vacuum tube power oscillators and tesla coils a...

Vacuum Tube Power Oscillators

and Tesla Coils

A Historical Perspective

ACKNOWLEDGEMENTS:

-Alex Tajnsekfor Shop resources and hospitality

-John Freaufor review of previous work

-Tesla Coil Mailing List for incredible technical (and

historical resource)

WARNING: This presentation is for entertainment

purposes only. The processes, HIGH VOLTAGES,

equipment discussed ARE LETHALand HIGHLY

DANGEROUS!!!


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I.Introduction, Goals

II.Patent Search, Review of Technology History / State of

the Art

III.Summary of Super Power Vacuum Tube Power Oscillator

requirements, goals, and similarities to VTTC’s

IV.Conclusions and learnings


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I.Introduction, Goals -1

�Review the history of Vacuum Tube Power Oscillators (VTPO) via

issued patents and public records (published articles).

�Compare to typically used single ended Armstrong oscillator to

determine if “forgotten”weaknesses exist compared to other

oscillator topologies

�Are there any technological advantages to push-pull topology

versus parallel topologies. Are there forgotten technologies?

�Review one of largest push-pull parallel Vacuum Tube Power

Oscillators ever built; and see what information is transferableto

VTTC’s

�Conclusions, opportunities and next steps for high power VTTC’s


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I.Introduction, Goals –2

REVIEW: Power Oscillator Topologies suitable as VTTC drivers:

Armstrong Oscillator (AOTG, AOTP) –Tuned Grid, Tuned Plate

Hartley Oscillator (HOSEF, HOSHF) –Series Fed, Shunt (Parallel) Fed

ColpittsOscillator (COSEF, COSHF) –Series Fed, Shunt (Parallel) Fed

Tuned Plate, Tuned Grid Oscillator (TPTGO)

Non-Tuned Plate, Tuned Grid Oscillator (TNTGO)


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Figure 1 --Armstrong Oscillator, AOTG. Used historically as a regenerative detector.


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Figure 2 –Armstrong/MeissnerOscillator, AOTP. Used historically as preferred VTTC

driver.


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Armstrong Oscillator Push-Pull, Tuned Plate

Figure 3 –Push Pull Armstrong Oscillator. Used occasionally for VTTC drivers.


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Armstrong Oscillator Notes1:

1. Developed by Edwin Armstrong, oldest VT oscillator, patent 1,113,149

awarded 10/6/1914.

2. Originally developed as a “Wireless Receiving System”. First disclosure

of the Regenerative Receiver, with tuned grid topology.

3. Tuned plate variant was independently developed by Alexander Meissner

(Germany –German patent DE291604) at approximately same time

during World War I, for RF power generation (transmitters).

4. Due to mutual inductance (k) between tickler and tank coils, oscillator

frequency stability suffers, oscillatorfeedback is touchy, and the oscillator

can be overloaded. But the Meissner/AOTP is most frequently used

topology for single ended VTTC’s.

1http://en.wikipedia.org/wiki/Armstrong_oscillator


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Hartley Oscillator (HOSEF) –Series Fed

Figure 4 --Hartley Oscillator, HOSEF. Used historically in first generation VT

Induction Heating Generators (Fo<= 1.0 MHz).


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Hartley Oscillator (PPSEF) –Push Pull Series Fed

Figure 5 --Hartley Oscillator, PPSEF. Used historically in first generation VT

Induction Heating Generators (Fo<= 1.0 MHz).


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Hartley Oscillator (HOSHF) –Shunt Fed

Figure 6 --Hartley Oscillator, HOSHF. Used historically in second generation VT

Induction Heating Generators and medical applications (Diathermy).


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Hartley Oscillator (PPSHF) –Push Pull Shunt Fed

Figure 7 --Hartley Oscillator, PPSHF. Used historically in second generation high power VT Induction

Heating Generators and medical applications (Diathermy). Start of IH industry.


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Hartley Power Oscillator Notes2:

1. Developed by Ralph V. L. Hartley, Western Electric, patent 1,356,763

awarded 10/26/1920. Used originally for early radiotelephone (AM,SSB)

Transatlantic communications.

2. Extensively used on all broadcast bands as a Local Oscillator for

SuperheterodyneIF conversion.

3. Also widely used in single ended and push-pull shunt fed variants for

Induction Heating RF power generators, medical diathermy applications,

early particle accelerators (particle kickers, cyclotron ‘Dee’180 deg

phased drivers), and illumination (UV, neon signs) applications.

4. More coherent frequency stability, more reliable, easier frequency

adjustment and circuit optimization then an Armstrong oscillator.

2http://en.wikipedia.org/wiki/Hartley_oscillator


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Colpittsoscillator (COSHF) –Shunt Fed

Figure 8 –ColpittsOscillator, higher frequency stability then Armstrong or Hartley Oscillator


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Colpittsoscillator (PPSHF) –Push Pull Shunt Fed

Figure 9 –ColpittsPush pull Shunt Fed Oscillator, higher frequency stability thenArmstrong or

Hartley Oscillator


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ColpittsPower Oscillator Notes3:

1. Developed by Edwin H. Colpitts, Western Electric, patent 1,624,537

awarded 4/12/1927. Reliable, only requires 1 tank coil.

2. Electrical dual of Hartley oscillator; series shunt fedand push pull variants

are possible, shunt fed most frequentlyseen.

3. Also widely used in single ended and push-pull shunt fed variants for

Induction Heating RF power generators, medical diathermy applications,

early particle accelerators (particle kickers, cyclotron ‘Dee’180 deg

phased drivers), HV multiplier applications.

4. More coherent frequency stability, more reliable, easier frequency

adjustment and circuit optimization then an Armstrong oscillator.

3http://en.wikipedia.org/wiki/Colpitts_oscillator


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Tuned plate, tuned grid / Tuned grid non-tuned plate oscillator

Figure 10 --Tuned plate, tuned grid oscillator. Developed for improved frequency and power

generation performance, feedback via inter-electrode capacitance (Cp-g) of vacuum tube.


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Push-pull tuned grid non-tuned plate oscillator

Figure 11 –Push-pull tuned grid, non-tuned plate oscillator. Used by D. H. Sloan for high power

(200kW) X-Ray Generator; Patent No. 2,009,458. Resonator provides signal feedback.


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Tuned Plate, Tuned Grid/ Tuned Grid, Non-tuned Plate Oscillator Notes:

1. Developed by F. B. Llewellyn, patent 1,976,570 awarded 10/9/1935.

2. Significantly improved frequency stability and power performance above

and beyond Colpitts, or Hartley Oscillator.

3. Non-tuned plate allows (requires) tight coupling to resonant load.

Another advantage is no tuning capacitors needed on primary (drive) coil,

resonant load provides feedback to anodes and via inter-electrode

capacitance, grid drive to tubes.

4. Used by D. H Sloan for high power X-Ray generator, was awarded two

US Utility Patents on application 2,009,457 and 2,009,458. At least two

X-Ray generators and cyclotron ‘Dee’drivers were built using this

technology.

5.This circuit design “may”be the optimal design for large high power

VTTC’s. An auxiliary bias (C-) supply will be necessary to limit anode

current if oscillator grid-leak bias is lost.


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II.Patent Search Review of Technology History / State of

the Art -1

�Review started with push-pull power oscillators, then expanded to

all VT power oscillators

�Object was the determine “family tree/timeline”of VT power

oscillator development

�Secondary object was to determine if forgotten or non-

commercialized patents exist that could be exploited today,

especially in allied fields of early pulse power (RADAR)

�Determination of strengths and weaknesses of various VTPO’s

topologies, and controlling methods


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the Art -2

�57 “Key Patents”were found with well over 150 additional patents

either cited or referenced to the Key patents; dated from 1914 to

2004.

�Utility Patents for VTPO’swere in broad categories:

�Radio (RF generation, UHF generation, cavity / resonant line oscillators,

modulation)

�HV generation (X-Rays, linear accelerators, pulse power)

�Induction (magnetic field—conducting/magnetic material) heating

�Dielectric (electric field—semiconducting/insulating material) heating

�Illumination (early predecessor of electronic neon light drivers, UV

sterilization, early gas laser drivers)

�Diathermy (RF interaction with body, medical applications)

�Plasma Generation (ICP Torches-spectroscopy, Hazmat material destruction)

�Controlling Means (duty cycle, staccato, keying, power controls)


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the Art -3

�Selected Historical “Luminaries”to the state of art who were

awarded patents

�David H. Sloan –

2009457, 2009458; HV resonator for particle acceleration

and X-Ray generation

�Dr. Eugene Mittelmann–2324525, 2490081, 2550584, 2545296 et al;

Induction / Dielectric Heating pioneer, IEEE considered “Father of Modern

Industrial Electronics”

�William Eitel, Jack McCullough –2026874; Oscillator Keying circuit, EIMAC

founders


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the Art -4

�Hartley and ColpittsOscillators were preferred oscillator topology

over Armstrong and cited most frequently as the “Preferred

Embodiment”

�Push-pull topology was listed as the preferred high power method

in well over 50% of patents and suitable alternative in the

remainder of patents. A circuit topology that was conspicuously

missing was parallel connection in any oscillator configuration.

Any oscillators cited utilizing two tubes was push-pull.

�One shortcoming listed with push-pull oscillators was difficulty in

self-starting, usually seen with high mu (“zero-bias”) vacuum

tubes. Several patents noted to correct this shortcoming in push-

pull configuration, by forcing an offset starting bias on one tube.

Author has not seen this problem with 3 push-pull VTTC’sbuilt

(0.2 -2kVA).


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the Art –5

�A Forgotten Technology / patent found was an early UHF push-pull

power oscillator, using a spark gap and storage cap in the anode

(B+) supply to create a UHF high power “burst or chirp”.

Technology was being developed for possible use in early RADAR

systems (pre-World War II). This concept may have use in high

power VTTC applications.

�Modulating the VT anode may be preferable for Staccato Controls,

since grid leak and filament cathode circuits would remain ground

referenced. Was sited in several patents that modulating cathode

circuit could lead to VT failure, due to HV arc-over to ground via

grid circuits. Grid is weak link in a vacuum tube and can be easily

destroyed by an internal arc.


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the Art -6

�Push-pull power oscillators most frequently referenced in

medical

(diathermy), UHF power

generation,

particle

acceleration/X-Ray generation, and high power industrial

heating (dielectric/induction) applications. Push-pull shunt fed

Hartley oscillators appeared more frequently in induction

heating or low frequency applications (100-1000kHz), whereas

pull-pull shunt fed colpitts

oscillators were more often

employed in high frequency service to UHF.

�Shunt feed topologies are frequently used for medical and

heating applications, so work coil or heating applicators can be

ground referenced; also limits damage in event of a tank coil

to link flash-over (RF versus RF + AC or DC power

component)


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the Art -7

�NOTE: All Armstrong Oscillators are series fed; there is no

circuit duality possible for a shunt fed Armstrong oscillator!

If a flashover occurs between the tank and tickler or load

coils with power follow through in an Armstrong Oscillator

will result in extensive thermal / arcing damage!

More dangerously, HV power supply pote

ntials could

appear

on

load

circuitry,

which

is

tota

lly

unaccepta

ble

in

medical

or

industrial

heating

applications!

Based on above revelations, using a Armstrong oscillator in

VTTC applications could be risky, even with previous

“successful application”by numerous experimenters!


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III.

Summary of Super Power VTPO, and similarities to VTTC’s-1

�Jan 1971 –NASA is performing research into heat transfer problems

associated with the nuclear air engine program.

�Rather then deal with radiological materials, an extremely large, high

power inductively coupled plasma heater was needed to heat gases.

Target power input was 4,000kW, operational frequency range 10 -50kHz.

�Arnold Engineering Development Center had a 19kV, 900A (30,000kW)

three phase input regulated DC power supply with adjustable auto-tap

transformer to grid.

�So this site was selected to house the new VTPO power supply and

supporting infrastructure.

�Development fast tracked (18 months), had to be proven technology.

�Alternative solid-state technology was considered (SCR/Transistors) but

were frequency restricted (3 -10kHz), and unable to support desired

operational range. Also large high power high voltage components did not

exist. Transistor part counts would have been in thousands (series

parallel arrays), with very poor MTBF’s. So VT technology was chosen.


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III.


�First task, what circuit topology and tube type should be

used. See matrix below:


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III.


�ML-7482 (MachlettLaboratories, Raytheon, others)

http://www.mif.pg.gda.pl/homepages/frank/sheets/026/7/7482.pdf

�Filaments 14.5VAC, 450A (1.2kA max, cold inrush)

�Amplification Factor –45, Vapor cooled

�Anode Voltage (maximum) –20kVDC

�Anode Current (maximum) –30A

�Final configuration

Push-pull colpittsoscillator stages (2 tubes) x 4 stages driving

tank load in tandem via striplines. Maximum total anode

dissipation is 3,520 kW (440kW X 8). Useful output to tank (ICP

heater) is 3,200kW. Block tuned by fixed capacitors over a

range from 10-50kHz, and reduced output to 100kHz.

Tank Q

nominal= 14, maximum tuning reactance nearly 50MVAR.


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III.


�Simplified schematic (one push-pull stage)


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III.


�Factoids

�Tank output voltage V rfrms~ 1.2*VdcInput

�Coupling Capacitors sized at ~20 -50X tank capacitance, WVDC @ 1.5 Vdc

�Plate Chokes sized at ~50 –100X tank inductance, BIL voltage

2.5-3X Vdc

�Grid chokes used were arbitrarily chosen as 100mH, ‘C’core construction

(1 per push pull stage)

�Tank capacitors sized as required for resonant frequency, WVDC 2.5 X Vdc

up to 50 MVAR tank capacitance needed to frequency andPF correction

�Itank~ 1.4 kA @ 24 kV rfrms.


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III.


�Factoids

�Push-pull chosen due to higher tank drive voltage, 2ndharmonic

neutralization, simpler/improved cathode current balancing, reduced tank

capacitor MVAR requirements, and high power. Parts count between

single-ended parallel and push-pull are virtually the same (no cost / design

penalty)

�Single ended parallel designs are difficult to dynamically current balance,

and can self stimulate (push-pull, push-push modes) UHF oscillations at

these power levels that can be highly destructive and dangerous.Push-pull

cancels 2ndorder harmonics during operation (simulation validated).

�First large scale Vacuum Tube Power Oscillator System that was computer

simulated in Fortran using early form of P-Spice, prior to construction

(validation and verification of performance).

�Specialized High Voltage training required with specific shutdown and

grounding sequences prior to maintenance or system startup. Tank caps

store SIGNIFCANT energies.

�Highest power VTPO found in literature, with supporting documentation.


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IV.

Conclusions

�One of the largest Push-pull VTPO ever built was successfully designed,

assembled, commissioned and operated at NASA Arnold Engineering

Development Center, Arnold Air Force Station, Tullahoma, TN.

�System was assembled and commissioned in 1972-74 timeframe, and

operated at least to 1976 (and may still be in service)

�System demonstrated the efficacy of push-pull shunt fed Colpitts

Power

Oscillator in low frequency (10 –50kHz, extended to 100kHz) operation, at

design power levels (to load) up to 3,200kW. Grid to load overall efficiencies

observed was 70-75%.

�Push-pull has an extensive historical lineage, and is preferred by scientific,

industrial and legal (patent) documentation for high power and /or high

frequency applications.

�This technology can and has been used for VTTC applications. The author has

built (1) 200W Armstrong, and (3) 600W, 1400W and 2200W Hartley PP

VTPO’sand all functioned well.


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QUESTIONS??

THANK YOU!

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