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I have one in my LTSpice simulation program... :-)

LTSpice is free from Linear Technology.

rogerw

Roger said: "That spike is what I was referring to as a "pulse." perhaps bad terminology"

That is indeed the pulse.

Where do I get an infinite impedance scope probe? Isn't the scope itself 1M Ohm load?

Geo

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From: "Roger Whatley" <rogwhat53@...>
To: "CDV700CLUB" <[email protected]>
Sent: Sunday, April 3, 2022 9:45:25 AM
Subject: Re: [CDV700CLUB] Question and an interesting site for those that modify CDV700s and more.

Geo,

If I were to put an infinite input impedance scope probe on the anode of the GM tube in, say, a Lionel, I am assuming I would see then a momentary -5V (or thereabouts, as low as -2V I think is spec'd on the 6993) spike in voltage when the tube discharges (conducts from anode to cathode) to ground. Then the tube quickly "quenches" such that the spike duration is short. I think I recall the "spike" duration as about 150uS.....?? That spike is what I was referring to as a "pulse." perhaps bad terminology..... it would be more like a fast-falling leading edge with an exponential recovery. This is because the modeled source impedance to the leading edge is low resistance, while the source impedance to the recovery is much higher resistance. At least that is what I recall thinking over two years ago....

In this sense, I would say, the GM tube's momentary short to ground "puts out a pulse" or a spike.......

I totally agree, GM detection is simple and fairly crude..... a wonderful device!

My point was that the negative spike from a properly biased GM tube is low enough impedance source excitation to drive a ~9K input impedance one-shot in a Lionel. So, if the impedance looking back into the HV source from the GM tube is significantly higher than 9K (and it is actually 3.3Meg in the Lionel), then functionality is probably ok.

rogerw

On 4/3/2022 9:12 AM, Geo Dowell wrote:

GM tubes don't "put out a pulse". They short to ground like any other neon tube, but recover quickly when the bias voltage drops down to the tubes conducting value (that's the? Quench gas at work).

That 3.3M is the actual load resistor and is added to the ENi during the LENi conversion. When the GM tube is not conducting, the probe side is sitting at essentially the full Voltage from the regulator element Corotron in Lionel, or Zener string in any upgraded units). When the GM tube conducts, current flows to ground through the 3.3M, which in turn drops it's probe side HV down, this change is sensed by the one-shot input capacitor, which on the other side represents that change as a sharp negative going pulse. GM tubes can easily deliver 5V (and more) pulses, which is why there metering circuits can be so primitive.

That same 3.3 million ohms, in addition to the inherent high impedance of the HV circuit in the first place,? prevents a common Fluke meter or even an oscilloscope from reading the true HV at the regulator by monitoring at the probe connector. Some Geiger Counters have even higher resistances, and no sort of standards exist, hence the need for a 1000 Million Ohm (1G-Ohm) HV Meter box.

With my version of the variable HV for CDV-700's the Zener/Corotron remains, but the variable Zener is applied to the primary side. This allows contiguous adjustment from the high set by the secondary regulator down to about 200V. When just the HV section is transplanted into its own box and the Zener set to 1350V, the box can become a useful bench power supply 200-1350V output. When married to a 1G HV Meter, even better.

The idea of a variable HV side Zener shunt regulator still greatly intrigues me in general, so please continue the effort.

?

Geo

---

From: "Roger Whatley" <rogwhat53@...>
To: "CDV700CLUB" <[email protected]>
Sent: Sunday, April 3, 2022 8:25:21 AM
Subject: Re: [CDV700CLUB] Question and an interesting site for those that modify CDV700s and more.

Now that I think about it some more, I think all the tube shunt regulators (or replaced with diode string) have a "dropping resistor" from the UnRegHV and then followed by another high value, say 1Meg to 3Meg, resistor to the GM tube. This is because the shunt-regulated node is also a low impedance (well kinda low....). However, the input resistance to the one-shot is not very high,either, depending on design...... I recall in the vicinity of 10K (just looked at a Lionel schematic, its about 22K//18K ~ 9K input resistance to the one shot).

So the dynamic resistance of those HV zeners in a 900+V string probably easily exceed 9K (this can be calculated from a regulation curve)....... but still there is no reason not to put the extra resistor to the GM tube, and Lionel put 3.3Meg there

rogerw

On 4/3/2022 8:09 AM, Roger Whatley via groups.io wrote:

there would need to be a series resistor, say 1Meg, after the series pass regulator low impedance output (well only kinda low) and going to a GM tube. That kinda points out that this is not practical for a GM tube and a shunt regulator is fine. In fact the only advantage I can see of the Shunt Regulator we have been discussing over a diode string is that it is easily adjustable. It could even have switchable values, each with a trim adjust.

However there might be some advantage of a Series Pass with the photomultiplier detector bias ring which requires HV dc current?

rogerw

On 4/3/2022 7:47 AM, Geo Dowell wrote:

Q: what happens when the GM tube fires, essentially going to ground?

Geo

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From: "Roger Whatley" <rogwhat53@...>
To: "CDV700CLUB" <[email protected]>
Sent: Sunday, April 3, 2022 7:40:32 AM
Subject: Re: [CDV700CLUB] Question and an interesting site for those that modify CDV700s and more.

Peter,

The reason I ask is that this Shunt Regulator can be turned quite easily into a Series Pass Regulator such that the "dropping resistor" can become, say 10Meg and the bias currents to the regulator remain small while the load gets only whatever current it demands.

Interested?

rogerw

On 4/3/2022 12:40 AM, peter via groups.io wrote:

Roger:
?PMT resistor divider resistance is all over the place. The lowest I encountered was a commercially made socket for Hamamatsu R647 pmt, and its total resistance was about 3.3 M ohms.The pmt base is tiny and the resistors are inside the potted pmt socket, so no chance in modifying it.?

Many surplus 1 x 1x 3" NaI probes made by Bicron/Alpha Spectra are around 13M ohms. My home made resistor dividers are around 50M ohms~ 240M ohms.?
Many pmts are 8 or 10 stages so 9 or 11 resistors, often all the same value. I often choose 4.7M, 10M or 22M ohm resistors for each resistor.

P

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On the Catclaw
ΜΟΛΩΝ ΛΑΒΕ

The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.