Important Safety Issues Related to Valve Equipment
I recommend that you must read through this page very carefully before you even think of starting this exiting past time hobby as there is a very dangerous side to it, regarding the use of high voltages and mains electricity. Mains electricity in the UK comes into the home at 240 volts AC, 60 cycles per second and there is a serious risk of electrocution and possible fires. Valve equipment, particularly high power audio amplifiers uses double the voltage ranging from 500 to 1000 volts of DC current stepped up with a mains transformer. Direct contact can course severe burns and your hands can also become locked and unable to let go until the power is turned off. Also the HT smoothing electrolytic capacitors can hold a lethal charge, many hours in some cases after equipment is turned off, so I recommend you discharge this voltage by using a bleed resistor of about 470K 1 Watt and a analogue multimetre to check that this voltage is fully discharged. Please note that when testing and setting up your valve equipment there are times when the power needs to be turned on. It is essential that you keep one hand in the pocket when using high voltage test leads, with the negative lead earthed to the chassis when doing HT voltage measurements by using a crocodile clip and be very careful not to accidentally bridge components by short circuiting with test leads, leading to possible damage. Be also aware that you should keep young children and pets out of the same room you are working in, as they may not fully understand the dangers and keep unattended work areas out of children's reach by locking the place up when you have finished. Provided these rules are always followed, Constructing valve radios and amplifiers can give you many hours of fun and pleasure. Please take time to read the following rules and guidelines listed below.
Safety in the work area
1. Avoid temporary electrical hook-ups such as multiple double adaptors and trailing extension leads, as these over a short period of time, tend to become permanent.
2. Try make use of a suitable spare room for this sort of work. The use of a outside shed or garage is suitable, provided it is kept heated and no damp or wet weather can get in, during the Winter Season. If this is not possible then the kitchen is the only solution, provided you have followed the previous warnings.
3. It is also important to have adequate power and lighting facilities regarding your work area. Make sure all sockets are the switched 13 amp modern square pin type. Also have a master switch that is in easy reach, particularly when first testing equipment, so that you can immediately turn off the power, in the case of danger arising.
4. It is important that the electrical wiring is sound and up to standard. If your home was built before 1962, it is still possible that your house was wired using rubber sheaved cable. If so, it is worth getting advice from a competent electrician and having your home rewired, as it may not become immediately apparent, the installation can work well for many years but the hidden danger of a fire or electric shock is waiting there. The use of a socket tester, available from most DIY electrical stores can reveal if your sockets are wired correctly. It is also very dangerous to not have any electrical safety earth. Under fault conditions it is possible for a metal case electrical appliance to become live without immediate warning. This is usually caused by a damaged flex or faulty mains transformer leading to a earth leakage.
Why fuses are very important
1. A fuse is designed as the weakest link in any electrical circuit for a number of reasons. It is a thin piece of wire enclosed in a ceramic tube or plug in type holder used in distribution boxes.
2. Without it, your electrical appliances would work OK, but should a fault, such a damaged cable or high current fault arise, there could be a high risk of a fire or electrical explosion.
3. The main function of a fuse, is when danger occurs the thin piece of wire melts, sometimes with a mighty bang, when a heavy short circuit occurs.
4. There is another two types of fuse that have been around for about the last 20 years or so.
5. Circuit breakers. A circuit breaker is a magnetic switch, now common in many household consumer units, it is designed to trip off and can be reset, should a fault or current overload occur and is more safer and reliable.
6. Residual Earth Leakage Current Breaker. Not strictly and not to substituted as a fuse, it used in many circumstances where water is involved such as electric showers and outdoor appliances. Should a outdoor appliance become faulty or the casing becomes live, it is designed to trip off, when there is a slight current leakage to earth of only 30 milliamps preventing a fatal electric shock.
7. Fuse rating. It is very important to use the correct size fuse for its protective purposes. For example mains plug fuses have about 3 different ratings. 3 Amps is uses on appliances up to 720 watts, such as hi fi equipment, table lamps and coffee makers. 5 Amps is used on appliances up to 1200 watts such as Vacuum Cleaners, small electric fires, computers and colour TVs. 13 Amps is used on appliances up to 3120 watts such as electric kettles, washing machines and heaters. Please note that this only protects the flex and the correct fuse in the appliance is probably lower, which should be the case, when valve equipment is concerned because it uses lower currents.
8. Mains and HT Fuses are usually employed in all high quality valve radios and Television Receivers. The purpose of the mains fuse is to protect the mains transformer from damage when a serious overload occurs, That can happen due to a faulty rectifier valve or smoothing capacitor. This is usually wired in the live side of the mains preferably before the mains switch of the equipment. However, This does not always give full reliable protection on the secondary side of things, as it is known that a output valve can fail, leading to burn out of the output transformer which can also lead a expensive repair on your hands. A HT fuse is usually wired in between the positive side of the second filter capacitor and the HT Rail on the equipment concerned to give complete protection.
Safety concerning Electrolytic Capacitors and the danger related with old Radio sets
As I mentioned earlier about safety regarding high voltages involved with valve equipment, electrolytic Capacitors pose the most danger. The reason being is that they behave like high voltage batteries which can store a very lethal charge over a considerable period, from months to even years when left idle. Also they can explode with a loud frightening bang, depositing there innards all over the place, particularly if connected in the Wrong polarity or if the voltage rating is exceeded. Electrolytic Capacitors also deteriorate with age when not used for a long time resulting with the same similar circumstances. Although modern electrolytic capacitors are now very reliable, this is not the case with a radio that has been asleep in the loft for the last 30 years and regardless of the old guy telling you it worked brilliant when last used, the chances is are that if you restored power to it again, The capacitors could explode causing more damage to the set and there could also be a risk of fire, if the set is left unattended. Why does this happen? Electrolytic capacitors are made of aluminium foil, rolled in the insulation form of an electrolytic past compound that forms the dielectric capacitance and when not used for a long time the dielectric becomes fragmented, Causing breakdown of the dielectric insulation which leads to leakage and short circuits. It is possible to reform these components by steadily applying a low current, Which can except in some cases heal the capacitor back to its useful life.
Safety Precautions Regarding Live Chassis AC/DC Equipment
As recently up to the mid 1950s, Many towns where not supplied with electricity by the National Grid and the mains was sometimes DC. Connecting a AC Mains set incorporated with a mains transformer to a direct current supply due to the incompatibility would cause serious damage, rendering the set useless and possible risk of a fire. To overcome this problem a series valve heater chain consisting of the heaters wired up like Christmas Tree bulbs was incorporated, Often followed by a series high wattage resistor to make up the required voltage. The High tension side of things mostly consisted of a half wave valve rectifier and simple capacitor input filter, followed by a surge resistor to limit the current. As a result due to no mains transformer, One side of the mains is connected to the chassis and there is a serious risk of electric shock if the polarity of the mains is unknown. To correct this problem it is best to make sure that the mains lead is fitted to a none reversible plug with the black lead connected to the natural terminal. It is also safer to confirm this by touching the chassis with a neon mains test screwdriver. The aerial and gram pickup sockets used high voltage isolating capacitors and it is not unknown for these to breakdown, leaving an additional shock hazard on the horizon. Always make sure these capacitors are replaced with the proper 1000 Volt Class, Designed for this purpose. Also beware of exposed metal knob shafts that can also be very dangerous by making sure the grub screws are tight and cant be touched by using special filler designed for the protective purpose. You will find in many old radio magazines some very tempting radio circuits that use this method as a way to cut cost, but my advice is to play it safe and build a design using a mains transformer.
Heat and Component Reliability
Compared to transistor circuits, valves generate more heat because of the hungry current consumption regarding the valve heaters and high voltage HT Line. It is this problem that results in component failure, in these circuits over the years of there working life. As a rule of thumb I always replace the lower power resistors with no less the 1/2 a watt, for improved reliability and as another example I always substitute a 450 Volt electrolytic capacitor for the 350 Volt type, particularly in power supply circuits, as that way you gain improved reliability rather then working close to the limit. There is a excellent website run by Paul Stenning and can be found by clicking on the following link UK Vintage Radio Repair and Restoration . It has some useful projects such as a capacitor reformer and a series lamp current limiter. Useful when testing your newly constructed radio for the first time.
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