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Modifying a dual-811A VTTC for different output

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cbfull

Wed May 16 2012, 06:13PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

I have been wanting to modify my VTTC for a while to lower the current and increase the voltage at the output terminal. As it is right now the sparks at the terminal are really hot and I can't get it to breakout unless there is a sharp point, which always gets red hot.

Since I don't want to wind a new secondary, I was thinking that I could retap the primary at the halfway point so that there would effectively be half the number of primary turns, then do the same for the feedback/grid windings. I'm not sure how this would affect the grid leak resistance however.

It seems that the next thing that would need to be changed is the primary tank capacitance, which I think would also need to be half of what it is now because of the reduced inductance.

Does anyone see anything wrong with this or have any ideas about this sort of modification?

Any insight is greatly appreciated.

Dr. Dark Current

Wed May 16 2012, 06:35PM

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

For this you need to lower the secondary resonance, I'm afraid there's no other way. Have you considered using a topload?

teravolt

Thu May 17 2012, 01:55AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Have you thought about adding a Staccato circuit to your tube coil. What cind of tubes are you useing?

cbfull

Thu May 17 2012, 04:32PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

I'm talking about changing the ratio of pri/sec turns, which is completely feasible.

They are 811As (it's in the thread title). The guy that used to build the coils for my hometown science center around 25-30 years ago had several experimental configurations based on the 811A pair.

One had 26 AWG wire on a 24" secondary and produced a firey output (this is the one I copied).

There was an older version that had a much taller secondary (about 4 feet long) and the number of primary turns was the same. This coil did not have the firey output at all. It had relatively cool sparks that were not nearly as hot against the palm of your hand.

I am hoping to approximate this difference by cutting the number of primary (and feedback) turns in half.

I forget how many primary turns I have but cutting this in half might create some problems and I'm just trying to anticipate whatever I can.

Dr. Dark Current

Thu May 17 2012, 09:07PM

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

This will not work. You will completely detune the tank circuit and get no output.

cbfull

Fri May 18 2012, 02:36PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

Adding a topload will also detune the secondary. I've already tried it.

Thanks guys, but I am really hoping for some more informative input rather than just a couple of "No" responses. It can be done, there's no question about it, that's why I mentioned the example where it already has been done.

I actually just remembered that when I originally wound the secondary, I had an identical back-up secondary wound at the same time since we were already in front of the lathe and it was super fast and easy. At one point I stacked them one on top of the other, adjusted the tank capacitance to half its original value and got excellent results. The sparks were thinner and slightly longer, greatly reduced heat as well. Just the results I am looking for now, but without the double-length secondary.

So I know that doubling the number if secondary turns is one way. But tapping the primary halfway is not the same as doubling the number of secondary turns. The turns ratio is the same, but the inductance becomes approximately half, which I assume means twice the oscillation frequency. Is that right?

So the next thing to consider is the primary tank capacitance. It doesn't make sense to me that the new tank capacitance would be very close to exactly half (with half the primary turns) just as it was when I doubled the secondary height.

I am also wondering if, since the number of turns on the feedback windings is the same as the primary, does the new higher osc. frequency affect the grid circuit in a way that will require me to change that too?

Thanks in advance for all of your comments and insight guys!

Patric

Fri May 18 2012, 05:57PM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 255

Sorry to interrupt! Do you get the same spark if you double the primary capacitor and halve the coil inductance? (resonance frequency remains the same)

Dr. Dark Current

Fri May 18 2012, 07:43PM

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

Patric: This way you double the Q of the primary circuit so theoretically you can get better oscillator efficiency, but this depends on previous circuit parameters. As for the sparks, you might need to re-tune the new configuration (+- a few % of primary turns) to compensate for the new coupling and possibly get longer or the same sparks. But surely the primary and secondary coils will heat up more, because of higher currents and stronger magnetic field (more eddy current losses).

cbfull

Fri May 18 2012, 08:56PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

Patric wrote ...

Sorry to interrupt! Do you get the same spark if you double the primary capacitor and halve the coil inductance? (resonance frequency remains the same)

Are you asking me? Or just throwing that out there for anyone? It kind of sounds like you might be trying to lead me down a certain logical pathway.

Either way, I don't have a very solid understanding of what the primary capacitor actually does in the circuit (at this point I realize I need to find a schematic to post here).

teravolt

Sat May 19 2012, 01:49PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

if you duble the cap and half the primary inductance and the resonant Fo stays the same one of the added benifits is that the primary ciurulating current goes up because there is less resistance. it may also change the impedance that the tube sees. what type of capacitor are you useing? high frequency low inductance like mica is the best

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