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500W Tube Coil gets Tuned





---------- Forwarded message ----------
Date: Sun, 19 Oct 1997 11:28:48 -0700
From: "Jeff W. Parisse" <jparisse-at-ddlabs-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Cc: Brent Turner <bturner-at-apc-dot-net>, Franklin Londin <swan-at-d2-dot-com>
Subject: 500W Tube Coil gets Tuned

All,

I've spent the last couple of nights tuning my 500W tube coil
and here are the results...

1. Grid Leak
I purchased several 50W resistors to swap in and out to find
the best settings for the grid leak. I also swapped the .002uf
grid leak capacitor for a .02uf to reduce the amount of RF being
passed by the cap. I found that the Tesla coil seems to work
best when I adjusted for full current draw (700ma) at 3500Vdc
rather that the tubes maximum rating of 4000Vdc. 18 turns on
the feedback coil and a 2500 ohm resistor (and the .02uf cap)
turned out to be the best (DC) combination. I now have 10%
headroom on the variac if I want to push the tubes to 800ma at
full voltage.

2. AC vs. DC
AC spark length equals 12+" at 1000 ohm gridleak setting.
DC spark length equals 9+" at 2500 ohm gridleak setting.
Filtered DC spark length equals 4+" at 2500 ohm gridleak.

The best looking output came from unfiltered DC. The bottom
of the discharge was hot and flame/plasma like and there were
twice as many sparks as AC. AC produced longer sparks and
a strong 60 cycle hum.

I'm moving to two 5K 50W (parallel for 2.5K 100W) resistors
for the grid leak. I think DC is burning twice the current.

3. RF Power
Given the tubes (250TH, 4kV, 350ma, 250w each), how do
my results compare to other coils of similar wattage?

I tried the "whipping wire trick" and got white hot balls of
molten copper flying off the end of the 26ga. wire I was
using. This coil really seems to put out some juice!

Can anyone suggest some interesting demos/effects with a
coil of this size/type?


Jeff W. Parisse, Art Director
Digital Design Laboratories
www.ddlabs-dot-com