Re: First VTTC light, and a few questions

["David Trimmell" <davidt-at-pond-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi,

good go on the VTTC. One thing though, it seems to me you could get
better voltage rise with more L in the secondary, at only ~8mH and running
around 700-800 KHz, you may do better with say 8" of windings to get you
around 12 mH. Not sure why your filament V is increasing, as long as it is
completly separate from the plate supply it really souldn't be affected by
the plate V, your ground isn't floating?

Don't be afraid to rough up those 811's, Iv seen them for a around $5 at
hamfests, so get a few spares and push 'em! Also, I would try to play
around with the grid feedback, adjust the number of turns, and see if you
can get a current meter (that works!) to watch grid I (I think its more
important than watching V).

Good luck,

David Trimmell 

On Tue, 18 Jul 2000, Tesla list wrote:

> Original poster: "Grayson B Dietrich" <electrofire-at-juno-dot-com> 
> 
> Heyas, All!
> 
> For just about as long as I've been building spark gap coils, I've wanted
> to build a vacuum tube based coil...  ...and now I finally have a
> functional one!
> 
> Following equations in Duane Bylund's "Modern Tesla Coil Design Theory"
> (a book that took me a year[!] of waiting to recieve-very unreliable
> publisher) I wired up a coil with a single 811A triode. 
> 
> At first it barely gave the tiniest spark, but with quite a lot of
> fiddling, rearrangement, and finally adding metering of  filament
> voltage, plate current, and grid voltage (the meter wired for grid
> current seems to be nonfunctional :p ) It now runs smoothly, apparently
> well within the specs of the tube.
> 
> The sparks are only 2" long at the very best, and I've read that an 811A
> coil ought to be able to give twice that....   ....so here are the
> pertinent specs as best I see them, and a request for advice.
> 
> (values measured)
> Plate Voltage: 1030VAC
> Current Draw: 120mA (I'd like to draw more, as the plates aren't even a
> dull red yet, but I don't want to reduce the grid leak resistor much
> more... it is already only 3K)
> Grid Voltage: -120VDC
> Filament Voltage: 6.6VAC (the meter reads a rising voltage as i increase
> the variac controlling plate voltage...  ...when the coil was ill-behaved
> it would increase to as much as 10V, now it only reaches 7V...what is
> happening?)
> 
> 
> Primary: 4" thin wall PVC form, PVC insulated #18 wire, 23turns, tapped
> at 18, windings are 1 7/8" tall.
> Feedback: on same form as primary, Cottenamel insulated #24 wire,
> 19turns, not tapped just wound on and off (excess spirals in solenoidal
> forms to reduce space taken up)
> Secondary: 3.5" dia. thick wall PVC, #28(a guess, worked right with
> WinTesla) enamel insulated wire, 5.75" winding height.
> Terminal: 2.25" aluminum ball with a 1" wood screw attached, pointing
> strait up.
> 
> Bottom turn of secondary is even with top turn of primary (gave better
> performance than with more coupling) Feedback winding is 3/4" below
> primary. Originally this was above the primary, and the coil performance
> was terrible, while the metered voltage and current values were all much
> too high.
> 
> Grid Leak Resistor: 3K to 4K, on a 20K 25W rheostat.
> Grid Leak Capacitor: 4000pF, 1600VDC PP film/foil (4 1000pF caps in
> parallel)
> Tank Capacitor: 1000pF, 3200VDC PP film/foil (mini MMC, 4 1000pF caps)
> 
> Plate voltage is variable, brought up slowly via variac.
> There is no filtering,
> 
> and....    ....Oh, yeah. Just how long ought I to let this thing warm up
> before I apply Plate Voltage? the filament gets orange-white in seconds,
> and doesn't appear to get any brighter if I let it sit there for a while.
> 
> 
> Also, is it acceptable to run an 811A with a dull-red glowing plate, or
> to what extent is such a thing allowed?
> 
> Thanks for any input, and I hope to hear from all you tube coil gurus out
> there. 
> 
> Thanks, 
> 
> Grayson Dietrich
> http://www.electrophile.8m-dot-com
> 
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