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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Just wondering about the theory, as I'm not gonna build a VTTC because tubes are out of my price range...
If the feedback was wound directly over primary, there would be no phase difference so the circuit could not oscillate. Right? If so, how does feedback position and proximity to primary affect tuning?
Registered Member #1837
Joined: Tue Dec 02 2008, 02:20PM
Location: NYC
Posts: 65
I'm a novice but i found good info here. In the #2 section he describes what is happening with the oscillation.
I would have said feedback position is pretty critical for proper firing of tube's plate. That the proximity must be related to amount of time AC sin wave takes to induce current in the feedback.
I would like to add I am completely guessing here but it kind of makes sense to me like that.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
The position of the feedback determines the the coupling to the primary. I think it would take the same amount of time for the signal to reach the coil no matter the position (it's just a magnetic field inducing voltage in some wire). Also, I don't think that the circuit actually "knows" what times to turn it on and off. It does it at the right times...
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Well, as soon as the pulse starts some of the energy is coupled into the feedback, which switches the tube off. The capacitor and grid leak resistor control how long it stays off. As stated on that site of Steve's, the tube is usually only on like 10% of each cycle.
Registered Member #229
Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506
Well I have an explanation, but dont shot me, I am not sure is correct. The capacitor c in the RC grid circuit polarise negatively the grid (it is charged by the feedback coil L3, in the first half cycle, using the fillament- grid space as a vacuum diode) . The resistence r is a leakage resistor that discharge the capacitor c at a slow rate, so the maximum voltage accross c start decreasing. In the second half cyle when the voltage produced by L3 is higher than the voltage on the capacitor c (decreased by the leakage resistor), the tube start to conduct for a short period of time, until the output from L3 decrease under the negative voltage of c.
But the rlc circuit, as a whole, is subjected to an AC current. The rc network include a lagging of the voltage behind the current, so the seried L3 coil is needed to be matched to compensate somehow the de-phasage between voltage and current. So, we have a parallel circuit RC in serie with an inductance L3 and we can get a phasorial description:
The coupling between primary (L1) and feedback coil (L3) affects the value of U(L3) vector (which is normal to the voltage on the rc network), so for the best coupling between primary (L1) and feedback coil (L3), U(L3) has the right value to reduce at minimum the angle phase between the current and voltage. BTW, the U(L3) has to be high enough to compensate the reactance of the rc circuit.
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How does a VTTC work?
