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Old 13th Dec 2020, 9:42 pm   #1
AdrianH
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Default A simple HT Generator project

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Ok several hours spent in the garage on the lathe, I was getting on so well had cut the groves, drilled 24mm hole down the middle and was parting off from the end when something grabbed and the unit shattered and bits flew all over the garage.

So after tea I thought I would have another go working in a different order, this time the last thing to do was drill the hole.
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Tomorrows job is to machine a small slot through the fins from end to end for wire pass though.

Hope it goes to plan or with the time spent would probably give up if this one breaks.

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Old 13th Dec 2020, 9:44 pm   #2
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Default Re: Ideas to assist with a simple EHT generator.

These are some of the CRT's I wish to try, well not all unless I go into magnetic focus and deflection.

1 x 2AP1
1 x 3BP1
1 x 4EP1
2 x 5BKP1 Think one is to air.
1 x 5BKP31
1 x 5CP1
2 x 5FP7
3 x 5NP1
1 x CR87
1 x CV1397
1 x CV1526
1 x CV960
1 x DG13-2 Has damaged valve base.
1 x Cossor 89J
1 x MW6/2
1 x VCR517B

Adrian
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Old 19th Dec 2020, 9:18 am   #3
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Default Re: Ideas to assist with a simple EHT generator.

Mods Please move to Home Brew.

Playing continues: -

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This is how I have initially wound the coil, going right to left in the picture, a 200 turn feedback coil, 500 turn primary coil and 5 x 600 turn secondary coils to make up 30000 turns in total for the overwind.

I have a few 500pF high voltage caps obtained from a radio rally of many years ago, so used two to form the EHT smoothing capacitor.
Rather than use valve rectifiers. I used a 16 KV, 5 mA diode for the EHT and a BYX10 to give the -Ve feedback voltage.
Although I like using 7 pin valves, the EL91 would appear limited to 16mA anode current at 250 Volts, so saving the extra current by not heating a couple of rectifier heaters seems worth while.
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I am using three ferrite aerial rods to form the core, this may be to much, I will attempt to do more testing later today.
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Testing is a bit of Heath-Robinson. I have 19 x 10 Meg Ohm resistors in series and use a Digital volt meter that has a 10 Meg Ohm input resistance, so in total a 200 Meg resistor across the output as a load giving a 1/20 Voltage reading on the DVM. Not sure on accuracy as all resistors are 5% tolerance.

But it indicates a peak of 3,200 Volts at present across 200 Meg. The feedback pot allows for dropping the output down to around 1000 Volts. If, as I did by mistake just put the DVM across the output then it indicates a maximum 1080 Volts into 10 Meg so 1 mA current.

With the 200 Meg resistance at 3200 Volts that indicates to me 16 uA (micro Amp). So perhaps I should be able to do a simple line graph to indicate what the max available is for the typical 5uA the CRT's I have require?

HT to the unit has been set at 250 Volts, max current drawn is 19.4 mA So think I am OK with the 16 mA EL91 anode current.

It seems I like building death traps.

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Old 19th Dec 2020, 10:32 am   #4
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Default Re: A simple HT Generator project

I plan to use the scope on the anode connection today to see if there is any ringing, it may be allow me to see the resonant frequency of the coil and perhaps I could add a capacitor or change the frequency of the 6D4 to match it better.

I also took some measurements of the coil with the Multi-function tester TC1 I have.

Without ferrite cores: -
Feedback coil 200 turns at 1.91 mH.
Primary 500 turns at 10.2 mH.
Overwind 3000 turns at 197 mH.

With the ferrite cores
Feedback 5.31 mH.
Primary 35.5 mH.
Overwind 1.28 H.

The centre former is 28mm and the slots are either 3 or 6mm wide.

I will try and fit these figures into some formula later to see if I can match the permeability for the ferrite rods.

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Old 19th Dec 2020, 1:30 pm   #5
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Default Re: A simple HT Generator project

OK the scope has been a great help, using close to the book values I found the Thyratron firing typically in the 500Hz range once every 2 mS to give the best voltage output, but with a lot faster ringing going on in the coil.

So a change to the C value from 10 to 1 nF and a play with the pot and the best I get now is at 50KHz frequency, the peak voltage into 200 Meg is now 4,200 Volts and with the 10 Meg load of the DVM the output drops to 1500 Volts so considerably better performance.

I am certainly going to have to encase this unit with the RF properties it has.

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Old 15th Jan 2021, 12:40 am   #6
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Default Re: A simple HT Generator project

After the holiday break I am now once again getting back into some construction. So I made a smallish metal enclosure to house the coil and HV caps. Adding some safety and I also hope also reduce RF emissions from the coil. The copper clad board is bolted to the case so I hope it acts like a closed shield.

Adrian

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Old 15th Jan 2021, 9:35 am   #7
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Default Re: A simple HT Generator project

Quote:
Originally Posted by AdrianH View Post
The copper clad board is bolted to the case so I hope it acts like a closed shield.

Neat little unit, Adrian, like it. That should do for screening. What sort of socket will you use for your EHT?
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Old 15th Jan 2021, 12:16 pm   #8
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Default Re: A simple HT Generator project

The CRT I hope to use with it is a 5CP1 which has a post deflection anode which is small snap terminal, It can run the final anode between 1500 and 40000 Volts, but a lot will depend on the second anode voltage which is only around 900 volts on my TV so the final anode voltage could be at the low end.
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I have some terminals thanks to Cobaltblue he pointed me towards some on Ebay so I have 4 or 5. A bit of solder some heat shrink and I can secure it to the co-axial inner I am using to bring out the EHT.

Adrian

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