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FAQ - First Timers Considering an amp or speaker build?

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Old 6th January 2006, 08:43 PM
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Default What are the safety issues?

All valve amplifiers run from very high DC voltages, and AC mains voltages, these voltages can be lethal. Before embarking on construction of any valve amplifier kit you must possess some fundamental understanding of valve amplifiers and electricity.

If you have any doubt in this area then seek further help and advice from professional persons like a TV repair engineer. If you feel you have this understanding and are determined to build a valve amplifier then please take the following, necessary, precautions.

Never be complacent, remember that until the fuse blows there is nothing between you and the equivalent of Battersea Power Station! During construction you will be soldering components and wires to circuit boards etc. Soldering irons get extremely hot and can give you nasty burns. Use a soldering iron stand, never wave an iron in the air and return the iron to its stand as soon as you have finished using it. Beware of molten solder, it can get splashed onto skin or into eyes. Wear protective glasses/goggles, especially when removing components. Solder fumes contain toxic lead and solder flux can be an irritant. If the fumes prove to be an irritant then wear a facemask and work in a well-ventilated environment, try to avoid bending over the soldering work in progress.

During testing the amplifier will be upside down so you can measure voltages. It will be switched on and the full AC mains voltage and High Tension (HT) DC voltages will be exposed. Never hold earthed metal work, such as the chassis, with bare hands when the amplifier is in this state and make sure you are isolated by wearing rubber soled shoes. Keep one hand in your pocket at all times as you probe inside the amplifier to avoid any shock traveling through your body and across your heart. If rubber safety gloves were supplied in the kit wear them! Obtain a pair from WAD if you don't have any.

Always ensure you have double isolated the amplifier after testing and before you begin work again. This means turning off the mains at the wall socket, removing the mains plug, and turning the amplifier mains switch to "Off". ALWAYS!

Beware of the power supply capacitors. They can hold a high DC voltage (charge) a long time after power "off" and possess enough energy to give you a nasty shock and possible burn. Large power supply capacitors should be discharged prior to working on the amplifier. If it does not already have 'bleeder' resistors across the power supply this can be done by connecting a 220K, 1 Watt, resistor across the capacitor terminals prior to working on it. Hold the resistor with a suitable insulated tool and, after a few seconds, check with your voltmeter that the reading across the capacitor is zero (or very small). Be aware that capacitors can spontaneously recover a very high charge, so you should tack solder the resistor in place and leave it there while you work. Remember to remove the resistor when you finish!

The valves themselves get extremely hot, just like a light bulb, and will burn you easily. Therefore place the amplifier out of the reach of children or pets during use. A general rule: If you drop it LET IT FALL. Valves or a soldering iron can be replaced more easily than you. Grabbing at a falling amp, which is undergoing live testing, is almost certain to result in some form of injury!

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