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I have a GA-9 amp that is acting strangely. It plays fine until you turn the volume up past about 8. Then the volume drops way down and remains fairly low. Although the volume is much lower, it's still adjustable. Also, the sounds get very distorted at certain points while rotating the volume control.
Turn it off for awhile, turn it back on and normal volume returns. It doesn't appear to be a heat issue because this behavior will show up right after turn on/warm up.
Here's what I've done to it:
1. Replaced all the electrolytics. They were old - original.
2. Replaced coupling caps - they were leaking DC.
3. Replaced all the tubes. Really old.
4. Tensioned and cleaned the tube sockets.
5. Replaced the volume and tone pots. Worn.
6. Checked power supply voltages and preamp tube cathode bias voltages - within spec.
7. Speaker has distortion at less than 200Hz - needs to be replaced.
8. I tried disconnecting the amp's speaker and playing it through my bass amp cabinet. The amp did not cutout when I turned up the volume to maximum. I played for several minutes at that volume and no problem.
Could the speaker be the problem? I keep suspecting something else in the amp because the problem heals whenever I turn the amp off for awhile.
Anybody have an idea what's going on here?
The tube compliment is 12AX7, 2 - 6v6 and 5Y3. The closest schematic match is the Gibsonette in Aspen Pittman's book.
Turn it off for awhile, turn it back on and normal volume returns. It doesn't appear to be a heat issue because this behavior will show up right after turn on/warm up.
Here's what I've done to it:
1. Replaced all the electrolytics. They were old - original.
2. Replaced coupling caps - they were leaking DC.
3. Replaced all the tubes. Really old.
4. Tensioned and cleaned the tube sockets.
5. Replaced the volume and tone pots. Worn.
6. Checked power supply voltages and preamp tube cathode bias voltages - within spec.
7. Speaker has distortion at less than 200Hz - needs to be replaced.
8. I tried disconnecting the amp's speaker and playing it through my bass amp cabinet. The amp did not cutout when I turned up the volume to maximum. I played for several minutes at that volume and no problem.
Could the speaker be the problem? I keep suspecting something else in the amp because the problem heals whenever I turn the amp off for awhile.
Anybody have an idea what's going on here?
The tube compliment is 12AX7, 2 - 6v6 and 5Y3. The closest schematic match is the Gibsonette in Aspen Pittman's book.
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