How to Change Amp Tubes. Your amplifier periodically needs to have the tubes changed. If you hear the volume begin to drop, then it's definitely time to change the tubes If you begin to blow fuses, it's a sign that the tube is on its way out. Listen for microphony, a low rumble, as a sign that you need to change the amp tubes.
Find out whether you have a self-biasing amp. There are two types of amplifiers. Marshall and Fenders have to be biased, while others are self-biasing, which means that they're a hybrid of transistors and tubes. If your amp isn't self-biasing, you need to bias after you change amp tubes.
Decide whether you want to change the preamp tubes at the same time. Many times, it's a good idea to change the preamp tubes when you change the amp tubes. This avoids another potential source of downtime.
Allow the amp to cool before you do anything. Make sure you turn off and unplug the amp before you remove the tubes.
Pull the tube up to remove it. The tube fits into the socket with little pins, which keep it secure. Once you remove the tube, check for burn marks to the socket. If you see burn marks, take it to a professional for servicing.
Use the numbers and symbols on the tube to find the replacements. Use the same brand for all the tubes and don't mix them. Change all of them at once. If you don't, the older tubes draw power from the new tubes and blow them out sooner.
Insert the tube into the socket. Turn it until you find the holes for the pins to go in, and then push down.
Take the amp into a qualified tech to be biased. You bias the tubes while the power is on and if you make a mistake you become the recipient of 350 to 450 volts of electricity. Save money on this and buy double what you need. If you buy a balanced set of four and only need two, carefully store the remaining tubes for next the change. You save the biasing on the next set of tubes.
You need to use a digital multi-meter when you to determine the cathode current when you bias after you change amp tubes. A formula of cathode voltage divided by ohms gives the current in milliamps. If you never biased amp tubes before, take it to a professional. If you play a lot and play loudly, you'll need to change tubes more frequently.
Use a cloth or gloves to remove the tubes and replace them. The oil on your fingers sticks to the tube and draws heat away. The cool spot on the tube creates a potential for a shattered tube. The cloth protects your hands if the tube breaks.