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VTTC question

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LAN_403

EvilTesla-RG

Sat Dec 05 2009, 12:02AM

Registered Member #1523 Joined: Sat Jun 07 2008, 02:05PM
Location:
Posts: 97

I know that SGTC's are disruptive coils.

But VTTC's are desined not to be, as such their sparks are much smaller.

Why is this? Why arn't VTTC's desinged to be disruptive? Am I missing somthing?

...

Sat Dec 05 2009, 04:38AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

There is no reason for VTTC's to not be disruptive, try googling staccato for some existing designs.

StevenCaton

Sat Dec 05 2009, 05:46AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

VTTC's can be run in a variety of ways.
A truly continuous VTTC would be fed with a smoothed HVDC supply on its plate. It would oscillate 100% of the time, consume huge amounts of power, and produce one thick hot display of plasma on its breakout. If im not mistaken, class E cathode modulation requires such a supply.

The most common way that people run VTTC's is with a voltage doubling arrangment for the HVDC supply. On one half cycle of the AC, a capacitor bank is charged, then on the other half cycle, the voltage from the capacitor bank adds to the mot voltage. This results in pulsed HVDC being fed to the plate. Note that with this arrangment, the coil is in oscillation 60 times per second, and is shut off 60 times per second, resulting in 50% duty cycle. A 60 Hertz Hum can be heard in the background during operation. This method of operation also consumes quite a bit of power.

Then of course, the stacatto circuit can be used to create a pulsed VTTC. This is incredibly desirable, as one can run the tube at higher powers and not have plate redness. For example, if the duty cycle gets cut down to 25%, then you can crank some more power into the system and not have to worry about the plate getting red.

Then, there are more elaborate schemes that pulse the tubes grid, creating different low frequency audio tones.

See this video for some of these methods of operation.

Dr. Dark Current

Sat Dec 05 2009, 09:50AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The problem with high peak powers in VTTCs is that I think the cathodes usually run very close if not over their current rating, and if you increase power even more and interrupt the coil, the cathode wears down quickly.

Steve Conner

Sat Dec 05 2009, 10:06AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Correct. Unless you can find some radar modulator tubes like the 4PR1000A, GMI-90 etc. These have a peak current rating of tens of amps. The GMI-7 can do over 50A.

You could also boost the peak output of an ordinary tube by turning the filament voltage up. It's not good for tube life, but we're not known for treating tubes nicely here.

Dr. H.

Sat Dec 05 2009, 10:49AM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

And as we speak about GMI-90 - have someone build a coil with it ? This thing can run at insainly high plate voltages (from the specs I have for my tube - 33kV at 40A pulse plate current ... nice).

Cheers

Proud Mary

Sat Dec 05 2009, 11:11AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

You would be better to use a device made for this kind of service such as a thyratron.

An inexpensive medium power H2 thyratron like 3C45 can output 43kW peak power 24/7 from a 1250VDC supply.

Even the miniature 1258 H2 thyratron - designed for small craft radar use - can switch 1kV/20A peak pulses indefinitely when correctly set up.

Stella

Steve Conner

Sat Dec 05 2009, 12:30PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Dr. H: Didn't you build a VTTC with some of these radar modulator tubes? GMI something or other. The GMI-90 obviously means business, with its corona ring top cap. I've seen one VTTC made with it, but powered by just a single MOT, it didn't get anywhere near realising the potential of the tube.

Mary: The problem with thyratrons is that they take a relatively long time to de-ionize, so they can only be used in a disruptive mode, unlike a vacuum tube that can directly generate RF power at the resonant frequency. Richard Hull built a hydrogen thyratron Tesla coil years ago, but the performance he got was pretty poor compared to even a VTTC.

He didn't make any provision for reverse current, instead he designed it for almost complete energy transfer in the first half-cycle. I think this was a mistake, and if he'd put a reverse diode across the thyratron, it would have worked better.

More recently, Chris Fredin tried to build a large coil powered by two back-to-back hydrogen thyratrons, so as to conduct current in both directions, but he never finished it.

vasil

Sat Dec 05 2009, 12:38PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

I have a project with a GMI90 vttc runing....but God knows when will be finished. I am in the collecting parts time yet....and it is a slowly time..

Dr. H.

Sat Dec 05 2009, 12:56PM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Hi guys

Steve: I've used the GMI-7 (two of them) for the project. The 50Amp pulse that you've mentioned is the criteria for "long lasting" ... less than 250hour from data sheet. I have a confirmed information of using those tubes (the 7s) with more than 100Amps on the plate. But lasting less than 40hours. Again in radar systems.

Vasil: Can't wait to see more of the project

Just for a reference from left to right - gmi-90, gmi-7, 6p45s.

Cheers :)

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