power transformer question

woodturner-fran · 8th March 2011, 10:35 PM

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

bob91343 · 8th March 2011, 11:48 PM

You should measure open circuit to all the other terminals. If so you can assume it's a shield.

woodturner-fran · 9th March 2011, 12:17 AM

Ah yes - a lightbulb moment!

I'll do that - thanks!

trobbins · 9th March 2011, 02:41 AM

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).

DF96 · 9th March 2011, 10:46 AM

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.

trobbins · 9th March 2011, 11:38 AM

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.

woodturner-fran · 9th March 2011, 11:04 PM

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....

8th March 2011, 10:35 PM	#1
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Power transformer question	keithgreenhalgh	Tubes / Valves	4	6th August 2008 07:37 AM
transformer power question	needtubes	Parts	2	9th April 2004 11:13 AM
PSU2 and power transformer question.	RockysDad	Tubes / Valves	1	5th February 2004 11:23 PM
i am new, power transformer question	i_drives	Parts	1	25th November 2003 12:53 AM
power transformer question	Bill Fitzpatrick	Solid State	5	8th June 2001 04:22 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

8th March 2011, 11:48 PM	#2
bob91343 diyAudio Member Join Date: Mar 2010	You should measure open circuit to all the other terminals. If so you can assume it's a shield.

9th March 2011, 12:17 AM	#3
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	Ah yes - a lightbulb moment! I'll do that - thanks!

9th March 2011, 02:41 AM	#4
trobbins diyAudio Member Join Date: Feb 2009 Location: Melbourne, Oz	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).

9th March 2011, 10:46 AM	#5
DF96 diyAudio Member Join Date: May 2007	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.

9th March 2011, 11:38 AM	#6
trobbins diyAudio Member Join Date: Feb 2009 Location: Melbourne, Oz	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.

9th March 2011, 11:04 PM	#7
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	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....