Troubleshooting Types of Amplifier Noise - Part 3
You turn on your amp only to hear a hum coming from it that you weren’t expecting. What can you do?
Hum in an amplifier comes in two types:
60 cycle hum
This type of hum more often than not originates outside of your amplifier. However, it can also come from the filament supply, a burnt bias pot or resistor or from the filament and grid wires being too close to one another.
In a fixed bias amp, 60 cycle hum usually comes from a bad bias supply filter cap.
120 cycle hum
Unlike 60 cycle hum, this type of hum does originate within the amplifier. It most likely comes from bad filter caps or a coupling cap becoming conductive.
Troubleshooting
Rob Hull Custom Guitar Cable
To determine if your amp hum has an outside cause, unplug your guitar cable from the amplifier. If the hum stops, you know the amp isn’t the problem. Instead, your cable or input jacks and plugs could be the culprit.
If this is not the cause of the hum in your amp, you can do a visual inspection of the components to look for burnt resistors or bias pots. Also, check the filament and grid wires proximity to one another.
If the hum is 120 cycle, you can find out if a coupling cap has become conductive by checking the plate voltage of the tubes. One of them will be noticeably low compared to the others.
Disclaimer: Unless you are an experienced amp technician or trained in electronics, please do not attempt to make repairs on your amplifier yourself due to risk of serious injury, including electric shock and burns.
Source: Weber, G. (1997). Tube Amp Talk for the Guitarist and Tech, p. 17-25. Kendrick Books: Texas
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