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Re: Tube coil ideas
Edwin,
This is yet another example that there is no free lunch. Your elaborate
scheme reduces the power waste in the output stage but merely redirects it
in powering all those added driver stages. Overall wallplug efficiency
would not be much improved if at all. Some of us have been experimenting
with pulsing the oscillator, still a self feedback driven configuration
with simple outboard means. These include a solid state, vacuum tube, or
thyratron switch in the cathode circuit of the oscillator tube, or pulsing
the screen grid of the oscillator (in the case of a tetrode). Of course
you can pulse modulate the B+ source as well.
The discharge characteristics obtained from a vac tube driven TC change
markedly as one goes from a true CW mode of oscillation to pulse mode with
narrower and narrower actual on-time in the duty cycle. The case where a
tube is pulsed for a very short pulse period followed by a long rest period
curiously enough makes a noisy discharge which closely mimics that seen
from a disruptive spark gap driven coil system.
A 100% duty cycle CW discharge looks like a gas burner flame with a ragged
hiss or rushing sound.
Many versions of tube TC's are possible, and many results can be obtained.
The best design is that which produces the output characteristics desired
by the builder. If you are after the longest air streamers versus system
cost or complexity, forget tubes and go for a spark gap driven TC.
Don't let my own opinion sway you away from building and experimenting what
you suggested however. Your hands might be smarter than mine.
Robert W. Stephens
Director
AREA31 Research Facility
www.area31-dot-org
----- Original Message -----
From: Tesla List
To: tesla-at-pupman-dot-com
Sent: Sunday, February 13, 2000 13:37
Subject: Tube coil ideas
Original Poster: "Edwin G. Buttell" <edd.b-at-snet-dot-net>
Tesla List wrote:
> Original Poster: "David Trimmell" <davidt-at-pond-dot-net>
> Problem is the ARRL Handbook (although, invaluable), and all "radio" theory
> goes out the door when designing a "tube coil"... At least that the way it
> seems to me, a "novice".
Edd.B writes;
Let's see if I can start a "new" thread...........
Just about all of the tube coil designs use the same very old
close coupled / grid feedback oscillator with a large triode
in class B service. The efficiency of a typical class B circuit
is about 60% to 65% ( power in / power out ).
An 833-A triode with cooling ( fan or blower ) has a plate
dissipation of 450 watts. With 3000 Vdc at 400 Ma ( .4 Amp )
power input is 1200 Watts, at 60% eff the power output is
720 Watts. 1200 - 720 = 480 Watts plate dissipation, this
over the max dissipation and the tube will run red hot with
a shorter life expectancy.
An NST design with a 15Kv/30Ma transformer has a 450 watt
output and will have a much better spark discharge, much longer
and thicker sparks from less power.
I think we all know that, But ..... WHY.....
The reason is ( I think ??? ) the NST circuit is a pulse
discharge circuit, with a very powerful pulse and oscillatory
ringing in a classic tuned circuit.
The tube circuit has a sign wave output not a pulse. After
all it's nothing more than an old fashion "ticker coil" design.
So what would happen if we pulsed our 833-A example ???.
A variable oscillator with a saw/tooth output from ... say
50Kc to 200Kc driving a voltage amp ( 6L6 ) biased AB1 to a
power amp ( 2 / 6L6 ) biased Class C .... this is how we change
from a saw/tooth to a pulse. Next is a driver stage having
about 25 or 30 watts output, an 807 in class C should do it.
It's not that complicated, it would be all tubes, nothing
small and modern to burn up, no tuned circuits in the amp
or driver and with the 50% duty cycle that we obtained by
pulsing the 833-A ( which is now biased class C ) a higher
input power can be used without exceeding the max plate
dissipation. The last benefit but certainly not the least
is the efficiency of the pulsed 833-A , it will be or should
be 90% or better.
Comments anyone ??????
Edd.B