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Re: efficiency of spark gaps vs tubes
---------- Forwarded message ----------
Date: Mon, 6 Oct 1997 10:36:44 -0500
From: Geoff Schecht <geoffs-at-onr-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: efficiency of spark gaps vs tubes
Guess I'd better qualify my statement (or opinion). I don't have a set of
curves available for an 811A, 4-125, 250TL or other medium-power
transmitting tube handy but I agree that there's little hope of those tubes
saturating at the low voltages quoted for a spark gap. Many gases sustain
conduction at surprisingly low values (like 70V for neon, air I can't begin
to guess about) so spark gaps certainly wouldn't be lossy compared to a
hard-vacuum tube. Thyratrons should also be fairly low-loss switches in TC
service since they're gas-filled devices. However, if you're aiming for
continuous, coherent power transfer at a fixed amplitude (i.e. single
frequency CW operation), a spark gap or thyratron isn't the way to go. Even
though the tank circuit certainly reduces the harmonic content of the
discharge energy as a function of the circuit Q, you necessarily wind up
with pulsed-mode operation at a relatively low repetition rate.
Sorry for my mis-statement as far as pure efficiency considerations go. I
am curious about something, though: aren't the higher frequency TC's
generally tube-driven? The few articles I recall seeing that used tubes to
drive TC's were generally operated in the Mhz region. Were MHz range, CW
TC's in vogue at one time (1960's) due to the wide availability of surplus
power tubes? One article pointed out that the sparks from an HF (13MHz ISM
Band ?) tube-driven Tesla coil had some striking properties (compared to SG
units) such as spectacular spark penetration through thick sheets of
plexiglass.
I would think that higher frequency TC's would be better for broadcast
power demos and you wouldn't get into hot water with the FCC as long as you
stayed in one of the ISM bands (speculation on my part as far as legality
is concerned!!).
Since we're on the topic. I have a question as far as SG efficiency goes:
how does a spark gap's efficiency vary with frequency? Tubes have
relatively constant efficiencies for a given class of operation until
they're operated near their cutoff frequencies due to transit times,
parasitic reactive elements, etc. What's the story with the plasma in a
spark gap (any references to literature, perhaps)?
Geoff
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: efficiency of spark gaps vs tubes
> Date: Friday, October 03, 1997 9:24 PM
>
>
>
> ---------- Forwarded message ----------
> Date: Fri, 3 Oct 1997 08:57:18 -0700
> From: Jim Lux <jimlux-at-earthlink-dot-net>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: efficiency of spark gaps vs tubes
>
> Geoff Schecht made a comment to the effect that tubes were more efficient
> than spark gaps. I made the same comment to James Corum and he pointed
out
> that really wasn't the case. With a Class C tube amplifier, the best
> practical efficiency is around 65-70%, that is, the switch (i.e. the
tube)
> is dissipating 30% of the DC input power. In a spark gap, the switch is
of
> very low resistance, and dissipates very little power, and with a low
duty
> cycle to boot. For a short gap in air, the voltage drop is going to be
> around 50 volts, so on a 15 kW coil with the primary at 15 kV, you are
only
> dissipating, at most, 50 Watts in the gap. This is 99+% efficiency. I
> suspect that the inefficiency in a typical tesla coil (i.e. Power
out/Power
> in)( considering that the arcs and corona is desirable power out) is due
to
> the loss resistance of the windings (pri and secondary) (i.e. you have a
> finite Q) and some heating of the primary tank cap.
>
> ----------
> > From: Tesla List <tesla-at-pupman-dot-com>
> > To: tesla-at-pupman-dot-com
> > Subject: Re: And what of the FCC?
> > Date: Thursday, October 02, 1997 8:44 PM
> >
> >
> >
> > ---------- Forwarded message ----------
> > Date: Thu, 2 Oct 1997 20:11:13 -0500
> > From: Geoff Schecht <geoffs-at-onr-dot-com>
> > To: Tesla List <tesla-at-pupman-dot-com>
> > Subject: Re: And what of the FCC?
> >
> > <snip>
> > >
> > > All of these posts regarding RFI etc have caused me to wonder:
> > > What is the difference between a rotary gap TC and the prehistoric
> > > rotary gap CW transmitters? Is it just a matter of tuning a TC to
> > > deal with an antenna instead of a toroid or sphere as the "load"?
> > > I have heard recordings of these signals from way back when, and I
<SNIP>
> > > I dont know much about sparkgap transmitters, but I am pretty sure
they
> > > contained no active device, if you will, such as a vacuum tube.... so
> > > mustn't they have been pretty much a TC? They were about the same
> freqs,
> > > as I recall.
> > > BTW, 500kHz is still very much an int'l distress frequency, the last
I
> > > heard. More of a ship freq than anything else, to my knowledge. So is
> > > 2182 kHz. Correct me if I am wrong tho.
> > > Randy
> > >
> >
> > Randy:
> >
> > The idea behind a sparkgap (or any other radio transmitter) is to
> > efficiently couple the RF energy from a source to an antenna. The
antenna
> > is then supposed to transfer this energy into free space. Sparkgap
<SNIP>
> > I've read about SG's being used as the power supplies in some
diathermy
> > machines and induction hardening apparatus at least through the Second
> > World War. Vacuum tubes were pretty much 100% war-critical materiel and
> > sparkgap RF generators didn't tie up those valuable devices when they
> were
> > needed the most. Sparkgaps aren't particularly efficient but they're
> cheap,
> > easy to build and they don't produce that much interference as long as
> > they're not attached to antennas.
> >
> > Anyway, sparkgaps and Tesla coils are inextricably linked. The old
> Russian
> > Woodpecker was a pulsed (I believe,spread-spectrum) OTH radar that was
> used
> > to detect missile launches in the Western Hemisphere. It certainly
didn't
> > employ a spark gap.
> >
> > Geoff Schecht
> >
>