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Well, hmmm......

1) I yield to your point, and,

2) I often find I do not understand everything I "know." This appears to be another one of those times........

btw, I totally agree with your comment about? the metering circuit input cap. Essentially its time constant with the input resistance is such that the capacitor is discharging throughout the period of the "pulse." That means that the fast fall on the leading edge does not get quite to the full peak of the input pulse.? My recollection is that it was about 90% in simulations..... and my recollection is that there was a good reason for not making it larger.... I forget what it was.

Can you perhaps do the same Vpp measurements on a 6993? I wanted to do that over two years ago but I did not have a HV scope probe.

So I am looking again at the 6993 specs (enclosed)........? If I understand this correctly, the "Pulse Amplitude when operating at 890V" is 2Vmin for the 6993/114 GM tube. That of course, does not mathematically preclude a full 890VPP pulse amplitude at the GM tube but I never found anything to suggest that either. Until now.

In the second enclosed document, a manufacturing test set up specified a 1Vmin pulse amplitude requirement for the 6993 tube. It specified a 50pF coupling cap, I suppose to a 1Meg scope input? I haven't read the document in over 2 years....

So I still think I do not understand everything I "know."

rogerw

On Sun, Apr 3, 2022 at 01:40 PM, Roger Whatley wrote:

k, in that case you will see a rather fast 900V rise to the neon bulb, except the neon bulb will begin conducting at something like 100V(?) and if there is no current limit on the HV supply, will probably explode in a coupla milliseconds, as you have previously alluded. If there is an appropriate series resistance, the Neon tube will drop in voltage as it conducts perhaps to remain on at 60V, also as you have previously said.

But (and assuming a Steady State operation with no turn-on transients) there is no such 900V transition in a GM tube plus detector circuit. There is only a -(2+)V transition (6993 spec). So I am having a hard time understanding what your circuit is illustrating that is relevant to the GM tube + detector circuit. In THAT circuit the capacitor only gets a -(2+)V pulse which it faithfully conducts to the detector input, while blocking the 900+V from that input.

?

btw, I said below that if it were not for the quenching gas a GM tube might discharge all the way to ground potential. This might not be accurate, using the neon bulb as example, a unquenched GM tube with only neon gas might discharge down to some similar potential as the neon bulb does, but that would still be a Yuge pulse magnitude pulse compared to actual spec.

While attempting to clean up the workbench today, I had to look into this. Using a Russian SBM-20 (because it was on the workbench for a different project) and one of my 900 to 450V adaptor bases, scope attached directly across the GM tube (scope rated for 2000V), and a mantle for a source, the peaks on the scope showed negative pulses measuring f 450V P-P. At the "Geiger Counter" actually a (Ludlum 2500 Bench scaler) the P-P was 900V. The Ludlum 500-2 actually adds pulses rather than discharging like a GM tube, and it read correctly as 5.5V P-P on the scope.

The M2500 was set so it took full output of a Ludlum 500-2 at 5.5Volts to start counting. The SBM-20 probe was providing twice that to the probe connector at least, as I turn the gain on the 2500 all the way down and the probe itself was still counting.

Many times people using my quartz crystal pulse calibrators complain some CDV-700's just can't be calibrated with it, even though it puts out way over 4V pulses, indicating that at least some models are pretty deaf, requiring at least 5V pulses to operate, but do operate on their 6993.

To me this indicate the value and quality of the metering circuit input capacitor has a great deal to do with the size and shape of the pulse presented to the electronics on its other side, more than simply DC blocking..

For what it's worth.

Geo

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rogerw
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