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Re: terry filter theory

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Bart,
Yeh, that works too. Either order will give you almost the same result. The only real difference is that you readjust the safety and I readjust the mains to keep the safety from firing during normal operation. I prefer to set the safety first and never touch it after that. I normally think of the safety as being in parallel with the main and which ever one breaks down first will "hog the show". If that is the main gap and it warms up, its breakdown voltage will decrease and it will hog the show even more strongly. If the safeties fires first, are set correctly, and one doesn't want to change them, the only recourse then is to reduce the main gap. I like to start with the main gap set too small and then increase it until the safeties fires occasionally.. A safety firing once in a while is not a problem with static gaps. Of course with SRSGs a different picture presents itself. A safety gap firing just once will screw up the SRSG timing and make things run rough. All of my comments below are in the context of a static main gap. 



Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Gerry, Todd,
The safety gap is typically a single electrode to electrode gap. The main gap should be built for temperature stability. It is the main gap that must be set correctly (first, before the safety gap). Once the main gap is set according to the NST voltage, then set the safety gap to "just not fire".
Main Gap Adjustment = Set so that it fires consistently with only the NST and main gap. Safety Gap Adjustment = Set so that it just doesn't' fire with only the NST and safety gap.
If during running, the safety gap is firing too often, "don't adjust the main gap". Instead, check to see if the main gap is getting hot (lowering the firing voltage) which is usually the case.
If the safeties are firing too often and the main gap is too hot and its breakdown voltage has been lowered because of temp, I believe adding cooling to the main gap wont help reduce the safety firing. It will only make it worst because the breakdown voltage of the main gap will be higher than without cooling.
If so, then increase the air flow and stabilize the main gap temperature. If after doing so the main gap remains cool and the safety gap is still firing too often, then the safety gap should be "slightly" increased (but only slightly, about 500 volts). 

My only problem with this is knowing what constitutes a 500V increase.
If still the safety gap is firing too often, the main gap cannot dissipate the heat fast enough and a rebuild of the main gap and cooling mechanism (or both) may be required. 

Here's a common scenario:
1) the main gap gets warm.
2) the firing voltage lowers.
3) the safety gap begins to fire.
It seems that lowing main gap's firing voltage will never cause of the safeties to fire more often. This lowing will only help reduce the frequency of the safety firing. Of course, the main gap needs to be properly cooled for other reasons. Like you say below, if the safety gaps are firing too much and heat up, their breakdown voltage will lower and maybe then will hog the show. 


4) the safety gap gets warm from the increased firing.
5) because the safety gap electrodes heat up, they now fire at a lower voltage, more and more frequently. 
6) the safety gap just turned into the main gap.
The coil starts out firing with the main gap and after a little time the safety gap takes over. The main gap is typically the problem, not the safety gap (unless you mistakenly created a needle gap for a safety gap). The safety gap should be able to either have the same cooling mechanism as the main gap or be slow to temperature variation. The later is usually the case by using a similar or slightly larger radius as the main gap electrodes but of a solid material so that it's mass is greater and it's rise to temperature is slower.
Yes, this is why I like to start the final tweaking with the safety gap set, the main gap sufficiently small, and then increase the main gap to get an acceptable ratio of safety to main gap firings. This process will get to the final setting with the safety gap cool and the main gap a little warmer. Of course, with sufficient cooling, neither the safety nor the main gap will rise in temperature significantly. 

Gerry R.