How to Repair Tube Amplifiers

The preamp houses the dials and potentiometers.

Tube amplifiers use vacuum tube technology to amplify the guitar signal. Tube technology has been superseded by solid-state, transistor technology, which is more robust and reliable. However, many guitar amps still use tubes due to a perceived higher quality of sound. But the higher sound quality comes at the price of reliability. Tube amps require more ongoing maintenance, such as biasing and tube replacement. Tubes burn out, which can cause other components in the amplifier to break.

Things You'll Need

• Screwdriver
• Replacement power tubes
• Replacement preamp tubes
• Replacement fuses
• Soldering iron

Instructions

1. 

   One blown fuse can cause the amp to completely fail.

   Troubleshoot your amplifier. Before you begin repairing your amp, it's smart to get an idea of where the problem might be located. With the power off, plug into your amp and turn the volume dial to zero. Turn the amp on and wait a few seconds. If the amplifier completely fails to power up, the problem is most likely in the power section. A blown fuse, power tube or even a loose connection to the power adapter could be the cause. If the amp powers up, gradually increase the volume. If there is power but no sound, the problem is likely located in the preamp section. If there is power but the sound is dull, crackling or intermittently dipping volume, the problem is likely to be with the speaker.

2. Unscrew the back panel and inspect the power section. The power section of most tube amplifiers contains power tubes, a power transformer and a mains connection. You can spot a burned out tube quite easily. The glass will have a brown discoloration. Replace all of the tubes with a matched set. This removes the need for biasing. Remove the old tube by pulling it straight out of its housing. Then slot the new one in its place.

3. Replace fuses in the power section. When a tube blows out, it can cause fuses in the same section to blow out too. Replace all of the fuses in the amp as a matter of maintenance, and power up the amp. If you still can't get any power to the amplifier, the power transformer may be damaged. Refer this problem to a qualified electrician.

4. Replace the power section and remove the preamp. The preamp in the vast majority of tube amps will slide out partially, certainly enough for you to access the tubes. If you wish to remove it entirely, disconnect the wires with a soldering iron. Dying pre-amp tubes make the sound fuzzy and harsh. If this is one symptom, replace all the tubes as a matched set. It's wise to swap like for like, but some tube types are interchangeable. The Tube Store website has a handy gain factor chart that lists interchangeable tube types. Replace preamp tubes as you would your power tubes.

5. Inspect the wiring and circuit board. If there is still a sound problem, check that the components on the circuit board are intact. Careless transportation can cause capacitors and diodes to come loose. Re-solder any loose ones back in place. Inspect the speaker for rips. If you spot any, unscrew the speaker, disconnect the wires and replace it with a new one.

Tips & Warnings

• Always leave your amp to cool down before moving it. The tubes remain hot after use and are more likely to shatter.

• Never open up your amp without disconnecting the power supply first.

References

• K Bapps: Tube Amp Tips
• Vox AC15: Vox AC15 Valve Maintenance -- How To Change The Tubes

Resources

• The Tube Store: Gain Factor