Ugly sounding distortion can take several forms.
- Harsh grainy sound
- Output tubes biased 'way too cold
- Rubbing or torn speaker cone
- Rarely, the amp can be oscillating ultrasonically and still get some sound through, with a
harsh, ugly sound.
- Sound cuts out or squawks on loud notes
- Muffled or constricted sound
- Failing preamp tube; find the offending section by tube swapping and see if a good tube
fixes the problem
- Low signal tube bias is pushing it into saturation or cutoff. Measure operating voltages
on the preamp tubes. The problem section will have tube pin voltages that are 'way off normal.
- Failing coupling capacitors from the preceeding stage.
- Failing plate resistor, cathode bypass capacitor, cathode resistor, or grid resistor
- Power supply problem; a dropping resistor may have drifted far from it's nominal value, changing the power supply voltage enough to cause this.
- Faint out of tune sound on every note
- Excessive power supply ripple, usually indicating that the power filter capacitors are going bad. This can also be caused by speaker cones and voice coils with problems so they just rub slightly, too.