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Re: My First Toob Coil
Original poster: "sundog by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sundog-at-timeship-dot-net>
Hi Carl, All!
Comments interseprsed and stuff snipped
----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
> Tube coils are very entertaining, and I'm sure you'll enjoy the experience
> with those 833s.
>
> >bottom-most turns I'll leave tappable so I can get the adjustment I want.
> The 5 nF capacitor sounds like it will work. I've used L / C ratios
> (henries per farad) of 50,000 up to 100,000 in tube coils.
>
> Changing the L / C ratio in such an oscillator changes the primary /
> secondary coupling, the circuit Q, and the impedance
>
> that the tube sees, so it's hard to know how to design the resonant
> circuit. Playing around a lot seems the only sure way
>
> to get the best results.
>
Heh :) Sounds par for the course. You have to love the "adjust 1 thing and
you change 200 others" properties of TC's (and everything else in life, come
to think of it!) I'm trying to keep it within the values I've seen on the
'net for other ppl's toob coils.
> >plate tranny - MOT with the voltage doubler in place - 2 MOT caps
in
> >parallel
> Here's something I learned from experience: don't use the oven
> rectifier. It doesn't handle high peak currents and
> chances are, you'll be looking to replace it soon. Make a string of 20 or
> so 1000 PIV / 6 A diodes, each bypassed with
> 10 megohms and at least a 0.001 microfarad ceramic cap. This setup has yet
> to cause trouble for me.
>
I have the 2 MOT diodes in parallel to help with the current spikes. I have
already been warned about using them, but I figure that I have them, why not
use 'em till they pop. They're too low voltage and i have too few of them
to use them for much else. Still, i'll start compiling a Digikey order for
myself.
And they'll at least get me running, so can't be all bad.
> I'll pass on some advice given to me by John Freau and Dave Sharpe a while
> back that may save you some of the problems
> I ran into. Make sure to bypass the transformer with about 0.5
microfarads
> or so- RF will cause burning and internal shorts
.5uf? Sounds a bit large to be dealing with RF. Most RF bypass caps I see
are small capacitance (1nf of so). Pardon me if my ignorance is showing ;)
My filament tranny is CT, and the CT is grounded, as is the core.The
filament tranny is a re-wound MOT, ugly, but it works.
> in the secondary otherwise. A nice added safety bonus would be a plate
> protection resistor (high-power wirewound) in
> series with the plate lead to limit the energy of accidental plate
flashover.
>
That idea I really like. Where's the best place to find those without
having to pay a small fortune, and what wattage am I looking for? (the
filaments are 10v10A, 2 toobs, so 200w total power needed, maybe 250-300w
resistor?)
> >the schematics James sent me calls for Mica caps, but I'm sadly lacking
those,
> >so I'll be stringing up MMC caps to fill this duty. I have enough
> strings to
> >get my to the necessary capacitancies. Though the voltage ratings
> will be far
> >(10-15x) overkill, I don't know about the current ratings. I'm not
> worried
>about toasting caps though, I'm more concerned about the tubes.
> I've had great success with MMCs in tube coil service, and I cannot tell
> the difference in performance between them and
> the high power mica caps from an induction heater. The mica caps are very
> hard to find at a good price now. You should
> give a thought to current handling, since the caps in tube coils do heat
up
> some. At least make sure you have two
> MMC cap strings in parallel.
Very cool. The MMC's are bulkier than the Mica caps, but I have the MMC's
on hand already. I figure "use what I got."
>
> >How critical is the value of the tank cap and grid leakage cap? The
grid
> >leakage R determines the "breakrate" of the toob coil.
I'm
> assuming you'd want
> >it to run the tube as hard as you can for the least plate redness while
> >charging the tank cap up as to full.
>
> Tuning is real critical in all the tube coils I have made. The effects of
> capacitor heating on detuning the coil are
> notable. I recommend a high-voltage air variable cap immersed in mineral
> oil for fine tuning, placed in parallel with the
> main cap. Otherwise, you can fine-tune the secondary to the primary's
> frequency by adding more or less topload. If you
> do use a topload, you can affect a bit of tuning by moving it up or down
> above the secondary. You should always have a
> breakout point if you use a toroid though; my tube coils seem to run into
> flashover problems if they aren't discharging.
>
Whooboy. I've browsed Ebay for a big variable cap, and not come up with
much.
Ideas? (i'd hate to have to build one)
> Regarding the gridleak, the tubes are most efficient running with class-C
> bias (that's the only way you'll get the mentioned
> 1600 W per tube.) The RCA handbook method for calculating the grid
resistor
> is good enough. This component can be a
My ignorance is showing, i know, but what's the method in the handbook (i
don't have that book or knowledge). For high powered resistors, my trick of
using black rubber tubing doesn't work too well. It's simply too difficult
to get down to a few kohm (or even 50kohm to parallel) without the copper
lugs that go into the tubing arcing to each other. Bummer, as rubber tubing
can handle *tons* of power so long as you don't let it arc internally (RF in
it is a Bad Thing(tm)), and are careful of routing it so it touches nothing
conductive (don't coil it on itself or the concrete floor).
>
> rheostat of the required power rating in order to play around with the
> value. If you have two tubes in parallel, take the
> value of the leak for one tube and cut it in half (since twice the current
> will be flowing) to get the right grid bias on both
> grids. If the gridleak RC (the product of the two) is too high, the
> oscillator blocks or "motorboats"; and thus R will determine
> the pulse rate. This mode of operation with high grid RC has been called
> "sputter mode." It's useful for limiting tube
> dissipation and increasing the spark-length-per-power efficiency. It is
one
> type of pulsed mode. "Staccato" is another
> type of pulsed mode that only applies to AC coils and level-shifted
> (doubler) coils. The advantage of staccato is that
> the coil is turned on and off in syncronization with the zero crossings of
> the AC plate voltage, reducing kickback
> problems.
>
I'm going to continue on my current setup (sputter), and maybe chance to
staccato later on. I've found you learn much more from a working coil, even
if it's not too awfully efficient.
> >do I have to ground *everything* around the coil that I don't want to get
> >shocked by?
>
> Non-grounded conductors near the tube coil will pick up RF power, which
can
> cause minor burns.
> The RF burn problem is more substantial with tube coils than spark gap
> coils. The biggest
> safety issue for tube coils especially is the plate supply, with high
> current capability. Unlike with an NST, you
> cannot expect to survive MOT current. The same safety considerations that
> apply to "regular"
> coils also apply to the tube variety- be extremely careful with allowing
> direct discharges to yourself because
> through arcing to the primary or a grounding mistake, the plate supply can
> show up at the secondary.
>
>
> Have fun! Definitely mention your progress and results.
>
> -Carl
>
I've worked a lot with my pole-pig powered coils (from the insane 2kva
2"x13" coil to the 7?8?kva 10" coil). Whenever I think i'm getting
complacant, i put the minimal ballast on my pig and draw out a foot long
flaming arc with a sissy-stick. That puts the fear of the god (Third Degree
Burns, the bbq god) right back in me. I'm no specialist, and because i
frequently work alone, i'm paranoid is heck. The only way to be :)
shad