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#11
10-19-2013, 03:42 PM
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Hum isn't necessarily caused by defective capacitors. A poor ground is a good possibility. Since it varies with the volume control then it's coming from an earlier stage. Short the middle volume control lug to ground and see if the hum clears up. It also may have more than one source. A leaky heater-cathode in a tube, a corroded ground lug mounting screw, lots could do it.
Describe the hum. Is it very low pitched, smooth? Raspy with lots of high frequency content? More like a buzz? 
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