Hum Bug

I have a 1963 brown Fender Deluxe that has a low hum on both channels. I checked the grounding, and even replaced the AC cord with a 3-prong type, but I’m still hearing a hum. Can you tell me where to look? —Jon Axford
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When troubleshooting hum in a tube amp, we must first identify which type of hum we are hearing, as there are two possible types: 120Hz and 60Hz. The AC electricity coming out of the wall in America is 60Hz. This manifests itself as a very low-pitched hum—lower than the low E string on a guitar, and in between a B and a Bb. When you hear this type of hum in a guitar amp, it is most likely coming from the 6.3-volt AC voltage that is used to heat the tube filaments. On all tube amps there are two wires, usually a twisted pair, that go from one tube to the next. These are the 6.3-volt supply wires. On your brown Fender Deluxe, these wires go to pin 2 and pin 7 of each output tube.

During manufacture, no one paid attention to the polarity of these two wires, yet this is very important in terms of hum. The two wires are terminated on pin 2 and pin 7 of one 6V6 output tube, and again on pin 2 and pin 7 of the other 6V6 output tube. If the wire terminating on pin 2 of one 6V6 terminates on pin 2 of the other 6V6, and the wire going to pin 7 of one goes to pin 7 of the other, then the 60 Hertz hum going into the tubes will phase-cancel in the output transformer and the amp will be relatively quiet. A problem arises when the wire on pin 2 of one tube goes to pin 7 of the other tube, and the wire on pin 7 of the first tube terminates on pin 2 of the second tube. In this case, the amp will still work. However, the hum induced in each tube is added together rather than cancelled. Correcting the polarity of the wiring will fix the problem.

The other type of hum that can come from an amp is 120Hz. Vacuum tube amplifiers use DC electricity, yet the wall only supplies AC electricity. When 60Hz AC is changed to DC using a full-wave rectifier, the DC is first changed into 120Hz pulsating DC and then the filter capacitors in the power supply of the amp smooth this pulsating DC to make pure DC. Oddly enough, this 120Hz pulsating DC is exactly one octave higher in pitch than the 60Hz signal. It is the pitch between the B and Bb found on the second and third frets of the A string on a guitar. All guitar amplifiers use electrolytic capacitors to filter this hum out. However, when a capacitor gets old, it will no longer do its job. Good amplifier hygiene requires that you change all of these filter capacitors every six to ten years. Make sure to use quality American-made capacitors, as the cheap Taiwanese caps will sound blown, even when new.

Here is a test to see if your filter caps are doing their job. First, turn the amp up really loud so that the output stage will be drawing lots of current. Next, play a Bb on the G string at the 15th fret. Hold the note, and listen as it sustains. If the note starts beating, like it is really out-of-tune, then it is time to change filter caps. On a vintage amp such as a brown Deluxe, I would increase the stock value of the filter cap going to the rectifier tube from 16uf at 450 volts to 100uf at 500 volts. When these amps were first made, no one played them very loud and most bands didn’t even have a P.A. Nowadays, players like to turn the amps up to get that good output tube distortion, and this higher current draw requires extra filtering to keep the amp sounding in tune and without excessive hum.
—Gerald Weber, Kendrick Amplifiers

To learn more about maintaining, repairing, and updating your tube amps, check out Gerald Weber’s new book, All About Vacuum Tube Guitar Amplifiers, which is due out soon.