Whats Cookin

October 1, 2009

I OWN AN EARLY ’60S AMPEG JET J-12 AMP AND when it gets hot, it starts to smell like electrical components burning. The amp was recently checked out by a repair tech and no problem was found. But when I got it back and played it for a half hour I could smell it burning again. I want my amp to be as reliable as possible without changing the tone. What can be done? —via email

If your amp smells like it’s overheating, it probably is. Power supply caps could be the cause, and any amp that is 40 years old needs to have them replaced anyway. It’s important to note that you can’t trust a meter check to determine if the capacitors are any good. This is because a typical capacitance meter runs off of a 9-volt battery and that isn’t the same as putting the caps under a 400-volt load. Also, capacitance meters do not check leakage current, which is probably the root of your problem. All electrolytic capacitors leak some current—it is the nature of that type of capacitor. New ones don’t leak much, but when they get old they leak a lot, and the sum of all leakage can tax the power transformer and cause it to overheat. If you have four caps in your amp that are each leaking 10mA, then four of them together will leak 40mA. Your particular amp’s power transformer uses about a 125mA rated B+ winding, so those old caps could be using up one-third of the transformer’s capability at idle. And when the rest of the amp starts using more than 85mA—like when you’re playing it—the transformer will overheat. So the bottom line is that nothing lasts forever, and you should replace those old caps right away.

Here is another possible cause for overheating: A transformer uses thin laminates of steel that are insulated from each other. If they rust and become conductive to each other, then the transformer “thinks” it has another one-turn secondary winding on it and electrical current begins to flow around and around causing the transformer to overheat. To check to see if your laminates are conductive to each other take an ohmmeter and put it on one of the lower resistance settings, say 200 ohms. Put one test lead on the edge of a laminate on one side of the transformer’s core stack and put the other test lead on a laminate on the other side of the core stack. If you read infinity ohms (open), then you don’t have a problem. If you get a resistance reading, then there is conductivity and the transformer will have eddy currents that can cause it to overheat after a few minutes of use. In this case, the only cure is a new power transformer or a rewind. —Gerald Weber, Kendrick Amplifiers

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