Ah good. Somehow this important Topic got?"locked" last night. If a moderator did that please let me know. It's unlocked again.

Here's the latest and more critical look at the Bias HV drop across the SBM-20 GM tube operated at 450V via a re-regulator base from 920V DC from a Ludlum Bench "Geiger Counter".

First the last night report- updated?

"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."

Looking again this morning with caffeinated eyeballs, a selection of eyeglass and more patience. Specifically looking to see if the "valley" goes all the way to zero before resetting. Best I can tell it does not go to zero, but the exact floor is difficult to determine with this 'scope, it seems to be somewhere around or just above 50V above ground. This would track very well with the NE-2 Neon Bulb analogy.? Someday (not this day for sure), we'll nail that down, with the storage 'scope, or someone else can.

By the way a GM tube activates with radiation because it is being operated on the knee of the gas discharge tube curve, in the Geiger region, and there is always a wide latitude of exact HV settings that allow that proper operation. + or - 50V or even more is normal, and is what Rad Techs call the "plateau" area (flat spot) of the curve.

Back to the discussion guys, sorry for the unintentional temporary lockout of us all.

Geo

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Peter/Geo/Michael/All,

Take a peek at the regulation curve enclosed..... Simulation is from 800V ramped to 1200V on the UnRegHV input and the RegHV output is a nominal 915V +/-2V over 1000V to 1200V input and -25degC to +75degC. Yes that plot contains three temp curves for regulation! This says nothing about component unit-to-unit tolerance, just that the circuit regulation has been wrung out fine and the circuit errors ameliorated. In simulation....... not breadboard!

Pretty danged good.... Penultimate in fact..... and definitely overkill to the requirement! but fun! I call it the "Penultimate Way-Better Cascoded Shunt Regulator."

Okay, now you can look at the schematic..... I am still using generic ideal JFETs for the cascodes, but everything below the cascodes is a real model of a real transistor. The best actual devices I have seen for the cascodes are either of the two that Michael found at Digikey in this thread below. Used a Rohm 11V Zener.

rogerw

PS - I can also later post the "Way-Better Cascoded Shunt Regulator" in its current final simulated form..... also NOT final breadboarded! That one has about 913Vto 926V over same temp and voltage. Used a Rohm 10V Zener. The Vbe tempco's just behave a little different with the local feedback pairs versus plain BJT's alone. The regulation curve for this one can be improved by using 3 * 6.2V Zeners and adjusting the output voltage appropriately, but the improvement to about 7V change over input voltage instead of 13V change is probably not worth the hassle or cost......

rogerw.

Sounds great!

btw, either of those small signal HV Mosfets that Michael found look good for this.

I have no idea how it might matter to reliability operating so close to the breakdown voltage, at least these currents are miniscule.....

I learned once, a long time ago the hard way, that a breakdown voltage is not a deterministic yes/no number depending on a threshold (or a spread of threshold values), rather it is a time/voltage/temperature dependent stochastic phenomena. When you test a device, say a capacitor, in the lab at room temp for breakdown, you merely found the voltage that raised the probability of breakdown to unity in a second or two of test time....... time, voltage, and temperature. For real high reliability, I learned to stay well below that voltage.......

I am working on a "Penultimate Way-Improved" version of the Shunt Cascode Regulator........ and btw I have yielded to reality and am using a single Zener as you do. However to temperature compensate the 3Vbe drops, either a 10V Rohm Zener (slightly negative tempco of RegHV) or and 11V Rohm Zener (slightly positive tempco of RegHV) works fine. I guarantee what I am doing for "Penultimate" is really overkill..... but fun!

rogerw

On 4/3/2022 6:42 PM, peter via groups.io wrote:

Roger et al:

Spent a few minutes in my junk drawer. Found a 2SK3265( 700V part)
I checked the break down of the body diode, no problem to 1150 V.
Mod'd the cascode to just 1 Mosfet.
Wow, regulation stiffness improved. Set to 900V, raise the input 1150 and may be 15V increase

p

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Sounds great!

btw, either of those small signal HV Mosfets that Michael found look good for this.

I have no idea how it might matter to reliability operating so close to the breakdown voltage, at least these currents are miniscule.....

I learned once, a long time ago the hard way, that a breakdown voltage is not a deterministic yes/no number depending on a threshold (or a spread of threshold values), rather it is a time/voltage/temperature dependent stochastic phenomena. When you test a device, say a capacitor, in the lab at room temp for breakdown, you merely found the voltage that raised the probability of breakdown to unity in a second or two of test time....... time, voltage, and temperature. For real high reliability, I learned to stay well below that voltage.......

I am working on a "Penultimate Way-Improved" version of the Shunt Cascode Regulator........ and btw I have yielded to reality and am using a single Zener as you do. However to temperature compensate the 3Vbe drops, either a 10V Rohm Zener (slightly negative tempco of RegHV) or and 11V Rohm Zener (slightly positive tempco of RegHV) works fine. I guarantee what I am doing for "Penultimate" is really overkill..... but fun!

rogerw

Roger et al:

Spent a few minutes in my junk drawer. Found a 2SK3265( 700V part)
I checked the break down of the body diode, no problem to 1150 V.
Mod'd the cascode to just 1 Mosfet.
Wow, regulation stiffness improved. Set to 900V, raise the input 1150 and may be 15V increase

p

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very kewl! that strikes me as an extremely good price. I bought one over 2 years ago for 3x that amount IIRC......

rogerw

I picked up a $25 ENI cdv700 on Ebay, I plan to convert it to a LENI.? What I learned from my last two ENI conversions - the LENI schematic shows 18k R18...? pretty sure in place of a wire wound choke original on the ENI.? The LENI instructions do not say to replace the choke.? The only way I was able to get the last two LENIs working was to replace the choke with the resistor.? I don't know why - I am not an electrical guy.? When this ENI arrives April 7,? I will give it a whirl again.? The rest of the conversion is easy... just being careful with the soldering iron and the very delicate PCB and pads - they lift if you look at them funny.? GEO ... I may order another one of your assembled Zener regulator boards - I built my last one, but it was a bit fiddly.? ?

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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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Well the question was how to model the GM tube source signal in Spice......? All I meant by that was that the only signal seen by the detector is the transient pulse caused by a radiation event, and so when I modeled the GM tube source excitation seen by the detector circuit, the 900+VDC is not present in the model. ie, "you don't have to worry about the 900+VDC" to model the signal the GM tube generates and which is coupled to the detector by the capacitor.

Sorry if I was not clear.

rogerw

Roger:
?I got lost. I didn't understand the commented direct to me:
??????????????????? since the 900+V bias is blocked by the coupling capacitor to the detector circuit, you don't have to worry about that.????

p

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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.

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ok, 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.

rogerw

"I respectfully disagree.......My modeling perspective is to "stand on the detector's input node and look back towards the coupling capacitor" and what do you see?"

I'm not talking about the GM tube right now, I'm talking about a capacitor with no charge (has been OFF) having 900 V applied to it. What do you or the neon bulb) see on the other side? e.g. GC being turned on.

Extend that then to a steady stream of on-off pulses from the PB switch.

Geo

----- Original Message -----
From: Roger Whatley <rogwhat53@...>
To: [email protected]
Sent: Sun, 03 Apr 2022 13:35:59 -0400 (EDT)
Subject: Re: [CDV700CLUB] Question and an interesting site for those that modify CDV700s and more.

I respectfully disagree.......My modeling
perspective is to "stand on the detector's input node and look
back towards the coupling capacitor" and what do you see?

You see a randomly occurring -(2+)V fast edge
with a slow exponential recovery to 0V. You do not see the
900+Volt bias behind the coupling capacitor. The one-shot
detector input never sees the 900+V bias on the GM tube.

Your circuit is as though the randomly
occurring pulse were not -(2V+)V but rather a 900+V pulse. But
the GM tube does not discharge all the way to ground. Not even
halfway......

The 6993 spec says that a properly biased 6993
tube will yield a pulse height of at least -2V minimum, so that
means it would go from, say 920V to 918V, or perhaps lower,? and
then slowly recover to 920V in about 175uS...... and on the
detector input node on the other side of the coupling cap, the
-(2+)V pulse is all you see.

btw, I think I recall that the only reason it
does NOT discharge all the way down to ground (ie, a 900+V
pulse) is because of the quenching gas mixed in with the neon
gas.....

So, yes, a GM tube is a lot like a neon bulb
but there are salient differences:

1) the quenching gas, and mechanism.

2) the triggering mechanism is a radiation
event, and not high voltage applied at the terminals. In fact it
is not good to trigger your GM tube with HV on the terminals!

rogerw

On 4/3/2022 12:12 PM, Geo Dowell wrote:

Roger said: "?Peter,?Since the 900+V bias is blocked by the
coupling capacitor to the detector circuit, you don't have to
worry about that."

Actually,
you do have to worry about that, in my opinion. Again, model
this? series circuit:? 900V power supply, series pushbutton
switch (SPST N.O.), capacitor Neon bulb, ground. Does the neon
bulb flash?

Geo

----- Original Message -----

From: Roger Whatley <rogwhat53@...>

To: [email protected]

Sent: Sun, 03 Apr 2022 12:40:03 -0400 (EDT)

Subject: Re: [CDV700CLUB] Question and an interesting site for
those that modify CDV700s and more.

Peter,

Since the 900+V bias is
blocked by the

coupling capacitor to the detector circuit, you don't have to

worry about that.

The simple notion of a GM I
modeled over 2 yrs

ago was a 2.6V (the minimum 6993 spec, and then plus one
silicon

diode drop) negative pulse voltage source of short duration
thru

a silicon diode to a parallel resistor/capacitor that gives
the

exponential tail with something like a 75uS time constant.
There

are other circuit ways to do this but it is basically just as

Geo said: It is a switch that turns on at low impedance for a

>2V fast negative edge, and then turns off with a slow
higher

impedance exponential recovery.

That is what I recall using
in simulations to

model the GM tube output and design the Silicon Retrofit

detection in both the Lionel and Vic's. basically, it was just

tweaking the respective input sensitivity of the two designs
in

simulation to respond reliably to the "pulse" or "spike" that

comes from the GM tube.

I have not really yet found a
motivation for

the Series Regulator although it is a simple change with one

additional HV pass transistor.

rogerw

On 4/3/2022 11:00 AM, peter via

groups.io wrote:

Roger:


?The series regulator flavor sounds interesting too!



I don't have any idea as to what a spice model of PMT or a gm

tube would be.?


I wonder if there are spice models for gas discharge tubes ?
ESD

and lightning protection tubes are still commonly used so I

would assume spice models would be available.


You could probably take a gas discharge tube's model and tweak

it ?

P

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rogerw
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ΜΟΛΩΝ ΛΑΒΕ

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ΜΟΛΩΝ ΛΑΒΕ

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