Warning · High voltage
Don't open the chassis. Valve amps run at several hundred volts inside, and the filter capacitors can hold a lethal charge after the amp is unplugged. Everything on this page is done from the outside.
Switch off and book it in if
- It smells of burning.
- It crackles or pops on its own, with nothing plugged in.
- It blows the replacement fuse (see step 1).
- It hums loudly with every control at zero.
- A power valve’s metal plate glows red (“red-plating”).
Don’t keep switching it back on to test it. Tell me what happened just before it went wrong.
1. Power
For both fuse checks, switch the amp off and pull the mains lead out of the amp first.
- Check the mains lead is pushed fully into the amp and the wall, then try a different socket and a different lead.
- Check the fuse in the mains plug.
- Check the amp’s own fuse. On most amps it’s in a holder on the back panel, with its rating printed next to it. If it’s blown, replace it once with exactly the same rating and type (“T” means slow-blow). If the new fuse blows too, stop. Something inside has failed, and a bigger fuse will turn a repair into a much bigger one.
2. Standby (valve amps)
Valve amps usually have a power switch and a standby switch. If the lights are on but there’s no sound, check standby is set to play.
3. Speaker connection
Switch the amp off before you unplug or change any speaker cable, and before you touch the impedance selector.
- Never run a valve amp with no speaker connected. It can destroy the output transformer.
- Use a speaker cable between head and cab, not an instrument cable. They look alike, but an instrument cable can overheat and fail under speaker current, and take the output transformer with it.
- Make sure the cable is in the right output for the cab’s impedance.
- In a combo, check the internal speaker lead hasn’t come unplugged. If you can’t see it without taking a panel off, leave it.
4. Things that mute the signal
- A cable in the loop return with nothing sending into it can cut the preamp off from the power amp. Unplug both loop cables. If that doesn’t help, a short patch lead from send to return is a good test of a faulty loop jack.
- On many amps, anything in the headphone socket mutes the speaker.
- Check the master volume, the channel volume and the footswitch, including which channel the footswitch has selected.
5. Look at the valves (valve amps)
With the amp on, look through the back without touching anything, because the valves get very hot. Don’t take off a back panel or cage to see in. If you can’t see the valves, skip this step. The small preamp valves glow faintly, and the power valves glow a bit more.
- A valve that’s dark when its neighbours glow may have failed.
- A silver, mirror-like coating inside the glass is normal. A milky white patch means the valve has lost its vacuum.
- If a power valve’s metal plate glows red, switch the amp off straight away and don’t use it again until it’s been checked. Red-plating can take out the valve and the output transformer.