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Unread 08-31-2009, 05:26 PM   #1 (permalink)
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Testing output transformer out of the amp

Would anyone know how to test an output transformer that is not in the amp?
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Unread 08-31-2009, 08:04 PM   #2 (permalink)
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Re: Testing output transformer out of the amp

Noting which leads connect to the B+ line, measure the resistance from the B+ leads to each plate of the output tube(s). Write the resistance reading down. Measure the other side if it's push pull. Plate windings are almost always in the tens to a few hundred ohms of resistance. A reading much over 1K is pretty sure sign that the winding is burned open. If it's open, the transformer is dead. While you're at it, measure the resistance of every lead to the housing. This reading should be very high, hundreds of K or preferably megohms. A low reading here indicates a short to the transformer core - again, dead transformer.
Do similar tests on the secondary’s - look for open windings, or shorts to the core. Finally, test for the resistance from primary to secondary. A low value, under a few K indicates an internal short, and a dead transformer.
If the transformer fails any of the simple tests, it's dead. Replace it. If it passes all of the simple tests, it may still have an internal short Sometimes a primary will have enough resistance that the transformer does not burn up or burn the wires in two , but will just bog down, have low power and sound bad. If you suspect that is the case and you don't have the skills to do the advanced tests, take it to a competent tech.
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Unread 08-31-2009, 09:11 PM   #3 (permalink)
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Re: Testing output transformer out of the amp

Thanks for the info Majornut. I was actually trying to test a tranny thats out of the amp. BTW, would you happen to have any close up pics of the output tranny on your 68 50 watt ? I have a few 784 139 transformers, one claiming to be out of a 68, that need some testing. The one that I have from the 68 is in unbelievable condition. Almost like it was pulled from the amp in 68. If you have some pictures of yours that would be great.
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Unread 09-01-2009, 12:56 AM   #4 (permalink)
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Re: Testing output transformer out of the amp

Quote:
Originally Posted by dread1 View Post
Thanks for the info Majornut. I was actually trying to test a tranny thats out of the amp. BTW, would you happen to have any close up pics of the output tranny on your 68 50 watt ? I have a few 784 139 transformers, one claiming to be out of a 68, that need some testing. The one that I have from the 68 is in unbelievable condition. Almost like it was pulled from the amp in 68. If you have some pictures of yours that would be great.
what I gave you was for out of the amp. I have one pic of a 68 trans, not even sure if its mine.

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