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Old 6th September 2012, 12:59 PM   #1
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Default Output Transformer Issue???

Hi, I have a 2 year old ultra-linear output transformer that I believe is malfunctioning but I guess I don't know how that could be or how to prove it is...basically it's been working fine the last two years but recently a tube malfunctioned (not bad, just a couple "sparks" inside after coming off standby and then I immediately turned it off). I replaced the tube and since then whenever I take it off standby I get a loud hum (it sounds exactly like a grounding issue) and then blows a fuse. Even with all the power tubes removed it does the exact same thing. I tried removing different components in the power section to see if something else was causing the issue but the only thing that consistently causes this "phenomenon" is when the B+ is connected to the OT's center tap so to me it's got to be something between that point and the outputs. The plate and screen pins of the power tubes do not show a short to ground, but the outputs on the secondary do (I can't remember if that's normal or not...). Any ideas? It seems like there's got to be a short to ground somewhere in order for the current to move through the transformer and create an audible signal on the speaker...how can I test an OT to verify whether it's working correctly or not? Thanks.
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Old 6th September 2012, 01:13 PM   #2
20to20 is offline 20to20  United States
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Originally Posted by howe0168 View Post
Hi, I have a 2 year old ultra-linear output transformer that I believe is malfunctioning but I guess I don't know how that could be or how to prove it is...basically it's been working fine the last two years but recently a tube malfunctioned (not bad, just a couple "sparks" inside after coming off standby and then I immediately turned it off). I replaced the tube and since then whenever I take it off standby I get a loud hum (it sounds exactly like a grounding issue) and then blows a fuse. Even with all the power tubes removed it does the exact same thing. I tried removing different components in the power section to see if something else was causing the issue but the only thing that consistently causes this "phenomenon" is when the B+ is connected to the OT's center tap so to me it's got to be something between that point and the outputs. The plate and screen pins of the power tubes do not show a short to ground, but the outputs on the secondary do (I can't remember if that's normal or not...). Any ideas? It seems like there's got to be a short to ground somewhere in order for the current to move through the transformer and create an audible signal on the speaker...how can I test an OT to verify whether it's working correctly or not? Thanks.
Which tube did you see the sparks in? Output or Rectifier?
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Old 6th September 2012, 01:18 PM   #3
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Output.
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Old 6th September 2012, 01:22 PM   #4
20to20 is offline 20to20  United States
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Output.
Is this a 2 channel amp? P-P or SE?

EDIT... Sorry...It has to be P-P with a center tap...

2 channel?
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Old 6th September 2012, 01:23 PM   #5
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Single channel and P-P
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Old 6th September 2012, 01:47 PM   #6
20to20 is offline 20to20  United States
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Single channel and P-P
It's possible you have a primary to secondary high resistance short that only occurs with power on. If you can read any resistance between them it's not good. You need to determine if your (-) speaker terminal is normally grounded or not.
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Old 6th September 2012, 01:58 PM   #7
palmas is offline palmas  Portugal
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disconnect the output transformer and check insulation of all windings (between wirings and iron, and from winding to winding.
My guess is you melted the insulation on the primary of the transformer, but this usually leaves a very characteristic smell...
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Old 6th September 2012, 02:39 PM   #8
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Any easy way of checking the windings or will this require taking apart the transformer?
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Old 6th September 2012, 02:57 PM   #9
palmas is offline palmas  Portugal
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Any easy way of checking the windings or will this require taking apart the transformer?
Don't need to take it apart, just disconnect all wires from PSU and ground, remove tubes and speakers, and measure...
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Old 6th September 2012, 02:59 PM   #10
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Cool, thanks.
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