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OTHER TOPICS.


	VALVE TOPICS.


	VALVE DESIGNATIONS. Mullard etc .(pro-electron code) 1st letter .= Heater voltage or current. A= 4 volt C= 200 mA D= 0.5 to 1.5 volt filament (usually 1.4 volt) E= 6.3 volt G= 5 volt H = 150 mA K= 2 volt O = cold cathode, then semiconductor device P=300 mA U=100 mA 2nd & 3^rd letters. A= diode B= double diode C = triode D= triode (power output) E = tetrode F = pentode H = Hexode, heptode, etc K = octode or (sometimes) heptode L= Power output tetrode or pentode M = tuning indicator N = gas filled valve (usually thyratron) Q= nonode X= gas filled rectifier Y= Half-wave rectifier Z= Full-wave rectifier. 1^st number. Type of base. 1= misc., side contact, etc. 2 = B10B, but was B8B (loctal) 3 = Octal 4 = B8A 5 = B9D, was B9G, or sometimes "oddities". 6 = subminiture 7 =subminiture 8 = B9A 9 = B7G 2^nd& 3^rd number = type. Mazda Valves. A = industrial valve B = double triode D = diode GU= gas-filled rectifier GT= gas triode (thyratron) H = signal triode (high impedance) KT= beam tetrode L = signal triode MU = indirectly heated rectifier N = output pentode P = output triode QP = quiescent push-pull double pentode S = tetrode (screen grid valve) U = rectifier VS = vari-mu tetrode W = vari-mu pentode etc. X = frequency changer (triode-hexode, heptode,, etc) Y= tuning indicator Z = HF pentode American Valves. 1^st number = approx. heater voltage. 1^st letter = single-ended if S.


	How Valves Work In these days of multi-legged silicon devices, many people do not know how the thermionic valve ("tube", to the Americans) works This is a simplified explanation Apply a voltage to the ends of a piece of thin wire If the voltage & wire thickness are correct, it will heat up. and then melt If, however, it is in a vacuum, it will stay at red-heat without melting


	The FILAMENT (as this wire is called) is coated with a material that emits elections when heated, and to enable the wires to come from one end, and to minimise space, is folded, as below If a cylinder is placed around the heater, and a voltage is applied so that it is POSITIVE with respect to the filament, electrons will flow from the cathode to the cylinder, which is called the ANODE.


	If, now, another cylinder made from fine wire is placed between the filament and the anode, it can be used to control the flow of electrons This is called the CONTROL GRID, or Just GRID.


	The whole assemble is kept in a vacuum, inside a (usually) glass tube. The filament is satisfactory if fed from a DC supply, such as a battery, but mains is AC, and this necessitates adding another part. the CATHODE, around the filament, which is then called the HEATER. The Heater is coated with the material that emits electrons when hot, and the heater just provides the necessary heat. Thus, looking at the construction from the end, we see a series of concentric circles (see below.)


	For simplicity, and by convention, a valve is shown as on the right, which can best be considered as the portion of the above drawing between the blue lines. (The CATHODE is usually abbreviated to K )


	Considering the circuit below, there is a positive supply which is fed to the anode via a resistance. The cathode is connected to the negative supply line. The heater has its own supply. The grid connection will be covered later.


	Transmitting valves. There a several systems of coding for transmitting valves. The one used by Mullard and some other manufacturers is primarily for high-power professional valve types and of little interest to the amateur, although the following may be useful. First 1 or 2 letters = type. e.g.. Q = beam tetrode Next letters = cathode type First figure (s) = maximum node voltage e.g., 06 = 600 volt Subsequent figures = max. anode dissipation in watts. So, QQV06-40A = twin beam-tetrodes, 600 volt maximum anode supply, to dissipate a maximum of 40 watts.


	When voltage is applied to the heater, it heats the cathode which emits electrons. Electrons are negatively charged particles and are therefore attracted to the anode, which is positive. A constant flow of negatively charged electrons will tend to make the anode "less positive". If the anode was connected directly to the supply the anode voltage would be fixed at the supply voltage, but as it goes via a resistor, the voltage on the anode can vary.


	The electrons will pass through the grid (which is usually constructed of fine wire with relatively large spaces in between) which will have little or no effect on them, until a voltage is applied to it. If a positive voltage is applied to the grid, it will attract electrons, and a negative voltage will repel them. The grid is close to the cathode, and so its voltage will have a greater effect than that of the anode. If a small negative voltage is applied to the grid, it will tend to repel the electrons, but allow the remainder through. Thus there will be a constant flow of electrons from the cathode to the anode, but this flow will be less than the maximum that would flow if there were no voltage on the grid. In this way it can be said to control the flow of electrons, hence its name of Control Grid.


	Cathode Ray Tubes etc. 1^st letter = type. M = magnetic deflection A= electrostatic deflection. 1^st Numbers = tube size = max diameter across bulb, not screen face. NOTES. Magnetic deflection. Ion trap magnet must be set correctly or ion burn will cause a black spot in centre of screen where ion's have destroyed the phosphor. MW 6-2 used in projection T.V's runs at 25 kV. There is a potential danger from X-rays. Also, some projection T V's use mains derived power so there is a high risk of electrocution. This was back in the early 1950's.!


	Although it is actually a flow of negative electrons moving towards the positive element, by convention it is always considered as a current flowing from positive to negative.


	To summarise, the current flowing through the circuit, positive supply-resister-valve-negative supply, can be varied by altering the the voltage applied to the grid, which in turn, alters the flow of electrons flowing from cathode to anode. A variation of the current flowing through the resister will, by Ohms Law, alter the voltage on the anode. If, to the slightly-negative grid is added a varying signal, (for example, an audio signal) the voltage will vary on the grid, which will alter the electron flow as previously described, which will, in turn, alter the current flowing through the resister and hence the voltage on the anode. As an example, a grid which varies from -4.9 volts to -5 volts (a change of 0.1 volt) may cause the voltage on the anode to change from 200v to 190v, a variation of 10 volts, or 100 times the original, thus giving an increase, or amplification, of 100 times.


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