I see two possibilities. First, it could be that the cabinet is resonating at the same frequency as the C note (130.81Hz or a multiple thereof), and this is disturbing a loose electronic connection in the amplifier or speaker wire. The sound would be like static that is vibrating with the note. If this amp came into my shop, I would take it apart, hook it to a speaker load, turn the volume up, and proceed to poke every wire connection with a wooden chopstick or other non-conductive instrument. When the offending joint was disturbed, there would be a loud static sound that I could again duplicate by disturbing the joint. This could also be an internally broken lead from a resistor or capacitor, so I would poke on the leads too until I found it.
What you are hearing could also be a very common phenomenon known as “cone cry” or “edge yell.” The first time I noticed cone cry, I thought I was losing my mind until I tested other 12" speakers and noticed that almost all brands of guitar speakers do this to some degree. What’s happening is that when certain notes are played and the speaker cone is vibrating back and forth, the speaker cone can resonate and generate a frequency of its own, which is almost never harmonically related to the original note. The result is a ghost note that doesn’t sound like it belongs in the sound. You could try some different speakers in your amp to see if it makes a difference, but about the only 12" speakers I’ve found that don’t seem to produce cone cry are JBL and Electro- Voice units. —Gerald Weber, Kendrick Amplifiers
I have an original blackface Fender Super Reverb that becomes so hot after playing it for a while that I can’t touch the switches. The power transformer is rusted, and I’ve heard that this can make the transformer— and therefore the whole amp—run hotter. Will replacing the transformer reduce the heat?— Rich Whiteman
There are two scenarios that will cause the amp to run even hotter than normal. First, if the tubes are biased such that too much plate current is flowing, then the tubes will generate excessive heat that gets transferred to the chassis. I recommend having the amp’s bias checked as a first step to make sure that’s not the cause. A rusty transformer can make the amp run hotter than normal if the laminates are shorted. Steel laminates are used to prevent the transformer from “thinking” the core is just another winding. By insulating the laminates from each other, current cannot flow through them. Rusty laminates can become conductive, however, which makes the transformer “think” the core is another secondary winding. The eddy currents that flow though the laminates rob power and create excessive heat as the electrical current flows round and round. To check for shorted laminates you need to test for continuity using a multimeter on the “ohms” setting. Touch one meter lead to any rusty laminate on the left side of the transformer core, and touch the other meter lead to a right-side laminate. If you see there is continuity, the transformer will have eddy currents and it should be replaced. —Gerald Weber, Kendrick Amplifiers
Weird resonances in an amp can be caused by faulty electrical connections, or by a speaker phenomenon known as “cone cry” or “edge yell.”
Heat rising from the power tubes in Fender amps (and other amps that sling their tubes upside down) is transferred to the chassis, so it’s important that the correct bias be maintained in order to minimize the amount of heat being generated.