have a practical clue about.  Suppose two coils
have the same dimensions, same # of turns, are coaxial and are spaced a
distance X.  How much will the mutual inductance add on to the sum of the
individual inductances?

Thanks for any help!

Carl