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Ampeg Jet J12-T volume problems


 
3/6/2006 3:14 AM
John L.
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Ampeg Jet J12-T volume problems
Hi,  
I have an Ampeg Blue Diamond Lead Series Guitar amp model Jet J12-T that has problems. Paul from this board has been helping me and provided me with schematic. He has been extremely generous with his time and knowledge, and I hope we can figure out the problem. I am at a stand still and I thought I would pick some of you fellow amp guru's brains. I can send you the schem if you like. All the controls work as they should, volume, tone, tremelo speed and intensity, and reverb. The main problem is there isn't very much output volume wise. The amp is 15 watts, but sounds like it is pushing out maybe 3 or 4 watts, volume wise. There is a slight rice crispy crackle sound that is intermittent accompanied with a slight change in volume. The tubes have been changed, no difference. I noticed the volume is the same with one power amp tube in or two in. The amp uses a 12ax7 for preamp, and one for phase inverter, and 2 el84's for the power amp section. It seems like maybe the voltages are screwy, but I'm not sure. The schems have some test points listed and I checked most. The plate voltage on the preamp tube was very low, almost half what it should be, but now its back up to being close to where it outta be. I checked most of the resistor values and connections all seem good. I saw online a case where a guy had bad screen resistors that kept fryin his power tubes, with this particular model. Has any one out there worked on this model amp? (I am a novice technician, but am learning all the time. I know enough not to fry myself so don't worry.)Thanks. Regards, John L.
 
3/6/2006 10:35 PM
John L.
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Hi all,  
I think we got it figured out. Gonna replace the filter caps as per Paul's suggestion. I just found out Paul's credentials so I don't think outside help will be necessary. What an awesome forum this is! Best Regards,  
John L.
 
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