Re: Redesigning/tuning a coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com> 


The nails will rapidly heat to red hot and then they don't quench (open the
circuit to let the sec ring freely) very well.  You might try replacing the
nails with 3/8th inch rounded carriage bolt heads.  These work good as a
sparkgap for smaller coils on a budget.  Use 2-3 gaps and experiment with a
100 CFM muffin fan behind them to see if increased airflow provides better
quenching and better spark output from sec.

With the doorknobs keep you runtime short --- 15 sec bursts.  Depending on
the dielectric type they will heat up and may not take the harsh 95% voltage
reversals common in TC oscillator circuits.

MMC caps are inexpensive at $3 each and will do a great job.  Contact the
"Geek Group" website for more info.

Sounds like you're off to a great start.  Have fun.

Dr. Resonance
 >
 > Hello, I'm new to this forum;  I've been devouring backposts from
 > the archives lately, and I really enjoy this valuable resource.  I
 > recently was offered the chance to tune up an existing Tesla coil,
 > and I jumped at the chance...I would like to build one soon, but time
 > and resources may not allow me to, so this seems like a pretty
 > good opportunity.  However, I have to get as much done as I can
 > by Wednesday, because I'll be leaving the area then.  I've checked
 > out the coil, and am making some recommendations to my contact
 > on what to change; I'd appreciate any comments to let me know if
 > my ideas are on-target, or if you see something you think I should
 > change.
 >
 > The current setup (all inch measurements are approximate):
 >
 > 15/60 NST
 >
 > Primary: about 19 turns of insulated wire, I'd guess around AWG 12
 > or 14, 18" diameter bundle
 >
 > Secondary: 8.75" OD, 36.5" wound length, 1500-1550 turns of
 > AWG 22 varnished wire
 >
 > Topload: currently either a metal bowl upside-down over the top or
 > a 8" x 23" toroid of aluminum flex ducting
 >
 > main cap: 16.2 nF (6 caps in parallel, each 2.7 nF -at- 40 kV--the
 > doorknob type, probably ceramic)
 >
 > Spark gap: a single gap using several nails
 >
 > Ground: I'm not sure if he has a good ground--I don't think so
 >
 > My main ideas for improvement based on what I've read here:
 > -switch the primary to 1/4" bare copper tubing, around 12-14 turns
 > to allow for plenty of flexibility when tuning, in a cone angled at 30
 > degrees for better coupling
 > -build a new toroid, 6" x 26" (aluminum flex ducting)
 > -main cap: see if I can persuade him to try something else, since I
 > know the doorknob caps are really lossy and their C changes with
 > temperature
 > -spark gap: an RQ/TCBOR style multiple spark gap
 > -connect a good ground
 >
 > Main questions:
 > -Will the 6" x 26" ducted toroid be too big for this coil?  I was also
 > considering 5" x 20", but someone my contact talked to said it only
 > comes in the even sizes, e.g. 4" and 6" but no 5", so I figured the 6"
 > was better than trying 4".  I thought this was strange, since many
 > people have mentioned 5", but perhaps they weren't using ducting.
 > Any thoughts on how appropriate this size is?
 >
 > -How acceptable are the ceramic doorknob caps?  I know they're
 > not the greatest, but I'm thinking we may just stick with them, since
 > we already have plenty, and replacing them would cost a fair
 > amount.  Also, the current C seems pretty reasonable; any
 > recommendations on either front?
 >
 > -The coil is currently located underground in a large warehouse-
 > style area (though I don't know if it'll stay there), so driving a
 > grounding rod isn't feasible--would connecting the ground to
 > structural steel members be acceptable?  (I don't think the water
 > pipes would be a good idea, as they're connected to the normal
 > electrical ground--is the structural steel also connected to the
 > electrical ground?)
 >
 > Any other thoughts/comments?
 >
 > Thanks!
 > Michael Johnson
 >
 >
 >