looking for a push pull schematic for VTTC / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

looking for a push pull schematic for VTTC

1 2 3 4 next

Move Thread

LAN_403

teravolt

Fri Jan 30 2009, 03:08AM

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

I am looking for a schematic for a class B or AB type (push pull) vacume tube coil. has anybody tried this. I'm thinking that it would work like the standerd VTTC but it would use a resonant primary with a center tap. the feedback winding would also have a center tap. I can draw an idia if any body wants. Thanks Nathan B.

Dr. Drone

Fri Jan 30 2009, 04:17AM

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

Coronafix

Fri Jan 30 2009, 06:15AM

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

Excuse my ignorance Doc, but would it have made any difference if you had wound it bifilar with a centre tap at the base?

Dr. Drone

Fri Jan 30 2009, 08:08AM

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

teravolt

Wed Feb 04 2009, 04:00AM

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

here is a basic scematic that I invented from a tube amp shematic and a a picture of a push pull primary. unlike Dr. Sparks primary mine uses only 10 tuns with a center tap with 12awg teflon wire. At one point I used 2 10000pf micas for the primary capacitors. the problem that I had was getting it to oscillate. should there be a quiescent biasing for the grids to get feedback? the plan would be to get it to work like a dual class C

1233720042 195 FT62684 Tube Test Primary

Proud Mary

Wed Feb 04 2009, 05:09AM

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

Why do you want to run a power oscillator in Class A, or Class AB, when Class C offers so much more efficiency?

If you let go of the vertical (contemporary) Tesla model, you'll see that what you are talking about is a high voltage radio frequency transformer, for which a push-pull driver would usually be the circuit of choice. During the valve era, devices of this kind were sometimes used to generate 100kV or so for charging large Marx generators, and driving industrial X-ray tubes.

I would choose one of the heavy-weight transmitting double tetrodes - a few hundred watts - as a good basis for an experimental model, but you will find making a stiff power supply for even a medium power valve circuit of this kind a very expensive business.

If you were able to give a clearer picture of what you have in mind, it might be possible to help you put together something interesting without having to sell your car to pay for it!

I would call the circuit diagram posted above a skeleton circuit, such as one sees in explanatory text books. None of the detail one would expect to see in an engineering drawing is shown. Nothing is done to protect the valves against premature application of the HT (i.e. a heater delay relay) nor against bias failure, against parasitic oscillation, and no meters are shown in grid and anode circuits, without which the tuning of the circuit would be all but impossible.

Valve Push-pull RF power circuits can be tricky to set up correctly, even for the experienced designer, as there is usually quite a big gap between the theoretical model and its realization as stable, effcient, reliable, engineering.

teravolt

Wed Feb 04 2009, 08:53PM

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

Hi Harry, I wanted to do push pull because using paraleling triods can lead to unequal current share and harmonic oscilation betwean the tubes in class C. I have 3 833c and 2 811h from rf electronics. I was reading that Class AB can be more eficent for amplifiers and I wanted to see if any body has done this. I have not decided to do push pull or not. I have read Steve Ward's VTTC page and when he went to 2 tubes the change was not a big one and he had some power balancing to do. any coments are welcome N.B.

Proud Mary

Wed Feb 04 2009, 09:38PM

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

teravolt wrote ...

Hi Harry, I wanted to do push pull because using paraleling triods can lead to unequal current share and harmonic oscilation betwean the tubes in class C. I have 3 833c and 2 811h from rf electronics. I was reading that Class AB can be more eficent for amplifiers and I wanted to see if any body has done this. I have not decided to do push pull or not. I have read Steve Ward's VTTC page and when he went to 2 tubes the change was not a big one and he had some power balancing to do. any coments are welcome N.B.

Rememeber that valves are high impedance devices, so small differences between them does not cause the runaway "current hogging" that we see with parallel, and slightly unequal semiconductors.

Precautions must be taken against parasitic oscillation whether valves are connected in parallel or in a push-pull configuration. Small chokes of dissimilar values hard up to anode and grid pins are your first line of defence here.

It is true that more harmonics will be radiated by a Class C oscillator unless suitable trap circuitry is used to by-pass unwanted frequencies to Earth.

But unless you place your entire apparatus inside a Faraday carriage you will be radiating considerable amounts of RF energy whatever sort of oscillator you use!

That being said, in a perfectly balanced push-pull oscillator, even order harmonics are, in theory, absent, but in practice will still be present but at low levels compared with those of the odd order.

With the skeleton circuit you have presented above, I would expect it to be quite unstable, the frequency wandering all over the place, with a sort of ragout of assorted RF emissions showing up on the spectrum analyser from LF all the way into the dizzy heights of the VHF.

My suggestion: build this circuit first of all with small power tetrodes, so you can find out how to make it work efficiently, which will certainly involve all sorts of small alterations and additions to the skeleton circuit. Put meters in the anode and grid circuits so you can see what is going on in the circuit. Without them you have only guess work and conjecture.
And why risk the ruin of expensive valves that othewise would have given years of good service?

But I can't do your experiments for you, lad!

Good luck with it!

Dr. Drone

Wed Feb 04 2009, 09:43PM

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

teravolt

Thu Feb 05 2009, 06:27AM

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

Thanks Herry and Dr. Spark for your input. Dr. Spark If you include my experience with teslas it would be 90 years. Herry and Dr. Spark, in a oscillator circuit the thing that creates oscilation is gain and positive feadback. My skeleton circuit is a crude way of getting out an idia. The plan is to feed energy from the secondary to drive the grids as moast VTTC's accept out of phase. Tuning will be a bitch and some expierimenting will be necisary. I have built the aperatus with 811h's and power supplies with mica caps and could not get it to oscillate. Getting it to oscillate properly meens that it may have pretuned to the fundimental of the secondary or lower pol in the event of frequency splitting. Also the Q of the circuit and the way it is built will help. In moast VTTC's the primary uses many turns in the primary but I found that in my later experimination only a few turns like in the picture the harmonics could be reduced. hopfuly I can get some meaning full data to extend this conversation. let me know if my ramblings mean any thing to you Nathan B.

1 2 3 4 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob