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VTTC primary Q, voltage (Steve Ward?)

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J. Aaron Holmes

Wed Feb 21 2007, 04:56PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

I'm having a hard time visualizing what is going on with voltages in the tank circuit of a typical Armstrong Class-C VTTC. In a prior thread on VTTC tank caps, Steve Ward said, "I dont think that the 40kV rating was overly engineered, given that the capacitor will see *at least* 5kVAC, plus some other voltage depending on the Q of the loaded tank circuit (might be something like 2-3X the plate voltage). I think i may run some simulations to see what is actually expected."

I'd love to know what those simulations say! I can sort of visualize the 2x thing. But 3x escapes my feeble intuition. As to what this question tends: I hear talk everywhere of the tube's plate voltage rating relative to the voltage actually being supplied in VTTC use (e.g., 2x the recommended max via level shifting), but aren't you then pushing *4x* the tube's plate voltage rating because of the oscillations in the primary tank? With a suitable tank Q, wouldn't a VTTC fed with a filtered DC source equivalent to the tube's plate voltage rating *already* be subjecting the plate to voltages nearing (at least) 2x the max?

And finally: I've just gotten ahold of some massive soda can-sized mica transmitting caps, rated 2400pF, 20kV, 24A @ 1MHz. From Steve Ward's VTTC page (http://www.stevehv.4hv.org/VTTCfaq.htm): "Higher Q gives higher tube efficiency, easier coupling, but more tank losses." Let me make sure I understand the pracitcal implications of that statement. Am I correct, then, in saying:
1) Higher Q means my cap is going to heat up MORE.
2) Higher Q means my tube is going to heat up LESS.

If these statements are correct, then because I've got some real beefy caps, why would I *not* want to shoot for a higher tank Q if I assume the coupling statement is correct?

Regards,
Aaron, N7OE

Steve Ward

Wed Feb 21 2007, 06:30PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Hi Aaron,

You ask some good questions, lets hope i answer them right

I'd love to know what those simulations say! I can sort of visualize the 2x thing. But 3x escapes my feeble intuition

The LC circuit acts as an energy storage device (on a short term time scale). If the VTTC primary circuit was very lightly loaded (remove the secondary for example), then the energy in the LC would continue to build until the energy dissipated in losses equals the energy supplied to the circuit (in theory, it could build to enormous energy levels). Increasing energy in an oscillating system implies both increase in current, and thusly, voltage. As to my actual simulations, with the secondary in place, and given what i would considerable a decent model for the streamer (150k ohms to ground) i see a voltage gain of about 3X on the primary circuit, so the tube would actually see peaks of around 3X the input voltage.

"Higher Q gives higher tube efficiency, easier coupling, but more tank losses." Let me make sure I understand the pracitcal implications of that statement. Am I correct, then, in saying:
1) Higher Q means my cap is going to heat up MORE.
2) Higher Q means my tube is going to heat up LESS.

At the time i compiled that FAQ, i got a lot of the information from John Freau. But, i can try to elaborate on the effects of Q on a circuit. A higher Q implies that for the same power through-put, the tank circuit will see higher voltages and currents circulating between the LC components. Higher currents cause higher IIR losses in the components.

But to be honest now, looking at the "math" section of that FAQ, i think its time for some revisions. I blindly accepted Johns suggestions for the tube load impedance, and i think there is something more going on there. The whole thing is highly dependent on what the secondary coil is doing, whether its producing long sparks or not. The tube load impedance is highly affected by the secondary output. Unfortunately, most people that have built VTTCs dont seem to care too much on the fine details of whats actually going on in the circuit, so there isnt a whole lot of information out there on how they work (in detail). Anyway, i think i will try to attack the analysis in a new way... its just gonna take a little time. Until then i can try to answer your questions directly.

Sulaiman

Wed Feb 21 2007, 06:43PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I'm not a VTTC builder so what follows may be incorrect;

Assuming that you are operating with a near sinewave across the Primary Tank (reasonable "Q")
then the peak to peak voltage on the Tank would approach 2 x Vdc.

If you are seeing 3 x Vdc then the loading must be quite high / low loaded-Q.
i.e. quite a lot of energy transfered from the power supply to the Tank inductor each cycle.

J. Aaron Holmes

Wed Feb 21 2007, 09:23PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Steve Ward wrote ...

As to my actual simulations, with the secondary in place, and given what i would considerable a decent model for the streamer (150k ohms to ground) i see a voltage gain of about 3X on the primary circuit, so the tube would actually see peaks of around 3X the input voltage.

Thanks, Steve. Ok, so basically, if I have any dillusions about "being nice to" my tubes by running closer to the rated B+, that's all they are

(Well, I suppose I'm being "nicer"

) I think, then, that for lack of any obvious reason NOT to, I'll go ahead and target the higher Q, since it's almost hard to avoid with caps this big (if my math is correct--always worth doubting). Otherwise the secondary starts to get too big or requires the use of real fine wire (funny how there always seems to be that one "seed" part that your whole project just kind of "grows" off of...)

I paid a visit to coworker and VTTCer Simon Winder's place this weekend and got a demo of his dual-Svetlana 572B coil. I was impressed that he dared to use a level shifter on those little tubes, but they seemed to be surviving and putting out some real good sparks. I doubt if 811A's would fare so well, but I'm reconsidering using those for my first VTTC. Maybe I'll just get four

It's amazing how cheap they are!

Regards,
Aaron, N7OE

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