12.11.2003
I'm deciding whether to buy a Gibsonette GA 8T guitar amp from a friend. It's not a GA 5T but it does have tremolo so I'm checking it out.
Right now it sounds gravelly at any volume and the tremolo does not work. I have it on loan for a couple of days to try to figure out what's wrong with it and then I can decide if I want to buy it.
It uses a 12AX7 pre, a 5Y3GT rectifier, and 2 6BM8 power tubes.
So far: I replaced the 12AX7 w/ no effect. I swapped the power tubes left to right (I don't have replacements) and it sounds even worse (squeals).
I found this at Schematic Heaven: Gibson GA 8T schematic
...fortunately I have a much clearer version from a book.
Here's some stuff from this cool amp troubleshooting page:
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
Failing coupling capacitor
Failing plate resistor, cathode bypass cap, cathode resistor or grid resistor
Intermittent ultrasonic oscillation
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.
I also posted the problems on Tubes Asylum.
Right now it sounds gravelly at any volume and the tremolo does not work. I have it on loan for a couple of days to try to figure out what's wrong with it and then I can decide if I want to buy it.
It uses a 12AX7 pre, a 5Y3GT rectifier, and 2 6BM8 power tubes.
So far: I replaced the 12AX7 w/ no effect. I swapped the power tubes left to right (I don't have replacements) and it sounds even worse (squeals).
I found this at Schematic Heaven: Gibson GA 8T schematic
...fortunately I have a much clearer version from a book.
Here's some stuff from this cool amp troubleshooting page:
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
Failing coupling capacitor
Failing plate resistor, cathode bypass cap, cathode resistor or grid resistor
Intermittent ultrasonic oscillation
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.
I also posted the problems on Tubes Asylum.
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