Don't put a beer on top of your Amplifier!

Here is a recent repair job I did on a B-52 AT-100 guitar amp head. The client had low volume output, two power tubes glowing really hot, and a story about spilling something on the amp. After opening it up I noticed that one of the screen resistors were blown. The screen resistor is used to control the amount of current from the bias supply to the screen of the power tubes.






Each power tube will have a screen resistor. After more inspection the underside tube socket had carbon build up all around it. This is a sure sign that something is wrong. I de-soldered the tube socket and to my surprise there was a hole burnt through the board and a substance in the hole. Apparently what got spilled was still there and it created a pseudo resistor. The resistance of this substance was much lower than 470 ohms which is the screen resistors value, and with the pseudo resistor and the actual screen resistor in parallel, the resistance would be decided by the pseudo resistor. A smaller resistance means more current.
Once I cleaned out the gunk and soldered everything back together the amp worked great! Something to take from this is don't set your beer on top of your amp when your playing a show.