BUILD A ONE ECC83 TUBE MW RECEIVER

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The main components of this simple receiver are the ECC83 tube itself, a variable tuning capacitor, an audio volume potentiometer, the coil mounted on a ferrite rod an a 2000 ohms phones. We have used an aluminium chassis, which is not essential; the receiver can be built on a plastic cage or even on a wood board. Small other components are detailed in the components list.

The energy to this receiver comes from two batteries, one of 12 volts to heat the filaments and another one of 36 volts to supply the plates of the triodes. This last battery is made up of four 9 volts cells connected in series. These valves fed with 12 volts absorb a current of about 150 or 160 miliampers to its filaments, what assures many hours of reception even with a rechargeable small lead-acid battery. The current taken from the 36 volts battery to feed the plates is about 1,6 miliampers, so this battery have a long life of service.

1 - 460pF Variable Capacitor	1 - 2K Ohm Phones
1 - Ferrite Rod Coil	1 - ECC83 Tube
1 - Megohm Resistor	1 - ON/OFF Interrupter
1 - 100pF Capacitor	1 - 1K Ohm Resistor
1 - 33K Ohm Resistor	1 - LED Diode
1 - 180nF Capacitor	1 - Phones Connector
1 - 500pF Capacitor	1 - Antenna Jack
1 - 180K Ohm Potentiometer	1 - Ground Jack

Aside from that as it is necessary: Cable connections, small bolts and nuts, input connectors...etc.

This project is an admiration homage to those scientists that created the electronic tube or valve, an invention that opened the doors to the world of transistors, the greatest achievement of the last century. Electronic tubes or valves are still alive and return again nowadays to the building of high quality audio amplifiers in competence with transistors.

To work with tubes is a fascinating job an a very rewarding hobby leading to a better understanding of Electronics. While in a transistor it is difficult to understand what happens inside of its junctions with electrons and holes, in a tube we have only a flux of electrons that travel through a vacuum space between the cathode and the plate. In its way they encounter a grid which allows us to modulate that flux and so the building of oscillators, amplifiers, radio receivers and a great number of others electronic devices.

The more basic electronic tube is the diode with only a cathode and a plate, but with the introduction in a diode of a grid we have a triode, the more easy and handy of valves to manage.

In our receiver we use an ECC83 tube or its equivalent, the 12AT7 which has the same characteristics. Though the manufacturers of these valves specify high voltages to its plate, these devices can work with voltages as low as 36 volts which allows us to build a radio receiver without any risk and safely. Nevertheless always we must pay attention to avoid the short-circuits, an essential rule all over Electronics.

The resonant circuit for this receiver is formed with a coil mounted on a ferrite rod and with a variable capacitor of 460 pF. The ferrite rod has a length of 120 millimetres, with a diameter of 9 millimetres. On this rod we wound 65 turns of 0,3 mm copper enamel wire to form the resonant circuit coil, and another small antenna coil with only 15 turns of the same wire.

The number of turn is not to be following slavishly, a few, and only a few, more or less turns will not affect to the tuning resonant circuits, which variations will be compensate by the variable capacitor.

This is a two stages receiver. The first is what it is called 'a grid detector stage' and the second one is an audio amplifier. The detector stage has a fixed polarize resistor of one megohm for its grid, while the audio amplifier grid polarization is changed by a 180 ohm potentiometer. So we get an easy audio control in the second stage.

An air isolation capacitor is used to complete the resonant circuit in parallel with the ferrite rod coil. This capacitor has two sections that are united in parallel to get a whole capacitance of 460 pF.

It is not essential to use an air isolated capacitor. Small variable capacitors of about the same numbers of picofarads can be connected in the tuning circuit, for example, capacitors salvaged from discarded commercial transistors.

The small number of components in this project invite one to follow the classic way of connecting them, as in the old time receivers. It is no necessary to used a perforated tracks board, or design a printed circuit for this set, due to the simplicity of the circuit.

Nevertheless we have to be sure that the capacitors and resistor, as well as all other connections, are electrically perfect avoiding any cold solders that would prevent the set to work. A good practice is cleaning carefully the capacitors and resistors terminals before soldering, and pre-solder the ends of all connection wires. A good soldering practice assure the success with any electronics circuit.

While with transistor receivers the phones or earpieces used have a low impedance such as 8, 16 or even 32 ohms, these types of phones can not be plugged directly to valves receivers unless a transformer changes the high impedance of the last amplification tube to the low impedance require for transistor sets phones.

So in our receiver we adopt a 2000 ohms phones or a crystal ear piece. Phones of high impedance are nowadays expensive and difficult to find. But if you are interested in radios with valves projects you can get 2000 ohms phones from the American Crystal Set Society where you can buy also variable air capacitors.

In the case you want to use phones of 8, 16, or 32 ohms, a transformer with a primary winding of about 2000 ohms and a secondary near to 8 ohms can solve the problem. Small transformers taken from discarded power supplies for transistors appliances are able to work relatively well as final audio transformers, though the solution is not technically perfect.

A more simple and quite cheap solution is plugging a crystal earpiece in the phones jack if we previously connect a 2000 ohms resistor across the terminals of the phones. Crystal phones or earpieces have what can be considered infinite impedance an no current can flow through them, so the 2000 ohms is essential to allow the flow of energy to the plate of the audio amplifier.

All valves devices needs two power sources, one for the high voltages of plates and some grids an another for the filaments that heat the cathodes. Generally a unique power source supply these two voltages and currents, but for reasons of simplicity we used in this project two batteries.

The ECC83 tube needs 12.6 volts for heating its cathodes so we used a small 12 volts rechargeable lead-acid battery with enough capacity to assure long hours of listening. The low voltage, only 36 volts to feed the plates of the triodes in this valve are taken from a battery make up with four 9 volts batteries connected in series. As the current taken by the plates is about 1.6 miliampers, this 36 volts battery will last also for a long period of listening and experimenting.

When you connect the filaments battery, and switch on the set, there is a few seconds, about 30 seconds, to wait until the cathodes reach the electrons emission temperature. After this time, opening or closing the air tuning capacitor blades you will scan the MW broadcast band.

In a medium size city, like the one in which I live, this receiver tunes three or four broadcast stations with only the ferrite rod antenna. If you have the luck of having a long and exterior antenna it is possible to tune at night, when propagation to MW band is better, some distant transmitters.

The quality of sound is quite good and clear due to the use of batteries which supply the cleanest of electricity.