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looking for a push pull schematic for VTTC

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Dr Hankenstein

Thu Feb 05 2009, 08:52PM

Registered Member #1642 Joined: Sun Aug 17 2008, 11:36PM
Location: Black Canyon City
Posts: 96

Last year I tried the push-pull configuration but ran into some interesting snags:

First of all, there was so much primary wire involved in the helix coil that it had a serious shunting effect on the power. That is to say the primary seemed to be way over-coupled (excessive mutual coupling) resulting in very low power output.

My next step around this was to try a pancake coil. This was abandoned due to inadequate L(uh) to C(uuf) ratio physically possible to tranfer any power to the secondary efficiently.

I found the classic single-ended design from the 1940's (The one that is on Steve Ward's website) to work the best.

All these brilliant ideas (or lack of performance of) eventually led to the construction of the "Super 833C" Multimode prototype coil, which is of classic design, and yet can be quickly configured for ticker coil, direct grid drive, and can be grid or cathode modulated. (This portion of the discussion is off topic)

Basically the push-pull configuration was abandoned due to complexity and excessive parts count for the theoretical "massive" gain in performance. Push-Pull Vacuum Tube Tesla Coils appear to be a completly different animal compared to their "audio" counterpart.

I have written some extensive notes on tests performed in my pursuit of a push-pull tank circuit and it boils down to:

If you want:
Least amount of headaches
Best performance
Least amount of parts
Simple design.........................stick with the classic VTTC!

Believe me, if RF push-pull circuits were so much better than singe ended; They (ham radio guys) wouldn't have abandened them in the early 1950's! They actually used balanced ladder-line to feed their antennas with a single-ended RF amplifier and a balanced-output antenna coupler....so it really wasn't push-pull in the first place! Check the ARRL handbook of the era.

Hank

Proud Mary

Thu Feb 05 2009, 10:41PM

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

Dr Hankenstein wrote ...

Believe me, if RF push-pull circuits were so much better than singe ended; They (ham radio guys) wouldn't have abandened them in the early 1950's! They actually used balanced ladder-line to feed their antennas with a single-ended RF amplifier and a balanced-output antenna coupler....so it really wasn't push-pull in the first place! Check the ARRL handbook of the era.

In fact, the last generation of medium power transmitter valves to have been designed were all designed for push-pull operation (Mullard's famous QQV0 double tetrode series for example only) and continued to be used in new RF designs well into the 1970s.

A necessary condition of RF push-pull operation - but one surprisingly often missed by beginners - is that the circuit requires twice as much RF drive as either a single valve - or two valves in simple parallel - which has obvious implications for oscillator design.

Coronafix

Fri Feb 06 2009, 01:58AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Don't know if this helps, but Tesla had a push pull configuration like this.

Herr Zapp

Fri Feb 06 2009, 02:08AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Coronafix -

As drawn, it looks like the electrode orientation on your RSG would simply short both tank capacitors simultaneously, with no current through the primary coil.

Regards,
Herr zapp

teravolt

Fri Feb 06 2009, 06:36AM

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

Herr Zapp, Coronafix has a good idia, can 2 tubes be done in half bridge? the driving of two half bridge tubes would take a lot of enginearing.

I was also thinking that it may be puasible drive the tubes with half bridge of mosfets and use another type of feed back using electronics eliminating the top pick off coil.

another thing that may be going on is frequency spliting like in a DRSSTC and tuning might be tricky with out a spectrum analizer or a clear undrestang.

I have found that it is pausible to replace the tubes with mosfets and use a standard push pull oscillator driver to make some analisis.

I have made a modified scematic of Steve's famus dual schematic for conversation sake and a schematic of the fet circuit that I was playing with. the mosfet circuit works and oscillates with 20 in and about 2000 out. I believe that the concept would work

Does any body know if there is a quiesant bias voltage for grids that will pramote oscilation in schematic 3?

please feal free to ask questions. N.B.

1233902160 195 FT62684 Test Push Pull Sircuit

1233902160 195 FT62684 Dual 833a Modified 2

1233902160 195 FT62684 Dual 833a Modified 3

Proud Mary

Fri Feb 06 2009, 08:46AM

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

It is difficult to know what more can be said in this thread, except to say that any of the basic valve push-pull power oscillator configurations could be made to work well to drive a resonant secondary.

What looks odd to me is the transition from a symmetrical push-pull driven primary to an unbalanced resonant secondary.

I am most certainly not a Tesla coil designer, but I should have thought the secondary should have an earthed centre tap, so the impedance presented to the driver primaries remains in balance. The full potential would then be developed across the opposing ends of the secondary, but with only half that in relation to Earth, a distinct advantage from an insulation point of view.

Herr Zapp

Fri Feb 06 2009, 06:17PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Harry -

I think you have raised a very interesting point here about the ability of a push-pull driver with "symmetrical" primary to drive a unipolar secondary.

I suspect that both Dr. Spark's and Dr. Hankenstein's push-pull VTTC experiments utilized a (more or less) symmetrical center-tapped primary driving a vertical, grounded-base secondary. Each "leg" of the primary coil may have have ended up with significantly different coupling to secondary, capacitance to ground, etc.

I think it would be interesting to look at the design of a horizontal bipolar VTTC, possibly with the secondary split into two sections that could independently be moved axially to adjust pri-sec coupling.

This could provide a truly symmetrical, "balanced" design with mirror-image inductance, capacitance and coupling characteristics for both halves of the drive circuit.

Regards,
Herr Zapp

Coronafix

Sat Feb 07 2009, 12:27AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Herr Zapp wrote ...

Coronafix -

As drawn, it looks like the electrode orientation on your RSG would simply short both tank capacitors simultaneously, with no current through the primary coil.

Regards,
Herr zapp

As noted this was Tesla's circuit, not mine. He also had a "regulator" between the RSG and primary, which I guess is a choke. Aug 17 1899

Dr. Drone

Sat Feb 07 2009, 03:06AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

teravolt

Sat Feb 07 2009, 06:42AM

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

Herr Zapp it sounds like you are describing a half wave VTTC witch sounds like a fun project, I may concider it. If it works with mosfets it should work with tubes. I just have to do some more expierimenting. I chose this epieriment not because it might be hard but because I think it has not been done. I may fail and come to the same conclusions, I may not.

can any body give me an idia about a tickler coil thanks

as a side conversation can any body tell me why sword like sparks only come from VTTC's and some SSTC's

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