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Registered Member #1845
Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211
Cool Chris
Have you solved the problem with RF getting into the pulser? What will you do to fix it? I remember at the thon, you had it in pulse mode, and it shot a spark out every second, and then it would go screech, and a bunch of sparks would shoot out. That was RF getting into the pulser and retriggering it right?
We'll see how that secondary works out. It will be interesting to see how a really high Fo affects things.
And how does the floating ground bus ensure all tubes will turn on at the same time?
Registered Member #1845
Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211
A VTTC requires a ground on the filament so current has a path to plate. With the ground floating on the filament, the tube will not conduct and this allows use of a Staccato setup. Triac/SCR are connected to the filament turning on to allow path to ground. I just connected the Triac/SCR to one side of the filament transformer. There was only .007 volts difference from last tube to first tube as used three 10awg wires and two ground straps per leg to ensure no voltage sage. However, there is a propagation delay issue as not in a star feed design. So now, all grounds tied to one slab of copper and Triac will set in middle feeding.
Allright, I think I get it now. Technically, the 833C that has the shortest loop around the RC grid leak circuit would actually see grid voltage before the others (the time difference would be so minute that it is almost pointless to mention so.) I think what Chris is getting at, is that the extra length of Copper contributes to a voltage drop when current flows through the RC grid leak circuit. This voltage drop would mean that the tube with the largest loop would have less voltage on its grid, and the next tube would see a bit more grid voltage, etc. A star pattern in the grid wiring would definitely fix this. If all tubes have an equal resistance through the RC grid leak circuit, this means that the tubes would turn on and conduct at exactly the same time, which is what a star pattern would accomplish. However, it would seem to me that slight differences in each 833C would make their conduction different anyway, so the fact that Chris is making them turn on and conduct at the same rate still doesn't make total sense, but maybe Chris was finding relatively more plate current on one end of the quad bank?
EDIT: I cleaned up my response, as I think I've got it now.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Chris, do you know how fast a SCR turns on, and how that compares to the propagation delay in a 2 foot long copper strip? I think the problem is more likely to be something else. Some things I can think of:
If triacs get overvolted between their main terminals, they may turn on. (What negative grid bias do 833s need for complete cutoff at 4.2kV plate voltage, and how does that compare to the voltage rating of your triac/SCR/whatever it is?)
If the main terminals see too much dv/dt then again the device will turn on. This is what the LCR snubbers in lamp dimmers, etc. are for, to decrease dv/dt when your triac turns off and stop it turning straight back on again.
Finally of course RF could be getting into your driver circuit, just have someone make you some ITO coated plexi!
Registered Member #1500
Joined: Sat May 24 2008, 04:38PM
Location: Ojai, Ca.
Posts: 44
Hi Dr. Spark, Can you share a bit more about your coil geometries. It seems you have set some new standards for performace. You mentioned you went to #18 on the secondary. Why so thick? must be currebt not skin depth. What is the diameter and length of the secondary and the wire gauge and windings of the primary. How tight is the coupling? what value are you using for the tank cap and lastly maybe some specifics on the grid coil. If you wish to keep these parameters proprietary, I'll understand. Thanks, Jim Mora
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