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Re: [TCML] Tube choice for a VTTC 350khz?
Hi Jim,
It was Steve Ward that spoke of the frequency issue. Perhaps others
spoke of it also. I never tried space winding for a VTTC. Some sort
of plate, toroid, or corona ring is needed when sparks go over about
29" in a VTTC, otherwise the sparks will tend to emit from the top
turn, instead of from the central tall breakout point. The variable
plate caps under oil are excellent for finding good tune points.
I used a 7.2kV pig for some VTTC work. These may be easier to
find than 4.4kv ones. Also, some of the 14.4kV ones have a switch
which permits operation on either 14.4kV or 7.2kV. Using 7.2kV,
you can limit the voltage to 5kV or whatever using the variac.
Some VTTC builders are using dual MOT's and doublers for 8kVAC
input. I'm not sure how the 833C's can withstand that voltage.
Of course if the tube plate is kept cool using staccato, that permits
a higher plate voltage to be used, but still...... who knows?
Dr. resonance often remomves a pig core from it's large housing,
and mounts it into a smaller square metal case, so it's more
convenient, but that takes some work of course.
But in any case if you run a large VTTC on raw AC, it needs a
very robust transformer with a low secondary wire resistance.
Otherwise it acts like a big resistor and kills the spark length
and efficiency. At least that's what I seem to see in my coils.
Amazingly, MOT's can do a pretty good job
up=2
0to a point, because they have so little wire that the
resistance remains pretty low.
John
-----Original Message-----
From: Jim Mora <wavetuner@xxxxxxxxx>
To: 'Tesla Coil Mailing List' <tesla@xxxxxxxxxx>
Sent: Fri, Jun 19, 2009 2:32 am
Subject: RE: [TCML] Tube choice for a VTTC 350khz?
Hi John,
Was it Dr. Spark ?? or someone noted that if their coil was running @
350khz
or above it had nice swords. Frequencies set below this were more
brushy. I
suppose geometries and power must play into this too. I am designing my
4x
833c to run at ~350khz and still have 17-25 turns on the tank coil on a
10"
acrylic former. I hope that works. The coil will be 6"x24 wound with
21awg
to keep the turns down. Have you space wound this scenario? My top load
may
end up being an edge rounded 7" .375 aluminum plate to keep the freq
up. I
also have one of your 6" toroids to attempt to duplicate the freq
difference
and arc shape changes. I have a very big tunable plate cap under oil
which
should assist in attempting replication of this observation.
I would like to find a 4400 5kva pig or PT! There are some 15KVA pigs on
ebay. Too Big! My big, mutitap dry transformer is way too big too (over
200lbs). Anyone?
Jim Mora
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf
Of futuret@xxxxxxx
Sent: Sunday, June 14, 2009 10:30 AM0D
To: tesla@xxxxxxxxxx
Subject: Re: [TCML] Tube choice for a VTTC
Dave,
I think as a practical matter, just about any kind of power tube can
work well
in a VTTC provided it's robust and can withstand the voltage, etc. I've
used tubes such as; 833A, 833C, 803, 805, 810, 845, 304TL, 4-250A,
4-125A, HF300, HF200, etc. All of them worked well. Some coil designs
seem to require a lot more tweaking to make them perform well. Some
designs tend to produce fuzzy sparks, whereas others produce spikey
sword-like sparks. It's not completely known why this occurs. There
is some evidence that a higher frequency helps to produce swordlike
sparks, but I know that other factors such as tube type, tuning,
coupling,
and feedback all affect this also.
I think the addition of a small toroid to a VTTC may help to stabilize
the operation. A toroid seems to help the spark output some also.
Perhaps at some point, someone will try to quantify all this, and maybe
make up some sort of computer design program to assist with VTTC
design. Some of the known rules of thumb, formulas, and general
principles could be incorporated into the program. I think there's
still
a lot of unknowns about tube coil design and operation.
Cheers,
John
-----Original Message-----
From: David Speck <dave@xxxxxxxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Sun, Jun 14, 2009 8:52 am
Subject: [TCML] Tube choice for a VTTC
0A
List,
I've read previously that the best=2
0tubes for VTTCs are ones with low
Mu's or amplification factors.
If one were to happen upon a high Mu tube at a good price, is there a
way to adapt the VTTC grid or primary circuits to make it work well?
Would using fewer turns in the grid coil or some sort of resistive
voltage divider be sufficient to optimize performance?
Does the high Mu make it harder for the system to remain stable when
the coil is loaded by the streamers?
Can anyone recommend a good reference book on the design of high power
vacuum tube amplifier/oscillator circuits? Seems all I can find are
books on guitar amplifier construction, a little small for my tastes.
Thanks,
Dave
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