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Re: [TCML] First light, small VTTC
There are so many factors to consider. Some tubes need more
feedback than others. My experiment was to test the idea
of using a high position for the grid coil which some folks
have found helpful. So far in tests with this small coil I didn't
see any advantage but of course this coil still needs more
optimizing in general.
I used 19.7 turns on the primary I think, and the coil height
is about 2" of 16awg wire. The secondary is 2.5" x 8.875"
wound with 28awg wire for about 620 turns or so. No toroid.
The grid coil and primary are both 4.75" dia. The grid coil
uses 20 turns of 24awg magnet wire tapped at 12 turns.
MOT is a small 2000V unit with level shifter. Filament transformer
is 6.3V output, so I use a 50 ohm dropping resistor on the 120V
input to drop the output to 5V for the 4-125A vacuum tube.
The RF bypass cap is 0.001uF at 5kV mica, the grid leak cap
and filament bypass cap are both 0.001uF at 2.5kV.
From: Vladimiro Mazzilli <V.Mazzilli@xxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Tue, 28 Oct 2008 5:07 am
Subject: R: [TCML] First light, small VTTC
So seems better to use thin wire to have a good grid driving, but not
in order to no decrease the spark length?
I powered up my new small 4-125A VTTC and got 4" sparks
so far. They are the fuzzy type sparks. More tuning is needed.
I tried removing the MOT shunts, but the sparks didn't get
much longer so far. The tube plate gets red which is not surprising
for a 4-125A tube. I tied the grids together on the tube, and
I'm using 1500 ohms for the grid leak R. I tried 2500 ohms but
that gave very weak sparks. On a previous coil which used
a 4-250A tube, I got the same spark output with grids
tied together as when feeding the screen grid from a
dropping resistor. That coil also used a 1500 ohm grid leak resistor.
There is no toroid on the coil. Some tests I did in the past
showed that even the choice of vacuum tube type can
make the sparks either fuzzy or sword-like. This may suggest
that some parameter was not optimized for each tube.
I did a quick experiment with a raised grid coil. Normally
I allow about 1/2" space between the primary and grid coil.
I raised the grid coil by about 3" as a test. The sparks
got much weaker. So I moved the grid coil tap so I used
20 turns instead of 12. This increased the spark length
almost to what it was with the lowered grid coil. This
secondary is only 8.875" long, so raising the grid coil
by 3" is a lot for this small coil. I tried various grid
coil heights and generally speaking, the spark output
varied with the grid coil height. A higher grid coil
gave weaker output sparks. These results are
preliminary because the coil still needs more tuning.
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