• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

GM70 psu transformer.

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Bas Horneman

Bas Horneman

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Joined 2002
basaudio.wordpress.com/
#1
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?
 
#2
Bas Horneman said:
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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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.
 
#3
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.
 
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#4
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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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.

Unless it is insulation breakdown. You could check this with a 1000V insulation tester.
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I'll have to google for an insulation tester. Thanks.
 
#6
i believe this is the reason why there are plate traffos and filament traffos, being separate entities make this issue go away.....

IMG_1302.jpg


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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