Hi Carlos,
Being someone who is only speculating about this take my comments
to follow with a grain of salt. If you get a fast rise time your
sparks will no longer be swords, they will branch like a disruptive
coil, if that is no problem for you, read on. I don't think a tube
will handle a high current surge associated with a spark gap, and if
it did I think it defeats the whole purpose. As I understand it the
oscillations in a valve come from ringing in the tank capacitor and
primary coil, that happen after the tube shuts off, just like in a
fly-back Armstrong Oscillator, and build as more cycles are added
until some kind of "equilibrium" is reached. If you were to put a
spark gap in series with your tank cap and primary, it would need to
trigger right when the tube shuts off, if you are going to do that
then the tube is pointless, it would have to always stay on to charge
your tank cap. You want massive sparks from a vacuum tube system, and
if what you say is true and the reason for small sparks is small rise
time, and rise time cannot be changed without completely redesigning
the tube oscillator which can only really take RMS current and not
pulse current like a spark gap does, methinks the golden ticket in
increasing those rise times would be a much more massive inductance in
your secondary, so maybe you should build a tube magnifier to process
more current? I wish I could be more helpful, I'm sure others will
have real advice.
Scott Bogard.
On 11/16/10, Carlos Van Camp <carlos@xxxxxxxxxxxxxxxxxxxxxx> wrote:
Hi Guys,
Following on from my problem of trying to get longer arcs from my 50kw
valve
coil, I have had an idea that I would like to run by you all...
I have thought about the many suggestions made, and aggree that the key
differences in the way the arcs are generated, between SGTC and VTTC is
the
slower rise time that the secondary coil sees, and the size of the tank
capacitor.
Even with a stacato controller pulsing the cathode of the valve, it still
means that the tank capacitor is starting from "empty" , meaning the
oscillations start small (as the tank gains momentum) and build up to
there
peak...
With a spark gap coil, the opposite occurs.
The tank capacitor charges up to the full break down voltage so when the
gap
breaks over, the oscillations are at a maximum and then ring down to
zero...
Because I want the coil to be sound modulated (at the least it needs to
be
break rate modulated) I am wondering if a hybrid system may be workable.
I could build a spark gap (Could even be a rotary gap with a high break
rate, but may need to be a quenched gap or at least a static gap) and
place
it in the circuit between the primary coil and Capacitor, as you would a
standard spark gap coil.
I would use an interface circuit to trigger the valve to turn on and off
at
the Audio frequency in pulses. Stacatto...
This would cause the Tank capacitor to charge up until the spark gap
breaks
down and then the valve would shut off, effectively disconnecting the
supply
from the tank circuit...
The valve would only be conducting the 4-5 amps required to charge the
capacitor, and would not have any RF feedback like a normal valve coil.
It
would just be acting as a switch between the 10kvDC power supply and the
tank cap.
Also I would be using a much bigger capacitor in the tank. (like a
standard
spark gap coil).
The Idea is that the spark gap would give me the fast rise time required
to
get the long arcs, but the valve would be there to "commutate" the pulse
rate to music and disconnect the power supply from the tank circuit. (one
of
the problems with spark gaps at higher power levels is that the power
supply
is shorted out by the spark when it fires, and causes it to "arc" which
destroys the HF oscillations...)
What do you think?
Comments?
Carlos
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