I did some *real* vttc coiling...

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Brett Miller <brmtesla2@xxxxxxxxx>

Fellow VTTC coilers,

Well, since I have been waiting for certain parts to
arrive in the mail for the 833A project, and also,
waiting for the hamfest, I have been seriously anxious
for some actual vttc experience.  After coiling for
nearly a decade, I realize that it can be foolish to
get eager and force things to run with substandard
components and ones with incorrect specs.  However,
yesterday I started thinking about all the old triodes
I have collected in the closet, and how many little
mica caps I have sitting around.  I also have a small
Zebra TAN-100TT MOT which still has its shunts.

So I decided to whip up something in a few hours from
my junk box and see if I could at least get it to
oscillate.  I based this attempt on a single 3c24 (I
have 4 of them in boxes) triode.  The datasheet looked
promising...plate voltage = 2000 volts.  An hour or so
later I had a 3.5" diameter primary/feedback over a
1.5" secondary (only about 2 inches long) which I had
wound a few years ago to demonstrate a crystal radio
to someone.

Well, I got an e-field that could light up a neon lamp
from a few inches, as well as a small flourescent bug
light, with very little sparks to speak of.  Obviously
I needed heavier jumpers, especially for the filament
and tank circuit...and it was *way* out of tune, but I
familiarized myself with the Class C Armstrong and
gained some confidence for my upcoming 833A coil.  I
eventually cooked the plate on one 3c24.  Another look
at the datasheet showed a low current rating.  Raw AC
and no staccato - bad idea.

It was a lot more fun for me than it looks:

http://www.hot-streamer.com/brett/vttc/junk_box_vttc.jpg

Later I substituted the primary and secondary for the
ones which were built for my 833A system.  About 1
inch sparks...still out of tune and I had not
installed primary taps yet.  Then I substituted the
3c24 for my 304tl.  Plate ran much cooler, 3 inch
sparks but still way out of tune.  With the 304tl the
skinny jumpers to the filament transformer started
smoking.  I won't be using anything smaller than awg10
or 12 in my 833A coil, not even temporarily.  One
thing I learned here is that you can't mess around and
jumper things together with tube coils and expect even
mediocre results.  I was able to experience the
consequences of higher RMS currents first hand in
these experiments.

I had a taste of VTTC action, I have now disassembled
that lashup and am focusing on nearing the completion
of my 833A coil.  I have a 28 turn primary with a tap
(added last night) on turn 20.  I will probably add
several more, possibly placing a tap on every turn
from around 18 to 28, something like what Cameron
Prince has on his primary.  John Freau has imphasized
how tuning, impedance matching, and overall efficiency
are hard to predict and model on a vttc.  I now have
personal experience with this dilema working with my
own coils.  This looks to be potential for a lot of
fun in the future.

Today I received an RCA 833A socket for the filament
pins.  You can look at a photo of it here:

http://www.hot-streamer.com/brett/vttc/RCA_833_socket.jpg

It looks a lot like Cameron's, but I believe his is a
bit more convenient to mount.  Nevertheless I have a
few ideas for this one.

-Brett





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