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Old 8th March 2011, 10:35 PM   #1
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Default power transformer question

Hi all,


I am restoring an ECL82 amp - mullard 4-7 design. Anyway, all the voltages on the transformers check out AOK - in fact exactly as per the mullard schematic, even the windings etc match up perfectly. One thing though, and I know a bunch of the seasoned valve people here will know this....

There is a bare wire coming from the power transformer - its bare all the way back into the windings. Now its not that the insulation has been burnt off or anything. I'm thinking this is a screen on the Tx..... in the original amp it was wired to chassis ground.

Anyway, was hoping to check with you before mounting it up - are there any checks I can do to verify that its a screen?

Fran
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Old 8th March 2011, 11:48 PM   #2
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You should measure open circuit to all the other terminals. If so you can assume it's a shield.
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Old 9th March 2011, 12:17 AM   #3
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Ah yes - a lightbulb moment!

I'll do that - thanks!
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Old 9th March 2011, 02:41 AM   #4
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It's an interesting topic as to where best to connect a PT screen (assuming just the one screen, and assuming it isn't a proper PE earth shield).
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Old 9th March 2011, 10:46 AM   #5
DF96 is offline DF96  England
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I would connect it to chassis/safety ground.

Incidentally, a related point is what to do if you mount a power transformer on rubber grommets in order to reduce mechanical hum. The transformer frame should be still be grounded. Normally the fixing would do this automatically, but you may have to add a ground wire to the frame.
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Old 9th March 2011, 11:38 AM   #6
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From a PE perspective, some transformers have to have a PE screen and chassis earthed - driven by certain safety standard requirements. Reinforced insulation systems typically don't require the core to be bonded to earth. In practise I would agree that grounding the frame is additional insurance against a fault causing the core to go hazardous without you knowing it.

From a non-safety perspective, then the core is somewhat similar to an internal screen, in that its main impact is capacitive coupling and screening, and the mounting becomes the return point for the capacitive currents.
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Old 9th March 2011, 11:04 PM   #7
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Just an update - it seems it is the screen, no continuity to any other winding. Its soldered to the chassis earth!



Great big thanks you to all....
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