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GM70 psu transformer.
GM70 psu transformer.
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Old 7th February 2011, 09:24 PM   #1
Bas Horneman is offline Bas Horneman  Netherlands
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Default GM70 psu transformer.

For my GM70 I got a power transformer. With the following windings:

900vac-0-900vac for the b+ ofcourse

and a 2,5vac-0-2,5vac (5A) for the 5c8s.

I wanted to test my 5c8s and so I wired up only the low voltage 2,5V. And never have been "bitten" or "tickled" by a low voltage.

However when I picked up the socket with rectifier tube that had just been wired with the 2,5v winding. I got "tickled" I.e. the feeling when you touch a high voltage exposed part (accidentally ofcourse).

So I decided to measure the voltage, finding it odd that I had just felt something weird. Funny thing was that when I got only one probe near it I could see a little blue "lightning" bolt reaching for the probe. (Around 1mm)

Is this somehow the 900vac windings introducing some kind of corona effect. Or some other high voltage artefact?
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Old 7th February 2011, 09:33 PM   #2
HollowState is offline HollowState  United States
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Quote:
Originally Posted by Bas Horneman View Post
So I decided to measure the voltage, finding it odd that I had just felt something weird. Funny thing was that when I got only one probe near it I could see a little blue "lightning" bolt reaching for the probe. (Around 1mm)

Is this somehow the 900vac windings introducing some kind of corona effect. Or some other high voltage artefact?
I do not like this at all! No way! I say, either the winding is incorrect or the insulation is defective. I've worked with enough transformers with high voltage windings in them to feel very strongly about this. That kind of reaction from a 5 volt winding is dead wrong in my book.
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Old 7th February 2011, 09:34 PM   #3
Matt BH is offline Matt BH  United Kingdom
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Most likely capacitive coupling between the secondarys. When its all wired up to the rectifiers and filter caps etc it should be fine.

Unless it is insulation breakdown. You could check this with a 1000V insulation tester.

If you put a grounded probe near where it arced before does it continually arc? or does it do it once and then takes a while to do it again.

I agree with Hollowstate. It could be a very dangerous situation if it is insulation breakdown.

Cheers Matt.

Last edited by Matt BH; 7th February 2011 at 09:42 PM. Reason: Safety
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Old 7th February 2011, 09:40 PM   #4
Bas Horneman is offline Bas Horneman  Netherlands
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Quote:
I do not like this at all! No way! I say, either the winding is incorrect or the insulation is defective. I've worked with enough transformers with high voltage windings in them to feel very strongly about this. That kind of reaction from a 5 volt winding is dead wrong in my book.
To be honest I must confess that they are a pair of transformers that were brought back to the winder with the complaint that they were vibrating. The winder did his calculations and re-did it properly. But I asked for the "rejects" thinking that if the only problem was some vibration I could get around it.

Quote:
Unless it is insulation breakdown. You could check this with a 1000V insulation tester.
I'll have to google for an insulation tester. Thanks.
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Old 7th February 2011, 09:42 PM   #5
Bas Horneman is offline Bas Horneman  Netherlands
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Looks like I'm going to have to buy the real deal. Sigh...just when I thought I hit a spot of luck. By the way. The winding does measure 4.6VAC.
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Old 7th February 2011, 10:25 PM   #6
TonyTecson is offline TonyTecson  Philippines
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GM70 psu transformer.
i believe this is the reason why there are plate traffos and filament traffos, being separate entities make this issue go away.....

Click the image to open in full size.

these are traffos i built over the year end, the black covered one is the plate traffo, the two on the extreme right are filament traffo and filter choke...
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Old 7th February 2011, 11:55 PM   #7
Gluca is offline Gluca  Italy
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well ... you always want to have the primary and the secondary properly insulted even if you split the main trafo into many.
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Old 8th February 2011, 02:24 AM   #8
TonyTecson is offline TonyTecson  Philippines
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GM70 psu transformer.
Quote:
Originally Posted by Gluca View Post
well ... you always want to have the primary and the secondary properly insulted even if you split the main trafo into many.
you are of course correct.......
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Old 8th February 2011, 03:12 AM   #9
johansen is offline johansen  United States
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900 volts across a couple layers of paper (tens of square inches) isn't enough capacitance to shock you...

i'd avoid using those transformers.
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Old 8th February 2011, 09:03 AM   #10
Bas Horneman is offline Bas Horneman  Netherlands
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Quote:
i believe this is the reason why there are plate traffos and filament traffos, being separate entities make this issue go away..
I have a separate plate choke, output transformer and filament transformer for the GM70. And a separate power transformer for the input and driver stage.

However I did not have a separate filament transformer for the rectifier tube.
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