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direction of sec and pri coils

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Herr Zapp

Sat May 22 2010, 06:44PM

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

doctor electrons -

I really don't understand the tone of your responses to my posts; please take a moment to actually read and understand my comments.

803's original question was: "are the coils of the sec and pri and feedback in the same direction?".

The answer is not a simple YES or NO; the real requirement is that regardless of the primary, secondary and feedback (grid) coil's winding direction, the feedback coil must provide a pulse of the correct polarity to the grid to the tube's control grid. If you happen to wind the feedback coil in the "wrong" direction and the coil fails to oscillate, simply reverse the feedback coil's connections and the system should now work, assuming all else is correct. With care, the polarity of the grid voltage can be monitored with an oscilloscope.

Additionally, you wrote: "A ct used for feedback is the only coil where the winding direction has bearing on anything, and only
if the currents are dc."

This is NOT true. While many solid-state TC designs use a current transformer (CT) to generate a low-voltage feedback signal proportional to the current in the tank circuit, or in the secondary's ground lead, VTTCs use a tertiary winding (not a CT) coupled to the secondary to genarate a relatively large voltage to drive the tube's grid.

A condensed statement might be that both SSTCs and VTTCs use feedback coils to generate signals that control primary switching; both these applications are sensitive to the output "polarity" of the feedback coil; SSTC's use current transformers, and VTTCs use potential transformers to generate the control signals; in both applications, simply reversing the leads from the feedback transformer will provide the correct signal polarity if the current transformer is installed "backwards", or if the potential transformer is wound "backwards". Note that commercial CTs designed for pulse applications will generally have a polarity mark (arrow, etc.) on the housing.

Do you find anything objectional or inaccurate in the above? If so, please calmly and rationally state where you disagree, and provide information to support your points.

And Steve - I'll make an honest effort to be less pedantic ("narrowly, stodgily, and often ostentatiously learned"). This pedantry might be a carryover from one of my job responsibilities, which is to generate extremely detailed, highly technical process instructions, status reports, etc. for people who might not be highly technical. This requires that nothing is left to assumption, that every detail, regardless of how self-evident it appears it should be, is carefully explained.

Regards,
Herr Zapp

doctor electrons

Sat May 22 2010, 09:33PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

803 wrote ...

are the coils of the sec and pri and feedback in the same direction? I've seen some in the same dri and others in different dir. i allways thought it was the same direction.:-?

Actually Zapp... He clearly said "sec and pri and feedback" that was his original question, which i answered to the best of my ability.
I did read it. Thats all i am going to say out of respect for steve and the original poster, im over it.

Herr Zapp

Sat May 22 2010, 10:50PM

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

Paul -

Attached is a very informative little presentation ("Vacuum Tube Power Oscillators and Tesla Coils - A Historical Perspective") put together by Dave Sharpe, comparing a number of vacuum-tube oscillator circuits (Armstrong, Hartley, Colpitts, Tuned Plate, etc) and their potential application in Tesla Coils. The presentation deals exclusively with triodes, but there is a lot of useful information here.

Since it sounds like you are already committed to dealing with the extra complexity of providing a bias supply for the screen grids, why not consider using your pair of 803s in a push-pull topology? This adds very slightly more complexity to the feedback coil, but could provide better performance and tube life than simply paralleling your tubes.

Dave's presentation also covers what was probably the largest vacuum-tube power oscillator ever built, a NASA plasma heater with 3.2MW output from eight ML-7482 triodes. Each of these tubes was rated at 440KW plate dissipation, and each tube's filament required 14.5 volts at 450 amps.

Regards,
Herr Zapp
]dave_sharpe_vtpo__vttc.pdf[/file]

803

Sat May 22 2010, 11:14PM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

thank you so much, I will try the push pull, however how is it better than parellel in a vttc?
2. armstrong or harly, what do you think?
3. also which is better shunt fed or series fed?
Best regards,

Paul:-)

Proud Mary

Sat May 22 2010, 11:45PM

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

Herr Zapp wrote ...

Dave's presentation also covers what was probably the largest vacuum-tube power oscillator ever built, a NASA plasma heater with 3.2MW output from eight ML-7482 triodes. Each of these tubes was rated at 440KW plate dissipation, and each tube's filament required 14.5 volts at 450 amps.

Here, out of interest, is a 1.5MW 110GHz gyrotron oscillator valve built in 2003. Bigger gyrotrons are in the offing:

803

Sun May 23 2010, 12:29AM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

also, do the tubes in push pull share the current? Some one from the 50s- 60s please help with your tube lore.:-)

Thanks,

Paul

Herr Zapp

Sun May 23 2010, 12:54AM

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

Paul -

Like everything else, the parallel vs push-pull topology is not a simple choice.

Some characteristics of the parallel circuit:
1. Approximately double the plate current rating of a single tube;
2. Possible difficulties in getting the tubes "matched" so they share the load equally.

Some characteristics of the push-pull circuit:
1. Max current rating is that of a single tube;
2. Potentially better coil output because of higher tank voltage;
3. Tubes run cooler because they conduct alternately.

Here's another of Dave Sharpe's schematics for a push-pull VTTC using 572B triodes, with a means of balancing the grid drive to compensate for differences between tubes, or slight asymmetries in the feedback tap points. Note that the feedback is derived from taps on the primary winding, and not from a separate feedback coil. This might help you layout your tetrode schematic.

Regards,
Herr Zapp
]dave_sharpe_572b_vttc.pdf[/file]

803

Sun May 23 2010, 02:10AM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

what do you mean higher tank voltage? the tubes have 2x the voltage rating?
I think I will first to parell then push pull

Thanks,

Paul

Herr Zapp

Sun May 23 2010, 02:34AM

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

Paul -

No, in the push-pull arrangement the B+ voltage supplied to the tubes is the same value as in the parallel circuit.

In the parallel arrangement, the tubes apply full supply voltage to one end of the primary, and then stop conducting. The primary never "see" anything greater than the B+.

In the push-pull circuit, the B+ is typically applied to a center tap on the primary. Each tube alternately grounds the outboard end of its half of the primary coil, so the primary "sees" an overall voltage "swing" of almost 2X the supply voltage.

Herr Zapp

803

Sun May 23 2010, 12:53PM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

So in a harley, the feedback is part of the primary?

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