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Old 25th August 2005, 04:06 AM   #1
diyAudio Member
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Angry Big Big hum ???

Hi, i was testing some mod i done to my amp and suddenly the only thing i heard was a big hum. What could it be ? Can my output transformer burn out ???

I took some voltage and found out that one of my decoupling cap fail. Instead of having 270v i was reading 50v. I change the cap and the voltage was up again. But there was this big HUM. Can the voltage peak that burn out the cap destroy the output transformer too ?

Thanks for the help....
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Old 25th August 2005, 06:37 AM   #2
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You don't mention the type of transformer/rectifier arrangement you are using for B+. If you do have a center-tapped secondary with full-wave rectification using two diodes (as ooposed tp a single untapped winding and a diode bridge), then it's possible one half of the winding could have burnt out as a result of a partial short-circuit caused by the bad cap. This would give you half-wave rectification and 50Hz/60Hz hum.
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Old 25th August 2005, 11:41 AM   #3
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The amp have a full wave rectifier ( using a 6x4 tube rectifier ). the amp is a single ended.

I have all the good voltage. when the amp is reaching is voltage, i heard the hum coming out of the speaker gradualy. But is a very big hum, like if there a ground missing.
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Old 25th August 2005, 12:06 PM   #4
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Default HUM

Remove the rec tube, and check for correct voltages at the anode connections on the socket. If one is missing, then the mains Tx could be faulty--check all connections to secondaries. If OK, try another rec tube--could have suffered when the cap went. If still hum, check all other smoothing caps or chokes in the supply--they could be bad.

Worth checking for heater/cathode shorts in all tubes except rec too! Unlikely the O/P Tx is bad, but possibly Mains Tx could be.
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Old 26th August 2005, 03:53 AM   #5
PRR is offline PRR  United States
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> Can my output transformer burn out???

Sure, but why would that cause hum? It causes silence.

> big hum, like if there a ground missing.

Good guess. Since signal flows, look for a lost power ground.
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