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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The question is concerning the primary capacitor, "we" (me and my friends) can't come to a conclusion.
Is the primary LC calculated so that it resonates at the same f0 as the secondary, or does it resonate higher?
I think that when the tube is conducting, it "brakes" the LC so in result the oscillator frequency is lower than the resonance of the primary LC alone. So the capacitor should be of lower capacitance than resonant. Is this true?
Here is a drawing illustrating my idea, first waveform is the oscillation of the primary LC alone, second is "switched" with a vacuum tube as in VTTC:
I also think that the capacitor should not alter the running frequency much, since the running frequency is mainly derived from the secondary resonance. Is this how does it work?
I am no expert on VTTCs and I have never built one so please correct me if I'm wrong.
Any help is appreciated
Edit: My point is that like this the efficiency will be highest, of course it will work when the primary LC is tuned exactly to the secondary resonance, but I think the efficiency will suffer this way...
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
I think it is more important that your capacitor matches the load impedance of your tube, tuning can be done on the primary, the capacitor is a vital part of deciding your Q of the system.
I can only advice you to read steve wards math section in the bottom here:
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The popular VTTC designs are nearly always Armstrong Oscillators
Wikipedia:
In the Armstrong Oscillator VTTC, the valve anode (US: tube plate) inductance also serves as the primary of the Tesla transformer. The feedback winding (US: 'tickler') couples a tiny amount of the RF energy in the resonant transformer back to the grid 180 deg out of phase, so oscillation is maintained.
Study the fundamentals of the Armstrong Oscillator - the simplest type of valve oscillator there is - and the whole VTTC action will become clear to you.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I have read the article, but I'm still not very clever from that, so I'll try to ask,
1) Does changing the capacitor change the running frequency accordingly? Or is the running frequency mainly dependent on feedback from the secondary so it stays fairly close to sec. resonance no matter of the primary cap?
2) (kinda related) Is the running frequency of a properly designed VTTC close to the primary LC self-resonance, or is the LC tuned higher?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
For optimum performance, the self-resonant frequency of the Tesla resonator secondary should be the same as the frequency determined (chiefly) by L and C in the Armstrong Oscillator.
In a real world oscillator, sufficient feedback from the 'tickler' coil is likely to 'pull' the circuit, so that it oscillates (perhaps erratically) despite optimum conditions not having been met.
A veteran lazy bones, I'd wind my secondary, determine its self-resonant frequency by sticking it into a quick and dirty oscillator - a P-JFET/N-JFET Lamda negative resistance oscillator, or one leg of an emitter coupled oscillator, for example. Once you have determined fo for your coil, it is a piece of cake to work out L and C for the Armstrong Oscillator, perhaps with a variable capacitor to really tweak it up to precision.
Registered Member #1223
Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133
Primary resonant freq is determined by primary tank circuit component valuses (L and C). Feedback circuit has no effect for this, it is only kind of load for tank circuit (adds some phase shift for tank current and voltage as any load will do).
Primary circuit has to be tuned to same freq as secondary resonant freq. Just like radio receiver works, it is tuned to same freq as transmitter station is.
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VTTC question
