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EL84 Push Pull Low Freq Humming.
EL84 Push Pull Low Freq Humming.
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Old 29th September 2003, 01:39 AM   #1
Steven Koay is offline Steven Koay  Malaysia
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Join Date: Aug 2003
Location: Penang
Unhappy EL84 Push Pull Low Freq Humming.

Hi, all diyers.
Recently I just completed my EL84 push pull amp which drive by 7044. It sound great & so natural. BUT it have low freq humming when I closer my ear to the speaker around 1 ft. I wouder where did the humming came from since I already having 2 stage of RC filter in PS. And all the signal wire did not cross over the heater wire or AC wire.
Could somebody provide me an advise.......
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Old 29th September 2003, 05:27 PM   #2
NickC is offline NickC
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Join Date: Dec 2001
Location: Earth
1.ground loop
2.rcrc ain't great if the tubes are not sharing equal current,
3.heater ac is not lifted ,
4.what is define as signal wire, is B+ wire included

you have to show the schematic for further diagnosis. Ground loop is the biggest culprit. the hum should be 50hz. In this a diyparadise circuit
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Old 30th September 2003, 11:06 PM   #3
richwalters is offline richwalters
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Location: Alps:Tube amp designs over 150W, SMPS guru.
Default EL84 hum

You don't mention rectifier, I presume it's EZ81 or sim. One side could be down or soggy. As per other suggestions, also check you have the correct value interstage caps fitted (from phase splitter or between driver tubes) as too high LF feedback can create a form of sub harmonic motorboating aggravated at line frequency. This is rare.

I've had this problem on a large amp using an aged 5U4.. That fault was assymetrical value 100Hz in the B+ choke, the pulsing stray magnetic field finding it's way in the input stage.
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