The Marshall TSL 60 is a three-channel, 60-watt guitar amplifier that has a crunchy, hot tone that is most suitable for rock guitar playing. The TSL 60 is available in three different configurations. There is an amp head, which you connect to an external speaker cabinet, and two combo amplifier options. Combo amplifiers have integrated speakers. There is a one 12-inch and a two 12-inch speaker version of the TSL 60. For best tone, it's essential that your amp tubes are biased correctly. Tube bias is the distribution of current to the tubes. Unequal distribution diminishes tone and shortens tube life.
Turn off your Marshall TSL 60, and let it to cool off. The tubes get hot when in use, and if you move your amp while the tubes are hot, they may shatter.
Unscrew the back panel with a Phillips screwdriver to expose the preamp, power amp section and the bias screws. On the amp head version, the bias screws are located next to three gold-plated wire prongs. If you are working on one of the combo versions, there is a black rectangular plastic plug that covers the trim screws. Lift up the plug to access the bias screws.
Put on a pair of latex gloves, and remove the far left tube, which is one of four ECC38 preamp tubes. Attach the first bias meter probe to the tube socket. Connect the second bias meter probe to the pins on the tube base. Connect the third probe to the volt meter. Set the volt meter to "Resistance."
Turn on the amplifier, and note the reading on the volt meter. A good reading lies between 80 and 90 millivolts. If it is lower than 80, adjust the left bias screw to the right. The bias screw is connected to a bias trim potentiometer, which is a variable resistor that adjusts the flow of current across a circuit, in this case to the tubes.
Turn off the amplifier. Remove all bias meter probes, and replace the tube. Remove the next tube, and attach the probes in the same manner. Repeat the bias meter test and bias screw adjustment to set each of the remaining three tubes to the same bias ECC83. For the two right-side tubes, use the bias screw on the right.
When replacing tubes, always replace with a complete set. Putting a new tube in with a set of worn tubes throws off the bias. The new tube wears out quicker because it works harder to compensate for the old, worn tubes.
Make sure no part of your body other than your finger is touching the amplifier when you turn on the power. This amplifier carries potentially lethal voltages. Consult a professional, if necessary.