Hi Carl, you suggest to make a third harmonic trap on the grid coil? Good idea.
Vladi
Vladimiro Mazzilli
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-----Messaggio originale-----
Da: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] Per conto di Carl Noggle
Inviato: martedì 1 novembre 2011 20.59
A: Tesla Coil Mailing List
Oggetto: Re: [TCML] valve coil problems
Hi, Carlos,
If you have a scope, you could shock-excite the secondary and measure
its resonant frequency. Take it away form the primary, then put a
one-turn coil around the bottom (bottom of the secondary is grounded)
and hook the one-turn coil up to the scope. then flick a 9V battery
across the leads to the one-turn coil. With persistence you should be
able to get a damped oscillation and measure the frequency.
As a fix, try resonating both the primary and the grid coil at the
operating frequency. Or you could make a series-resonant circuit to put
across the grid coil--the L should be 2 or 3 times that of the grid
coil, with capacitor to resonate at the third harmonic
Or for that matter, you could try running it at the higher frequency.
I still suggest driving the grid with a low-impedance square wave. Then
you can run in class C. Also, make sure that your screen grid supply is
well-bypassed.
Good luck--sounds like a cool coil.
---Carl
Yes, thats pretty much whats happenning here. (also yes, standard positive
feed back, self oscillator)
But somehow the secondary coil is taking over control of the valve. I have
not seen this happen before...
The tuned circuit in the plate (even when designed to have a high Q) , or
in the grid, is having very little sway over the driving frequency.
The valve I am using is a Tetrode 4CX15000A. It can dissapate 15kw, and is
good for 50kw in class C.
The screen is well bypassed, and the cathode is well bypassed. the only
thing that I can think of is that the ground cable is fairly long, and
could be getting a voltage on it from the secondary base driving into it.
However, The secondary, cathode, and grid etc, are all grounded to a large
aluminium ground plane, so am I correct in saying that even if the ground
cable that earths this plate had a voltage accross it, as long as the
components are all seeing the same potential at the same time, then the
voltage should not effect the valve.
In other words, it is the difference in voltage between cathode and grid
that is important right?
So:
How is the secondary triggering the valve?
Why the third harmonic and not the resonant frequency?
How do I test to solve this?
Kind regards,
Carlos
On Tue, 01 Nov 2011 07:58:57 -0700, Carl Noggle<cn@xxxxx> wrote:
Hi, Carlos,
If you are running it as a self-oscillator, it could be that the
secondary is resonating as a 3/4 wave--I had a TC that did that once.
If you can, I would advocate driving your final valves from a
driver--better control and better efficiency. What kind of monster
valves are you using?
---Carl
Hi People,
I am in the final stages of my giant valve coil (50kw).
I have changed the circuit that I am using since the first time I fired
it
up, in an effort to get longer arcs.
To cut a long story short, I have a peculiar tuning problem.
I measure the secondary resonant frequency, then Tune the tank to
match,
then run the oscillator with no secondary and make sure it is
oscillating
correctly, then slowly introduce the secondary inch by inch.
Even with a very loosely coupled coil, as soon as the tank sees the
secondary, the valve starts to oscillate at about the 3rd harmonic of
the
secondary coil...
To put this problem into perspective, I have built many valve coils,
and
never had this problem.
The coil was built over a year ago, but has sat around for almost a
year
since its first tests and admittedly suffered some water damage to the
valve cabinet.
Over the last week, I have tried 3 different secondarys each with a
different frequency, different coupling factors, circuits with the tank
in
the plate, and circuits with tank in the grid...
But for some weird reason, every one of them gets swamped by the
secondary
coil, and runs at the 3rd harmonic.
Any Ideas as to what could be going on?
Thanks,
Carlos
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