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Re: Microwave Xformer

> Date:          Sun, 12 Jan 1997 22:25:00 -0700
> From:          Tesla List <tesla-at-poodle.pupman-dot-com>
> To:            Tesla-list-subscribers-at-poodle.pupman-dot-com
> Subject:       Re: Microwave Xformer
> Reply-to:      tesla-at-pupman-dot-com

> Subscriber: brad-at-greenepa-dot-net Sun Jan 12 22:01:45 1997
> Date: Sat, 11 Jan 1997 23:48:21 -0500
> From: iLLuSioN <brad-at-greenepa-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Re: Microwave Xformer
> 
> >I have a nice clean high voltage Xformer from a microwave. Variac
> >testing at low voltage indicates 2520 V. at line voltage. The microwave
> >was rated at 600W.(I assume that is input to the Magnetron), so you
> >should be able to draw 240ma. with no strain.
> >	
> >I'll be happy to trade it for a nice clean(no tar babys please) neon of
> >any rating  or any other TC stuff of equal value.
> 
> All this talk of microwave transformers and tube coils, etc.. has raised a
> few questions for me. I have some schematics of tube coils, etc, provided by
> a few kind people on this list (thank you again). But I am wondering what
> the best tubes to use are... Also, are microwave oven transformers the best
> fitted transformer to operate a tube coil? They are fairly easy to get ahold of.
> 
> Thanks,
> Brad

Brad,

Microwave oven transformers are attractive because they will do the 
job and they are easy to get.  A better transformer is a real plate 
transformer easily obtained at many hamfests for $10-$100.  Tube TC's 
can be made from about 100 watts up. 

The tubes best suited to work with here are triodes.  Forget tetrodes and 
pentodes.

You could make a small system with an old B&W or better yet color, all vacuum
tube TV set power  transformer with the full plate winding employed (about 700
volts, 350-0-350) and a 6.3 volt filament winding just right for a 
single 809 tube. Such a transformer could actually power a pair of these tubes for 
more performance. You could also use one or a paralleled pair of 
beefier 811A's in this circuit succesfully.  Check a number of TV repair shops
and one of them will surely give you for free a complete tube TV chassis if they
have one taking up space.  I think a secondary wound with #28 wire on a 3 inch
form about 15 inches long would be good for this.  Use just a 1/4 -20 bolt 
on a porcelain bushing as the top terminal.  Use #12 solid PVC 
covered wire as your primary.  Use #18, enamelled or PVC as your grid 
tickler (feedback) coil.  Employ a solenoidal primary arrangement.  
Flat spirals don't work in these tabletop vac tube TC's.  For a 3 
inch secondary diameter, a  6 inch primary diameter would be a 
good match.

Good results really start with paired 811A's at 800-1200 volts on the 
plates.  805's are slightly more powerful, slightly higher voltage 
range say 1000-1500V.  810's are the tube I have the most experience 
with in singles, twins or three in parallel.  These start to work at 
1500 volts and are terrific at 2200-2500V, a good match for a wide 
open microwave oven transformer.  833A's are about the same 
as three 810's for power in one bottle.  One of these would be 
terrific on a large microwave oven transformer run wide open.  
Whatever transformer you get, a variac to vary the plate voltage is a real aid in
tuning and running your coil.

It is normal and safe for any of the above mentioned tubes to run all 
day long with a dull red glow on the plate surface, visible in 
subdued room lighting, as long as there is ample convection air 
available to the tubes. The 833A can actually show a bright cherry 
red to dull orange color with forced cooling! (John Freau !)  All of these tubes 
will  operate safely in the straight up vertical position.  Other mounting positions
are possible in some cases but reference must be made to a tube manual to 
be sure.  It would be a good idea to attempt to get the old RCA 
Transmitting Power Tubes handbook (covers all these) or any older 
year of the Radio Amateurs Handbooks,  especially through the 50's 
and 60's.  Tube specs are given in charts at the back, plus there is 
a wealth of RF and power supply construction tips and theory as well.

Since the output is escaping at the same rate as the input is 
being generated, the system tank cap does not have to store energy for a 
repetitive big bang in a vacuum tube system.  Since it does not 
determine the output energy it can be much smaller than that required 
for a disruptive coil.  Several hundred pF is all that is needed in a 
small system.  Air variable, aluminum finned transmitting caps 
(cheese slicers),   transmitting micas, and fixed or variable vacuum 
caps are the caps of my choice for these systems.

There are some useful vac tube TC build it project articles in the 
popular electronics magazine of the '60's as I recall.  These are 
good for reference, although I have one article that has a long 
candlestick design secondary powered by a pair of 811A's that is a 
real dog, just 2 inch brush discharge from a coil about 3 feet long!  The article 
_was_ a  success though in that I'm sure the author got paid.

If you have further questions, I'm sure there are a few of us here 
now that will gladly hold your hand until you can put it next to the 
events horizon of your new brush discharge.  Touching it will then be up 
to you! : )

regards,
rwstephens