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Old 4th Jun 2024, 11:09 am   #81
Dazz100
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Default Re: GEC Miniscope model BW 464

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Originally Posted by GMB View Post
I think that is not right and unnecessarily complex.
The forward voltage drop is going to be about the same as 50 Si diodes I presume, the rest being a resistive loss.

The exact characteristics seems bit hard to deduce. I found the attached data sheet on this kind of rectifier but it is expressed in an odd way.
Hi
When the circuit is acting as a forward biased diode allowing current to flow, the zener diodes are reverse biased. They act as switches. As the voltage rises, the zeners turn on and allow current to flow through the associated resistor. As the voltage rises, the overall resistance reduces as more resistors conduct in parallel. More current, less resistance.

The transformer has high winding resistance. As the secondary current rises, the transformer output voltage droops. Also as the load current increases, the average voltage across the smoothing capacitor decreases. The net result is DC voltage droop as current increases. So what is needed is a current dependent resistor. More current, less resistance.

This is one of the reasons why I think the GEC engineers were good designers. I suspect they custom made the selenium diodes to match the impedance of the transformer and smoothing capacitors to regulate the HV DC voltage across a range of load currents. It is very likely that the transformer was custom designed to match the selenium diodes. All a very elegant design if my analysis is correct.

I have already measured and characterised the transformer and selenium diodes. The results are in earlier posts. With this data, I can design a silicon based selenium diode emulator and exactly match the actual selenium diodes. I just need to select the right values of zener diode voltages and resistor values.

The missing information is the load characteristics. Once I get the scope working I can characterize the load. The use of low loss, low leakage film caps may result in a mismatch with the power supply and poor regulation (excessive voltage). It is also likely that the selenium diode circuit was very conservative (significant droop by design) to allow for the temperature sensitivity of the selenium and to avoid thermal runaway. I will be able to tune the selenium emulators to closely regulate the output voltage. In theory I could achieve zero droop.

I anticipate the key design parameter will be power dissipation in the zener diodes. The solution to that problem will probably be additional resistors, parallel zener diodes in series with low value resistors, and other techniques to allow for the use of low power low cost zener diodes. A practical circuit is likely to look quite different to the one I have drawn that shows the principle of operation, but not the practice.

I can't be the only engineer on the planet that has looked at this problem, and there are no secrets in the use of diodes as switches. I fully expect someone to say they saw this principle applied in silicon to regulate HV DC many decades ago.

So the next step is to model the power supply circuit to figure out a practical circuit for the selenium emulators. I still have plenty to do rebuilding the Miniscope so modelling will be on the back burner. Probably something I will do when I have to pause the rebuild.
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Old 4th Jun 2024, 1:03 pm   #82
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Default Re: GEC Miniscope model BW 464

I think your actual diodes are bad, so replicating them is not a great idea.
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Old 5th Jun 2024, 6:20 am   #83
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Default Re: GEC Miniscope model BW 464

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I think your actual diodes are bad, so replicating them is not a great idea.
Hi
I agree so the plan is to model the silicon version of the power supply and test it in my PC before exposing it to electrons. Once I get things to the point of where I can safely apply full mains voltage, I can start tuning the silicon version.

I anticipate that the tuned silicon version will end up with a significantly different transfer curve because I won't have to worry so much about high temperature coefficients and thermal runaway.

This is my first hands-on contact with valve gear so I am learning as I go.
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Old 5th Jun 2024, 4:12 pm   #84
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Default Re: GEC Miniscope model BW 464

Valve gear is not very fussy about exact voltages etc. and any quoted values will be with large tolerance. Your only worry is not to take capacitors over their ratings.

Maybe next week I will find time to try the EHT with original rectifiers. The HT has already been crudely changed to pairs of silicon diodes.
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Old 6th Jun 2024, 11:39 am   #85
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Default Re: GEC Miniscope model BW 464

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Originally Posted by GMB View Post
Valve gear is not very fussy about exact voltages etc. and any quoted values will be with large tolerance. Your only worry is not to take capacitors over their ratings.

Maybe next week I will find time to try the EHT with original rectifiers. The HT has already been crudely changed to pairs of silicon diodes.
Understood but if I just substitute in silicon diodes, combined with near zero leakage new caps, everything will be operating over the volt limit.

I am aiming to achieve a functional restoration so the circuit operates close to the original design. If successful, nothing on the outside will reveal what has been done on the inside.

I expect the actual practical circuit for the selenium emulator will be a lot simpler than the circuit I posted above.
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Old 7th Jun 2024, 12:17 pm   #86
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Default Re: GEC Miniscope model BW 464

Hi
Two potentiometers were so tightly seized that I had to use a lathe and collet chuck just to move them. I was worried I would break them. I suspect the lubricant had turned into a gum.

After some experimentation I found that hot kerosene dissolved the gum. I put the kerosene in an egg cup, in a bowl of boiling hot water. I worked out the dissolved gum and kero with an absorbent cloth. The gum was soaked and worked out over a period of several days.

The pots ended up like new. I then applied aerosol chain lubricant. This is thick viscous grease in a light solvent. When applied, the solvent carries the grease into fine spaces and soon evaporates leaving the grease. The grease has just the right tactile feel to it.
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