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Re: Tests: Thin wire vs. thick with fewer turns, etc.

Hi John,

	From the information you give and making a few guesses, I would estimate
that the output impedance of your coil went from 327K ohms down to 111K
ohms with the changes you made.  

If one assumes that a streamer presents a load of 220k in series with 5pF
(Z=416K at 90kHz and Z=263K at 200kHz) then your old set up was probably
impedance matched better to the streamer load than your new setup.  

Assuming a bunch of stuff, your streamers would have been receiving 225
watts before and 140 watts now.  Assuming streamer length is proportional
to the square root of output power (and input power too assuming more
stuff), I would expect your new system to give streamers 80% of the length
of the old setup.

Admittedly, the techniques used to determine all this are a bit new and
barely tested.  However, in your case, the results seem to follow the
predictions.  With better streamer load data, such impedance matching may
really help in determining what the best size and type of secondary to make
to produce the best sparks.  I still find it hard to believe that 220K ohms
in series with 5pF is the impedance of ALL secondary arcs.  However, it
always seems to work in every case so far...

Thanks for providing this valuable information!  It has great meaning to me...

Cheers,

	Terry



At 10:43 PM 11/13/1999 -0500, you wrote:
>Hello coilers,
>
>Today I decided to wind a new secondary for my 42" spark TC.  I
>re-used the same form, but rewound it with 660 turns of #20 magnet
>wire, compared with the original 1500 turns of #28 magnet wire.  The
>winding is 6.5" by 23.5" (same as before).
>
>I then removed the old primary which used 22 turns (out of 44 total)
>of #12 stranded close wound PVC insulated wire.  I installed a 1/4"
>copper tubing primary (spaced 1/4") in it's place.  This new primary
>is unusual in that the bottom 6 turns are coned, and the top 3.5
>turns are flat.
>
>All other components were left unchanged; 0.0147uF cap, toroids,
>xfrmers, ballast, 120bps sync rotary gap, etc.  The new frequency
>is about 200kHz, the old freq was about 90kHz.  The new secondary
>is 17mH, the old one was 95mH.  The DC resistance is much lower,
>as can be imagined.  The new primary is about 50uH, the old one
>was about 200uH at the tap point.
>
>I tested the new arrangement at 580 watts, and the sparks seemed
>a little weaker than with the old set up and did not reach 42".  I 
>wasn't sure if I had enough primary turns, so I installed a slightly
>larger cap (.0164uF).  I was able to determine that it's tuned best at
>about 1/8th turn from the end now.  After adjusting the ballast, sync
>phase, coupling, etc., I was able to barely obtain 42" at 590 watts.
>
>I still have to play around with the sync phase, and ballasting some
>more, and I'm hoping I'll be able to at least equal the performance of 
>the old set-up.  The sparks just look short and sluggish now although
>they did hit the 42" with difficulty.  In any case, the thicker wire does
>not seem to make the sparks brighter, hotter, whiter, or anything
>like that, but this does not surprise me.  I will also try higher power 
>levels.
>
>In any case, I think it's pretty safe to say that there's not a great deal
>of difference between; thick wire or thin, higher freq or lower, more
>turns or less, thick primary or thin, etc.  Again, the main keys to
>performance appear to be; power input, break-rate, and toroid size.
>(A caveat; it is possible that some of the new parameters are cancelling
>other ones, thereby completely hiding various good or bad features.)
>Of course a larger TC should use thicker wire in general, etc.
>
>In any case, the coil is still mostly obeying:
>
>        spark length (inches) = 1.7*SQRT input power (watts)
>
>My secondary is much heavier now, because of the thicker wire, but
>it was easier to wind (in a way).  I should strengthen my variable
>coupling post to handle the heavier weight. The primary looks a lot
>nicer now, and is easier to tap.
>
>The primary and secondary run pretty coolly, but the tank wiring (#12)
>gets warm, so I may beef it up.  
>
>When I did a similar comparison some time ago, I saw a great 
>reduction in spark length when I used fewer turns on the secondary. 
>I think this other secondary used about 550 turns of #24 pvc insulated
>wire, and I used a coax primary that time.  I attribute the poor results
>of that test to the use of the close wound coax primary.  I don't think
>the secondary would make that much difference.  The spark length
>dropped from 42" to 29" I seem to remember, in that test.
>
>The work will continue.
>Cheers,
>John Freau
>