Re: Redesigning/tuning a coil

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

Original poster: "john cooper" <tesla-at-tesla-coil-dot-com> 

Regarding the primary angle:  I ran through that experiment on my largest 
coil a few years ago and 18 degrees of slope was the maximum allowable 
primary slope on that coil.  Primary is about 19 turns of 1/4" cu and I 
believe it was tapped at 17 turns.  1 degree over that 18 and the primary 
field wouldn't cover the top of the secondary and, believe me, you could 
tell where it was hitting the secondary, an inch or two below the top 
turn.  Secondary form is 33" long with a o.d. of 6.3", with 31.5" wound 
with 21awg.  With a shorter secondary it seems that I may have been able to 
use a steeper angle on the primary but I have not yet performed that 
experiment.  You can see pics and specs of that coil on this link:

http://www.tesla-coil-dot-com/Quarterwave.htm

Hope this helps


---------- Original Message ----------------------------------
From: "Tesla list" <tesla-at-pupman-dot-com>
Date:  Fri, 16 Apr 2004 12:32:04 -0600

 >Original poster: pepperman-at-SoftHome-dot-net
 >
 >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
 >
 >
 >