If your Marshall tube amplifier seems to have lost some of its power, is cutting in and out, or is beginning to sound dull or buzzy, there is a good chance that some or all of the tubes need to be replaced. The lifespan of tubes varies greatly and depends on how often an amp is played and how hard it is driven. However, you will get the best tone out of your Marshall when its tubes are fresh. Marshall recommends replacing all tubes at the same time for the best results, but this can be expensive. You can typically get away with replacing only the power or preamp section.
- Skill level:
- Moderately Easy
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Things you need
- Replacement tubes
Power off the amplifier and unplug it from its power source. If the amplifier has recently been used, give it several minutes to cool down, as the tubes can get quite hot.
Loosen the screws holding the back grille in place and remove it from the amplifier so that you can access the electronics compartment and tubes. The exact location and number of the tubes in a Marshall amplifier varies from model to model, but there will be a section of smaller preamp tubes as well a section of larger power-amp tubes. Depending on your model, there may be additional tubes for driving the reverb and rectifier sections of the amplifier.
Using a towel, grasp the tube you wish to remove with one hand and use the other hand to spread the metal clips at the base of the tube (avoid touching tubes that will not be replaced, as residual oils could cause tubes to overheat and blow out). Pull the tube upward and out of its socket.
Align the pins of the new tube using the notch on the base of the tube and socket as a guide, and press the tube directly down into the socket. Always replace tubes with the same type and rating. In Marshall amps, preamplifier tubes are typically 12AX7 tubes, and the power section will most likely run on EL34 tubes. You can find this information on the old tubes.
Repeat steps 3 and 4 for each tube that needs to be replaced.
Reclose the metal clips back around the bases of the tubes, and reinstall the metal grille onto the back of the amplifier.
Tips and warnings
- When replacing power tubes, it is best to replace them as a set. Having one new tube with one old tube can cause the newer tube to burn out more quickly. Power tubes are typically sold as a matched set.
- From time to time, an amp will need to be biased after power tubes have been changed. If you find that the life of your tubes has been reduced drastically, your amp may need to be biased.
- Never attempt to replace amplifier tubes with the amp powered on. The voltages running through an amplifier are lethal.
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