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Tube Tesla Impedance



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> From: Tesla List <tesla-at-poodle.pupman-dot-com>
> To: Tesla-list-subscribers-at-poodle.pupman-dot-com
> Subject: Re: Tubes
> Date: Thursday, January 23, 1997 12:25 AM
> 
> > Subject: Re: Tubes
> 
> Subscriber: unknown-at-apc-dot-net Wed Jan 22 21:37:36 1997
> Date: Wed, 22 Jan 1997 02:02:47 -0800
> From: Open Minded <unknown-at-apc-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Re: Tubes
> 
> Tesla List wrote:
> > 
> > Subscriber: FutureT-at-aol-dot-com Tue Jan 21 23:10:35 1997
> > Date: Tue, 21 Jan 1997 16:50:44 -0500 (EST)
> > From: FutureT-at-aol-dot-com
> > To: tesla-at-pupman-dot-com
> > Subject: Re: Tubes
> > 
> > > Stephen,
> > 
> > >I am not at all familiar with the nomenclature ECC803S, but I was
> > >able to look up a type 803 transmitting tube in the RCA databook.
> >  >Although this tube is a pentode, at 2000 volt plate rating and 125
> > > watt plate dissipation it is a heck of a lot of tube for $15,
> > >assuming of course that what you have found is truly an 803.  If it
> > >is, I'd get one and try it.  Tie all three grids together and pretend
> > > it is a triode, might work.  The filament on the 803 is 10 volts at 5
> > > amps.  The tube has a plate cap and a giant 5 pin base (same as a
> > >4-125A/4D21, with a straight sided diameter of 2-9/16" and an overall
> > >length of 9-1/4".
> > 
> > >snip
> > >rwstephens
> >   >>
> > 
> > Steven, Robert,
> > 
> > I also do not know if the ECC803S is the same tube as an 803.  I used
an 803
> > in a tube coil, (found 3 of them at hamfest for $3 each),  I connected
a 15k
> > resistor from the high voltage supply to the screen grid, and a 45k
resistor
> > from the screen grid to the suppressor grid, the coil gave a 5 to 6"
spark.
> >  By the way,  an 845 tube gave a 7" spark, and a 304TL tube gave a 10"
spark
> > in the same tube coil, which happened to be powered by a microwave oven
> > transformer.
> > 
> > John Freau
> 
> 
> John -
> 
>   I just had a flash(?) of inspiration - some fuzzy part of my brain
> started screaming "load impedance". Perhaps the 304TL gave maximum
> bang due to tube's lower internal resistance???? (plate volt/current)
> 
>   I recall that there is a bit of an art to matching the plate load
> impedance to the tube's internal resistance. Too high a load impedance
> and you don't get full poop. Too low a load impedance causes the same
> thing, only the excess energy goes into heating the plate in the
> tube. This might explain the one post about how the tubes ran nice and
> cool with the incandescent (resistive) load and really warmed up with
> the Tesla tank circuit (*highly* reactive.)
> 
> - Brent

Let's Hear It for Brent's Fuzzy Brain Cells! YES! When running an
incandescent or
purely RESISTIVE Load, the frequency is basically Inconsequential. BUT a
Tesla Tank Circuit in a Tube Type Tesla Coil is generally A Parallel Coil
and Capacitor. At RESONANCE the overall Impedance (resistance) of the
CIRCUIT is
MINIMUM. Off-Resonant Frequency On EITHER SIDE results in a DRASTIC
DECREASE In Overall Impedance. This means INCREASED CURRENT. Since Power is
a function of Current (P=E*I  -or-  P=I*I*R), Increased Current Results in
Increased Power CONSUMED (and therefore DELIVERED). How Drastic? Depends
on the Q of the circuit. The Greater the Q, the more Drastic the
Difference, because the Resonance Curve becomes Narrower and Steeper with
higher Q. 

There is a Simple Trick you can use to help prevent you from Destroying
your Tubes as you TRY to find Resonance. Include a RESISTOR in Series with
The Paralleled Coil/Capacitor Tank Circuit. Some Old Timers used Light
Bulbs as this Resistor, so they could SEE the effects of tuning. (High
Power Ham Radios and Tube Tesla Coils are Kissin' Cousins). Have a Good
Heavy Duty (INSULATED!) Switch connected in Parallel with the
Resistor/Light Bulb. Open the Switch. Turn on the Tube Tesla Coil. Wait for
the Heaters to Do Their Stuff, and then TUNE. The Bulb will be DIMMEST at
Resonance. Now CLOSE the Switch! The Tesla Coil will be VERY near Optimum
Resonance. Fine Tune for Perfection.

The More we GO FORWARD, the More Important it is to KNOW OUR PAST. Tesla
Tubers would do well to review OLD Copies of HAM RADIO Books. The OLDER the
Better, because some of the Really Old Articles show some REALLY SIMPLE
Circuits that Work.

As to actually Matching the Impedance of the tube to the circuit, THAT is
One of the Things That You Are Doing when you Adjust the Plate Voltage. And
THAT is really one of the MOST Effective ways to do it. A Variac on the
Primary of the HV Transformer is all it takes. By the way, if you use a
variac you probably won't need the trick mentioned above. Instead, begin
with a LOW plate voltage and adjust for resonance. (Eyeball the Corona at
the Secondary). Once it is Tuned at Low Power, Slowly INCREASE the Plate
Voltage. As you do so, Rock the Fine Tuning Back and Forth, and just keep
the Resonant point in the MIDDLE of your rocking. In this way you can
SAFELY bring your Tube Tesla Coil up to FULL POWER without damaging your
Tubes. The Plates will say "Thank You" with a Warmth that will make you
Smile. If  you find that a Further INCREASE in the Plate Voltage Causes the
Output of the Tesla Coil
to go DOWN **OR** the Tubes get TOO HOT, then Reduce the Plate Voltage to a
SAFE LEVEL.

If all else fails, try something else.
Fr. Tom McGahee