There is often considerable concern out there about the safety aspects of building valve amps, and with good reason. Even the simplest valve amp designs like my Lamington Junior generate a high voltage supply of over 250V DC to operate the valves, and accidental contact with this voltage will be unpleasant at best, and potentially more dangerous.
However, building valve amplifiers need not be dangerous if you are informed about the risks, and take the necessary precautions to ensure the safe construction, testing and operation of valve amplifiers. Being aware of a few simple strategies can make amp building safe for you and for those around you.
Here are some practical strategies that you can employ to ensure a safe environment for building and using your own valve amp:
1. The use of an ELB (earth leakage breaker) in the work environment is highly recommended. ELBs work to protect you if there is an accidental contact with the 240V mains. Most houses and workplaces have them installed – make sure you have one fitted.
2. Don’t work on your amplifier in a damp environment and make sure you are wearing shoes with insulated soles and preferably a floor that is insulated e.g. carpet or wood.
3. Make sure that ALL 240V mains wiring in your amp is properly insulated. I always use heatshrink tubing to insulate the primary leads of power transformers, fuse holders and mains switches. A good check is to follow all of the connections in your amp from mains active to mains neutral and make sure that every connection is covered with heatshrink or otherwise insulated.
4. Make sure that your amp is properly earthed by checking for zero resistance between the earth pin of the mains plug and your amp chassis with a multimeter.
5. Make sure that ANY wiring on the top of the chassis that is in contact with high voltage (transformer lugs, top caps of output valves if you are using them) is properly insulated.
6. Use the colour of wiring underneath the amp to indicate the presence of high voltage. The convention is to use red for HT supplies, and other “hot” colours like orange and yellow for other connections that have high voltage present. This assists you to keep away from these points when checking a working amplifier.
7. Don’t work on an amplifier when you are tired or distracted – you need to have all of your attention focussed on the job at hand and to avoid potential danger. Ideally, don’t work on your amplifier alone – have someone nearby to assist you if needed.
8. Before working on or making changes to an amp, disconnect the mains lead from the amp and wait for the high voltage capacitors in your amp to discharge. You can then safely work on your amp.
9. If you need to check voltages in your amp with a multimeter, with your amp turned off first clip the black (negative) multimeter lead to an appropriate earth point on the amp chassis. Then turn your amp on, and with one hand safely away from the amplifier use your other hand to hold the red multimeter probe and check the appropriate voltage point. This ensures that even if you were to accidentally touch a point in the circuit at high voltage, no current can flow through your body causing electric shock.