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Old 16th December 2010, 04:45 PM   #1
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Default Mains Transformer Hums under load

I just completed my 2A3 amplifier, and when the B+ comes up the mains transformer hums real bad, the amp works and the B+ voltage is what it should be. Do you think the mains transformer is bad? or could it be something else?
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Old 16th December 2010, 06:02 PM   #2
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The two most common causes of transformer hum (apart from overload) are:

1. Loose laminations. If the stack is not properly compressed then individual ones can buzz. Sometimes you can check this by squeezing the stack with a clamp.

2. Bolted directly to a steel chassis. The magnetic filed couples to the chassis and can cause a buzz. The solution is to preferably use a non-ferrous chassis or mount the transformer on non-ferrous stand-offs.

Cheers

Ian
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Old 16th December 2010, 06:09 PM   #3
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Old 16th December 2010, 06:14 PM   #4
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Tube rectifier or solid state? HT fuse or just incoming power?

At least you're in the States, so it can't be the 50Hz-60Hz thing (lots of American gear overheats and buzzes in Europe because a 60 Hz transformer has less metal in it than its European cousin)

Unplug all the tubes and HT fuse, plug in amp, turn on. If tube rectifier, and/or there is HT fuse, and transformer still hums, probably duff transformer or something silly like a loose washer vibrating in the magnetic field. Probably it doesn't, which proves nothing. Turn off. Plug in rectifier tube, and HT fuse, but no output tubes. Turn on amp. Does power transformer hum? If yes, check HT voltages, check for ripple (multimeter on AC volts, 1,000 volt paper capacitor in series) should be negligible. Measure current in HT line, check whether anything is getting hot (apart from the tube). (All right some of this involves "turn off amp. Unplug power line. Short out reservoir capacitors. Wait two minutes and short out reservoir capacitors again.")

If transformer did hum at this stage, could be a duff rectifier, a leaky capacitor - something that's pulling too much current. If it doesn't, check grid voltages on output sockets; it could be that a tube is pulling massive amounts of current, but is still within working limits (although the fact the anode is glowing dull red on no signal could have been a clue. If these are correct, we're going to have to put the tubes in and check for RF instability; unfortunately the transformer hum symptom can have a lot of causes. Best to replace speaker load with resistors here; even if you can't hear that high the tweeter's not enjoying it. An oscilloscope is useful here. Put it, or the multimeter on AC volts, across the speaker terminals. Set the volume to minimum and short circuit the input. If there are any volts there, there are a lot more before the output transformer. Check all RF suppression components.

If output tubes not heating up, but transformer hums on quiescent current, in all probability power transformer duff. Does it get hot, too?
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Old 16th December 2010, 06:56 PM   #5
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Loose secondary winding?
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Old 16th December 2010, 07:14 PM   #6
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Thanks I'll check some of this out. this is a rebuild from another chassis and worked fine before I rebuilt it. The tubes don't appear to be glowing extra hot and the voltages are correct. I never checked the current before so I'm not sure what it should be, it does seem like its stressing the power trans. It seems like I am hearing something from the one of the output trans too, just slight though.
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Old 16th December 2010, 08:16 PM   #7
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is the transformer a toroid ?

maybe you should check this out :
Mains DC and Transformers
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Old 16th December 2010, 09:07 PM   #8
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Its a hammond 302AX rectifier is 2 5Y3GT's
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Old 16th December 2010, 09:55 PM   #9
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Is your power supply well dimensioned?
You could check with Duncan software.
Once I had a problem with a humming power supply transformer (which was well impregnated a.s.o.). At the end I put a choke right after the tube rectifier making a choke input power supply, and the transformer was completely silent (though there was less DC voltage of course). Capacitor input power supplies can sometimes cause resonances upsetting the power supply transformer.
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Old 17th December 2010, 02:45 AM   #10
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Quote:
Originally Posted by mark02131 View Post
I just completed my 2A3 amplifier, and when the B+ comes up the mains transformer hums real bad, the amp works and the B+ voltage is what it should be. Do you think the mains transformer is bad? or could it be something else?
Hello,
Hum is the nature of the beast. Forces alternate with the AC voltage/ current. Choke input power supplies are worse.
Recommendation; Take the fasteners out of the transformer at the chassis. Isolate the transformer from the chassis at each of 4 corners on a Pink Pearl eraser (like used for pencil marks on paper). The noise will most likely be much attenuated. If the noise is reduced with this test you need some vibration isolation between the transformer and chassis. Closed cell Neoprene maybe, the screws used for attachment should remain loose and be double nutted at the bottom. Ground the frame of the transformer for safety.
DT
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