Fixing an amp that turns off or drops in power after warming up.
It's pretty hard to fix an unseen amp, as there are so many factors and so many questions that have to be answered.
When the amp dropped in power, was it real sudden? and did the power come back later on - then drop off again?
If so, it may be a heat-related thing, such as a bad capacitor or tube.
Often an old amp will have bad filter capacitors - these trouble makers are very important to the operation of the amp. On a Fender they are in a "box" under the amp chassis. Other amps may have “can” capacitors, or other locations for these fairly large parts.
When they are too old they become undependable - after they get hot, one may short out - then when it cools down, the short goes away.
Look at the "filter caps for your tube amp" picture and section on our web site (http://www.torresengineering.com/filcapforyou.html ), then compare to your amp. If you see filter caps with the same brown color, they have very little chance of being good, and should be replaced. Also look for "dimples" or leaks of white powder - you are not likely to see as extreme a case as shown on the web page, but you may be able to see something wrong.
Next possible problem - power tubes. Old tubes, even super good stuff wear out. Since their whole "thing" is being hot, they can easily have an intermittent, or heat related short circuit. If it is in the screen grid circuit, one tube will turn itself and the other off - Also old tubes will certainly have drifted off "bias" the all-important adjustment of standing current to the tubes. We have a flyer available to help you set the bias correctly
Part# (Bias Note) Bias instructions $5.00 (http://www.torresengineering.com/biasnotes.html )
If bias is really off, (very possible) the tubes will run too hot and shut off after they get past their tolerance for heat.
In installing a kit, it is possible to make a small error that causes the amp to turn off.
Fender and others havee ground wires from the circuit board to the brass strip under the pots. When amps get old, the circuit board stretches, and pulls on these wires (they are often too short) eventually they break, or become separated from their solder connections. This causes the cathode of the tube(s) to become disconnected - turning the amp, or parts of the amp off like a light switch.
Check out all these things, and you should be able to get the amp running again.
For more info check out "Inside Tube Amps" by Dan Torres, available direct from Torres Engineering (http://www.torresengineering.com/inbydantor.html).
$45.00 + shipping.
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