Shorted secondary windings of transformer

Edvard · 29th December 2010, 08:40 PM

Hi

Unfortunatelly I shorted the secondary windings of my power transformer. I was attempting to solve a hum issue and thought I was connecting the transformer shield to ground but instead I connected a tap of the B+ winding to ground by mistake. The transformer is a Hammond 300BX with a 400-50-0-400 winding. This caused both the 50V tap and the center tap to be grounded. After about 1 min after powering on the amp, the fast blowing 3 A fuse blew( I live in europe 220V and my amp has 2x 300BX, one for each channel).

I'm trying to understand what damage I may have caused and I dont understand why the fuse didn't blow immediatelly. The power supply is fairly standard: transformer, rectifier, cap, choke, cap... the rectifier is a 83 mercury rectifier if that matters.

hope you can help me find some damage or suggest what to test before powering on again, thanks

Joshua_G · 29th December 2010, 09:19 PM

Does the secondary have now the right output voltage?
If yes, possibly no permanent damage was done.
If not, the transformer would need to be rewound, or replaced.

Tom Bavis · 29th December 2010, 09:24 PM

Probably no damage in one minute. But check the resistance from the 50V tap to ground against the other transformer - if it's less, you have shorted turns.

Do you have a separate fuse for each transformer? That would give better protection...

Protecting transformers fully is NOT easy. For example, if the 50V winding has 20 Ohms resistance, it will draw 2.5A when shorted - that's 125 VA or about a half amp at 240V... hardly a noticeable increase! Worse yet, the wire resistance goes up when it heats, so the current DROPS from there. The loss of bias (if you used it) would cause the tubes to draw lots of current, and that may blow the fuse.

Edvard · 29th December 2010, 09:40 PM

I' going to remove all tubes and test the secondary voltages.

Tom, I was using the same fuse for both channels. I will add separate fuses if I manage to get the amp working again, good idea. The amp uses cathode bias which was probably upset, I think your theory is correct. It wasn't the shorted secondary that drew enough current to blow the fuse it may e.g. have been the output tubes because of wrong bias.

Will test voltages and let you know, how it goes.

Edvard · 29th December 2010, 10:20 PM

Tested the resistances of the windings and one of the transformers seems to be broken. Resistance of 400 to CT is 10 ohms but the other side is about 50 ohms. The other transformer shows about 50 ohms on both sides of the center tap. Will test voltage tomorrow - need a new fuse... And yes was using the same fuse for both channels, good idea to have separate fuses for each channel.

Tom, i think your theory is right, bias got upset, amp uses cathode bias.

Thanks for the help. Hope the output tubes are ok.

Edvard · 29th December 2010, 10:22 PM

Sorry about posting the same reply a few times, did not notice that moderator must approve reply.

29th December 2010, 08:40 PM	#1
Edvard diyAudio Member Join Date: Dec 2010	Shorted secondary windings of transformer Hi Unfortunatelly I shorted the secondary windings of my power transformer. I was attempting to solve a hum issue and thought I was connecting the transformer shield to ground but instead I connected a tap of the B+ winding to ground by mistake. The transformer is a Hammond 300BX with a 400-50-0-400 winding. This caused both the 50V tap and the center tap to be grounded. After about 1 min after powering on the amp, the fast blowing 3 A fuse blew( I live in europe 220V and my amp has 2x 300BX, one for each channel). I'm trying to understand what damage I may have caused and I dont understand why the fuse didn't blow immediatelly. The power supply is fairly standard: transformer, rectifier, cap, choke, cap... the rectifier is a 83 mercury rectifier if that matters. hope you can help me find some damage or suggest what to test before powering on again, thanks

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
adding and using secondary windings	jrenkin	Power Supplies	0	19th April 2010 03:05 AM
Transformer Windings	impsick	Chip Amps	6	2nd February 2007 11:00 AM
cap btween primary and secondary windings?	sam9	Power Supplies	8	19th May 2005 06:34 PM
Transformer windings identifications - how to get it?	Alfetta87	Solid State	7	21st February 2005 04:01 PM
Parallel secondary windings	jlm	Pass Labs	6	27th March 2004 10:39 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

29th December 2010, 09:19 PM	#2
Joshua_G diyAudio Member Join Date: Nov 2008 Location: Israel	Does the secondary have now the right output voltage? If yes, possibly no permanent damage was done. If not, the transformer would need to be rewound, or replaced.

29th December 2010, 09:24 PM	#3
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	Probably no damage in one minute. But check the resistance from the 50V tap to ground against the other transformer - if it's less, you have shorted turns. Do you have a separate fuse for each transformer? That would give better protection... Protecting transformers fully is NOT easy. For example, if the 50V winding has 20 Ohms resistance, it will draw 2.5A when shorted - that's 125 VA or about a half amp at 240V... hardly a noticeable increase! Worse yet, the wire resistance goes up when it heats, so the current DROPS from there. The loss of bias (if you used it) would cause the tubes to draw lots of current, and that may blow the fuse.

29th December 2010, 09:40 PM	#4
Edvard diyAudio Member Join Date: Dec 2010	I' going to remove all tubes and test the secondary voltages. Tom, I was using the same fuse for both channels. I will add separate fuses if I manage to get the amp working again, good idea. The amp uses cathode bias which was probably upset, I think your theory is correct. It wasn't the shorted secondary that drew enough current to blow the fuse it may e.g. have been the output tubes because of wrong bias. Will test voltages and let you know, how it goes.

29th December 2010, 10:20 PM	#5
Edvard diyAudio Member Join Date: Dec 2010	Tested the resistances of the windings and one of the transformers seems to be broken. Resistance of 400 to CT is 10 ohms but the other side is about 50 ohms. The other transformer shows about 50 ohms on both sides of the center tap. Will test voltage tomorrow - need a new fuse... And yes was using the same fuse for both channels, good idea to have separate fuses for each channel. Tom, i think your theory is right, bias got upset, amp uses cathode bias. Thanks for the help. Hope the output tubes are ok.

29th December 2010, 10:22 PM	#6
Edvard diyAudio Member Join Date: Dec 2010	Sorry about posting the same reply a few times, did not notice that moderator must approve reply.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)