R: [TCML] valve coil problems
cn at q.com
Wed Nov 2 07:08:52 MST 2011
I think the major problem with the trap is to make the impedance high
enough to give good rejection but not so high as to make the response
too narrow. This will probably require some cutting and trying.
> Hi Carl, you suggest to make a third harmonic trap on the grid coil? Good idea.
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> -----Messaggio originale-----
> Da: tesla-bounces at pupman.com [mailto:tesla-bounces at pupman.com] 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
> Good luck--sounds like a cool coil.
>> 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?
>> 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,
>> On Tue, 01 Nov 2011 07:58:57 -0700, Carl Noggle<cn at q.com> 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?
>>>> 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
>>>> 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
>>>> then run the oscillator with no secondary and make sure it is
>>>> 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
>>>> secondary coil...
>>>> To put this problem into perspective, I have built many valve coils,
>>>> never had this problem.
>>>> The coil was built over a year ago, but has sat around for almost a
>>>> 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
>>>> the plate, and circuits with tank in the grid...
>>>> But for some weird reason, every one of them gets swamped by the
>>>> coil, and runs at the 3rd harmonic.
>>>> Any Ideas as to what could be going on?
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