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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

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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rogerw
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rogerw
On the Catclaw
ΜΟΛΩΝ ΛΑΒΕ

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