Re: Toob coil questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi John,

At 08:25 AM 9/19/2003, you wrote:
>Terry,
>
>I'm glad to hear that you're doing some tube coil work.
>Most folks let the 811A plates get red-orange or so, although the
>specification is for it to be barely red for intermittent use.  The thinking
>of a tube coiler is usually that it's OK if the tube lasts just 500 hours
>instead of 5000 hours.  Since the builder used a voltage doubler (it appears),
>the tube is being run way over its ratings, but that's the tradition.

The DC plate voltage is about 1500VDC with another 1500 VAC riding on 
it.  Indeed, way over the rating but the tubes don't seem to mind.  When I 
blow these up, I will definitely want to go to 572B types >:-))  The plates 
run a little red but no problem.


>Inductive resistors work fine.  I too prefer a variable resister to make
>it easy to make adjustments.  A range from 1500 ohms to 12k ohms
>or even 20k ohms is a good range.  Usually 2500 ohms to 5k ohms is
>what's needed for most coils.

Mine has 8.1k ohms now but the three 10 watt resistors see 35 watts, so 
they get really hot.  They are mounted on wood with a Teflon insulator 
which is stressful.  I could not find a nice say 100 watt 10k variable 
power type in DigiKey.  Big power resistors seem to go only to 1k ohms 
since the high voltages needed for higher powers in high value resistors 
are rare.  The grid current is ~75mA total.  I will have to keep looking 
for a big high value variable.


>I like to be sure the heater voltage is either exactly correct, or perhaps
>a little high, but not more than 5% high as measured at the tube socket.
>Some of these tubes don't need much heat-up time, but I like to let
>them heat for  30 seconds or a minute.  When selecting a tube for a tube
>coil, I generally select a tube which is rated for a high voltage.  811A's
>and 572B's are rated for 1200VDC and 2500VDC respectively I think.
>833A's are rated for 4000VDC.

Svetlana says 6.0 to 6.6 Volts.  I think I am ok there.  If I go to 572Bs 
the plate voltage will be "sort of" near the ratings :o))


>The breakout point or spike on top of the coil should be  1) physically
>sharp and  2) high enough above the toroid.  If the spike is too short,
>the toroid shields it, and prevents breakout.  If the spike is rounded,
>it can't emit an ion stream to initiate breakout.  It should be sharpened
>with a grinding wheel so it's very sharp.  I'd try this before raising it.
>As the coil runs, the spike will get dull again and have to be resharpened.
>Some folks use a tungsten tip so it tends to stay sharp longer.
>A fluorescent tube held nearby will show that the coil is oscillating.
>How the coil breaks out also depends on how the coil is tuned.  It can
>be tuned to break out more easily, but then it will give shorter sparks.
>Basically it's being tuned to account for streamer capacitance for
>longest sparks.

It has a tungsten tip that was a little dull.  I sharpened it and that 
seemed to help a lot.  It may be a little low into the toroid too.



>A big factor with tube coils is impedance.  They generally run in a
>much higher impedance mode than SSTC's.  Many of the coils are not
>optimized.  The high impedance of the tube coils means that they
>cannot break-out well without a breakout point.

Interesting!


>I use the formula
>
>   spark length (inches) = 0.5*sqrt input power (watts)
>
>for non-staccato tube coils.

I have about 1000 watts input so...  15 inches is right on.  Whoever made 
this coil (I don't know who the original builder was) seemed to know 
exactly what they were doing ;-))  I found very little that needed 
adjusting.  I mostly just beefed up some mechanics so it could ride in the 
car easily.


>In any tube coil, tank impedance is the key to efficiency.  The
>tank impedance must be within a certain range, based on the
>operating parameters of the tube and other factors.  Generally
>speaking if the tank impedance is too low, the coil will draw
>heavy current, the tube will get red, but the sparks will be short.
>If the tank impedance is too high, the coil will not draw enough
>power, the tube plate will stay cool, and the sparks will be short.
>Unlike a spark gap coil, it's not enough to simply have a coil that's
>in tune.  The tank impedance must also be matched to the overall
>system.  Tank impedance is one of the factors that controls the
>current draw in a tube coil.  There's a range of tank impedances
>which generally work OK for a given coil.  Grid-leak network
>parameters also control the current draw.

Interesting.  Computer modeling may be able to help optimize such things 
along with some measurements.  Tube coils are pretty steady so impedances 
are far more stable and predictable.

We had great fun with it at the Tesla meet.  We ran an inverted jar over to 
top filled with argon that was really cool!!

>John

Many thanks for your, and others, thoughts!!  I am getting it pretty well 
figured out now ;-)

Cheers,

         Terry