Hi Dick,
That is a very respectful output for your 120 VAC input to your single
811-A :^) I am actually surprised that I have never experienced arcs
inside my small form. But being acrylic is probably a factor I would say.
You have somewhat beaten me to the subject of RF induction heating. I was
going to bring this up a bit. As you know this heats up all kinds of things
besides metal, especially as the frequency goes up. I actually took a
couple of pictures last night of a favorite old Eimac ad from 1943 of the
304 tube which has to do with RF heating of materials, which is also what I
suspect is a possible factor of my warping acrylic tube. I do not think it
is just resistive losses to heat here. It is a combination of the copper
and the acrylic alone itself heating up mostly.
Link to couple pictures of ad:
https://photos.app.goo.gl/ZV9M9Tza1QPbaftm8
Very interesting about your friend repurposing that piece of equipment :^)
and using rectification.
That is extremely great output, even if just done for short periods.
I am sure this must be a "nice and healthy" transformer inside to achieve
this, which would make sense to me for this kind of piece of industrial
equipment, to have transformer "reserve" for longevity in this environment
use.
For me, on this 304er, my record is 42" for this really small 2"x13.75" of
#28. And realistically, this could only be done for really, really short
periods. Everything extremely hot. The worst was the MOT, as I was also
giving it over voltage of 140V, which heats it up on its own of course,
besides the now drawing excessive current loads also. Decided to not do
this anymore, before I burned up the MOT. It would be better to build a
proper bigger coil and have a proper beefier transformer to match, than
keep doing this, so I have scaled back to what is realistic here.
No, I have not been to Roger's yet. I was planning two years ago and my
coil broke down the night before, tried to repair, and was up all night and
failed. Sun now coming up, tired and shot, decided not going, since so
disappointed, need sleep instead now.
As soon as he holds one again I am planning on going.
This will be my first at any one of these. Be great to be around other
coil-heads.
Chris Reeland
Ladd Illinois USA
Sent from my Samsung Galaxy Tab® S
On Sun, Apr 25, 2021, 4:30 PM <hooverrl@xxxxxxxxxxxxxx> wrote:
> Hello Chris,
>
> I have an old coil that runs directly from the AC output of a Thordarson
> 1500 VAC plate transformer (300 mA). It runs on 120 VAC. The circuit does
> not use the diode and capacitor voltage multiplier that you are running. I
> have a single 811-A triode for the oscillator. The primary has a second
> tickler coil for feedback to the grid. The secondary is 18" of close-wound
> #22 heavy Poly-Thermaleze (formvar) magnet wire wound by hand on 2" ID PVC
> pipe. With no top toroid (just using a 1/4-20 on a through-wall insulator).
> I get single dancing brush discharge up to 6". Originally, I didn't seal
> the ends of the PVC and it arced down the inside of the pipe and burned a
> serious black trail, so I had to make another secondary. For the
> replacement, I sealed both ends closed and have had no further problems
> with arcing inside the PVC pipe.
>
> "304" - I asked about your application as I used to run a LECO carbon
> tester at work. It ran on 120 VAC, but maxed out at 22 amps. It was an 18
> MHz induction furnace. A three turn primary would turn a crucible of iron
> filing red hot in 30 seconds. Co-worker and local HAM Ed Smith (RIP), used
> parts from one of the old furnaces for his Tesla Coil. He just changed the
> primary and added a big secondary. With a high voltage ring (from high
> tension work) he could get 4 to 6 ft continuous brush discharge for
> intermittent use. He swore by the twin 3B28's for his rectification. The
> third tube looked like yours, but was the 304-TL. The circuit was enclosed
> in the original induction furnace steel enclosure with two large cooling
> fans. The running fans were nuisance noisy, but bearable to be around when
> running.
>
> Have you been to Roger Smith's Tesla-Fest in Chillicothe? I gave Roger one
> of my other secondaries with elephant stand primary. We have movies of both
> his big coil and the one I built (which has spaced secondary turns like the
> original coils that Tesla built.)
>
> Remember to use your discharge stick when approaching the coil after use.
>
> Dick Hoover
>
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