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R: [TCML] valve coil problems
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
>>> _______________________________________________
>>> Tesla mailing list
>>> Tesla@xxxxxxxxxx
>>> http://www.pupman.com/mailman/listinfo/tesla
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>> Tesla mailing list
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