RE: Redesigning/tuning a coil

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

Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> 

Hi Michael:

My comments interspersed:

 >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


The toroid size sounds good, and the toroid is much better than the
metal bowl!


 >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

The gap sounds like a weak spot in the design.  The electrodes should be
rounded, not pointy, and thermally massive so a hot spot does not
develop at the point of arcing.  Forced airflow through the gap arc
generally improves performance by a large margin.  A pair of parallel
copper pipes will work much better.

 >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

There is no need to make a conical primary.  A flat one will provide
adequate coupling and have better clearance to the top load.  Too much
coupling may result from a conical primary - more is not always better.

 >-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 capacitance seems OK, although I use a 20nF with my 15/60.  As far
as doorknob suitability, your mileage may vary...   Some are better than
others, but none are as good as a good polypropylene cap.

 >
 >-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?)

The choice of ground will have very little impact on performance; it
mostly affects how much RF get back into your power mains.  I think
connecting to the steel frame is a good idea.

 >
 >Any other thoughts/comments?
 >
 >Thanks!
 >Michael Johnson

Regards, Gary Lau
MA, USA