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Re: [TCML] Maximum arc length from 700w VTTC?


Yes when the doubler is added, retuning is needed.  If you
do not have a tap-able primary, then an easy way to re-tune
is to add capacitance to the tank circuit.  You may need to
add as much as 40% more capacitance if you tune that way.
Or you can tap in more primary turns.  If you change the
number of primary turns, that can affect the grid feedback, etc.  
I always make my grid (feedback) coil on a sliding section that
I can slide closer or farther from the primary to get the
best results.  Usually I use at least 1800 ohms for
the grid resistor, and often about 2800 ohms or more.
Of course that all depends on the type of tube, etc.
In one setup for 36" sparks, I used 40,000 ohms for
the grid resistor.  A too-high grid resistor can cause
problems however.  You don't want the grid voltage
to go higher than the plate voltage.

My old (2) 833A tube coil gave 24" sparks using
2400VA without staccato.

BTW, the tuning of a VTTC is more critical than it
might seem at first.  Proper tuning often won't increase
the spark length very much but it will greatly reduce
the input power draw.  If your tube plates get red, then
the degree of redness can be used to judge the tuning.
With proper tuning, the coil should give maximum
spark length with minimal plate redness.  Tuning a
VTTC is similar to tuning a radio transmitter.  You can
also monitor the input power draw as you're planning 
to do, as a way of judging proper tuning.  Also final
tuning must be done at full power.  A coil that is 
tuned for full power/full spark length, will actually
produce very short sparks when run at a low power
input.  Tuning for best results at a low power will never 
give a good result at high power.  (I'm assuming your
controlling your input power using a variac.)





-----Original Message-----
From: William Howard <snakeprior@xxxxxxxxx>
To: tesla <tesla@xxxxxxxxxx>
Sent: Mon, May 19, 2014 8:56 pm
Subject: Re: [TCML] Maximum arc length from 700w VTTC?

Dear John,
Thanks for the explanation of my questions, I see that alot in the design
is there to protect the tube.

I hooked the coil up last night with a voltage doubler and got slightly
longer arcs but much hotter and louder. It kept overheating my diodes.

So I agree it must be more than 700w. I will increase my grid leak from 180
ohms to 500 or more and focus on tuning to get more spark length instead of

That is until my new mot transformers arrive and I use a battery powered
rectifier tube in the doubler!

But I am more satisfied than before now probably because fear has crept in

A guy on youtube made 50cm sparks with 2000va by putting 600v on the screen
I think this will be the next mod.

I'm going to put a power meter on the mot soon and check the current draw.

I would like a stable coil I can run for 30 mins but understand its quite

Thanks again.

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