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Ampeg Jet J12-T problems Help!


 
3/19/2006 9:25 AM
John L.
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Ampeg Jet J12-T problems Help!
Hi, I have a re-issue Ampeg Jet II J12-T blue diamond series guitar amp that has some problems. I have the schematics if you think they might help in assisting the diagnosis, just email me and I can email em to you pdf file.  
The main problem is the amplifier doesn't put out like it should. All the controls work fine, volume, tone, tremelo speed and depth and reverb. The amp breaks up nicely when the volume knob is cranked, but the over-all volume of the amp is very quiet, it sounds like maybe its a 3 watt amp at best, where it should be a loud 15 watt. There is a rice crispy kinda hum like sound that accompanies minor intermittent changes in volume. The tubes were changed with no difference made. I also changed the filter caps, because the schematics show test points, and some of the voltages are off. The first of which is the DC voltage of the preamp tube at the plate seems to run about 1/2 of what it should. Some of the other dc voltages seem a bit screwy, like some kinda oscillation might be going on. Also, when I was changing out the tubes, I notice there is no difference in volume change whether there is one power amp in or both in. I'm at a standstill.  
Any help would be greatly appreciated. I am not a professional tube amp repair person, but I know a little and am learning more all the time, especially from people over the internet. Thanks. John L.
 
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