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Re: Looking for plans/materials for a tube type coil and a question.
Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <cwillis-at-guilford.edu>
I have a schematic of my current 833-a tube coil at
This coil is much inspired from John Freau's 24" spark coil he described in
an earlier post, using a level-shifted MOT supply. I also describe a
non-level shifted 811-a coil on another part of the site. As I had to
learn the hard way, a microwave oven diode is not the best rectifier for
these tube coils- they cannot handle the momentarily very high currents. I
now use a string of 1 kPIV diodes.
About getting cheap parts: check E-bay every now and then for "diathermy"
machines. These usually have some tubes that handle 2500 V and 150 W
dissipation on the plates easily, plus a plethora of other RF goodies
including a plate transformer that is much better built than an MOT,
chokes, capacitors, insulators, tube sockets, etc., and I have gotten them
for $20 on e-bay. A resource that could be exploited, but frequently the
sellers of these machines don't list them where the people who want their
guts will find them. Probably well worth your money.
Sack old microwave ovens for their capacitors, if nothing else (but I
usually take the other goodies as well.)
There is a great used parts dealer, Allen Bond, in the Atlanta area. His
company's website is www.mgs4u-dot-com and his personal part of the page lists
many tubes, oil capacitors, vacuum caps and other stuff. I just bought six
big oil capacitors from him at a few bucks each, and they are already in my
new level-shifted supply. He offers some decent deals on big tubes (and
what I consider a real good deal on the 4PR1000A, a pulse-rated tetrode
that I think can handle plate voltages of 10 kV.) Hamfests get you the
best deals, but I hate having to wait half a year for the loot I want NOW.
About cheap tubes in general: the 833-A is one of the very best deals in
tubes, even new. Check out www.rfparts-dot-com.
If you cannot get out at all, McMaster-Carr will sell you pipe and magnet
wire (search www.mcmaster-dot-com for these items). I was interested in their
clear PVC until I saw the price. PVC is a trustworthy plastic for winding
tube coils (and all other coils). Probably the best is the thin-wall white
drainpipe that is sold in 4" diameter in my hardware store. It's much
cheaper than Sch. 40 and will have less losses due to being thinner.
I really encourage the use of an MMC in tube coils. There are some nice
mica transmitting caps out there (these are traditional favorites) but they
are hard to come by it seems. Take Terry Fritz's recommendations regarding
what capacitor types to buy. I got mine from Digi-Key. I'm a very happy
Hope some of this info helps.