Safety

Tube amplifiers and audio equipment contain voltages that can kill. Read this page before you open, build, repair, or modify anything.

Ten rules to know before you start

1. Unplug the amp from the wall. Don't rely on the power switch.
2. Discharge the filter capacitors with a proper discharge tool (a resistor with insulated leads), then confirm with a multimeter that they read close to 0 V.
3. Never discharge with a screwdriver or bare wire.
4. When the amp must be on, work with one hand and keep your other hand away from the chassis.
5. Never work alone on a powered amp.
6. Remove jewellery and work on a dry, insulated, uncluttered surface.
7. For a first power-up, use a variac, isolation transformer, or current-limited tester such as a dim-bulb tester.
8. Always connect a speaker or dummy load before powering on.
9. Let tubes and transformers cool before you handle them.
10. If you're unsure at any point, stop and ask a professional.

Before you open the amp

• Switch off, then unplug at the wall. Remove the mains lead from the amp if you can.
• Discharge the filter capacitors, then measure across them with a multimeter rated for the voltage. Don't assume the bleeder resistors are working; they can fail.
• Check the work area: dry floor, good lighting, no liquids nearby, and children and pets kept away from the open chassis.
• Wear safety glasses. Use insulated tools and, where the job calls for it, insulated gloves.

Working safely

• Treat every part of the circuit as live until you have measured it.
• Biasing and some testing are done with the amp powered. This is where many injuries happen, so use the one-hand rule, have another person nearby, and use insulated probes and clips.
• Plug the amp into a circuit protected by an RCD (safety switch).
• Check mains leads and plugs for damage before every session. Don't use damaged leads.
• Never bypass safety grounding, and don't defeat or remove an earth connection.
• Use the correct fuse type and rating. Never substitute a higher rating or bridge a fuse.
• Use heavy parts, especially transformers, with care. Lift with your legs and get help with large units.
• Solder in a ventilated space, and wash your hands after handling lead solder.

Handling tubes and parts

• Heat: tubes, transformers, and resistors get hot enough to burn. Allow time to cool.
• Glass: tubes can crack or implode. Handle them gently, and wear eye protection when installing or removing them.
• Ratings: use parts rated for the voltage, current, and power in your circuit. Electrolytic capacitors can rupture if over-volted or fitted the wrong way round.
• Operation: running an amp without a speaker or dummy load can destroy the output transformer and cause arcing.
• Fire: never leave a newly built or repaired amp powered and unattended. Keep a suitable fire extinguisher nearby.

If something goes wrong

If you're not sure, stop

There's no shame in handing a job to a qualified technician. A repair bill is far cheaper than an injury. If a procedure is beyond your training, or something doesn't look or measure the way you expect, walk away and get help.

You work on tube amplifiers and audio equipment at your own risk. This page is general guidance and isn't a substitute for training or for the instructions that come with your equipment. Specifications and information on our site may change without notice. For our full terms, see our Terms & Conditions.
Last reviewed: October 2026