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Once upon a time, long ago, British and European transistors were commonly coded as valves: 0 = no heater, C = triode. However it was quickly realised that everything would end up as an OC something or other (likewise with diodes, which would all become OAxx) and could be confused with existing cold-cathode valve types. So the ProElectron organisation developed a new system of coding semiconductor devices and the one in use today. Basically, British and European transistors are issued with a unique combination of letters and numbers.
The first letter identifies the semiconductor type:
· A = Germanium
· B = Silicon
· C = Gallium Arsenide
· D = other compound semiconductor material
The second letter indicates the intended use:
· A = Small signal diode
· B = Varicap diode
· C = Small signal LF transistor
· D = LF power transistor
· E = Tunnel (Ersaki) diode
· F = RF small signal transistor
· K = Hall effect device
· L = RF power device
· N = Optocoupler
· P = Radiation sensitive device (e.g photo transistor)
· Q = Radiation emitting device (e.g LED)
· R = Low power SCR
· T = High power SCR or triac
· U = High voltage switching transistor
· Y = Rectifier diode
· Z = Zener diode
Any third letter indicates the device is primarily intended for professional and industrial use. This letter is usually a V, W, X or Y and can be ignored. Many devices have a suffix letter after the number: A = low gain, B = mid gain, C = high gain.
Devices with no suffix can have an hfe anywhere within the part's specification. Military devices are generally marked with a "CV" (Common Valve) number rather than the civilian code. For instance the CV7003 is an OC44 in uniform.
Audio medium power types OC72 OC81D OC81 can be replaced with an AC128, a relatively high voltage, high current device. Type AC127 is the npn complementary to the AC128. Low power AF amplifier transistors like the OC71 can be replaced with the economically priced AC125. The pnp AC188 and npn AC187 are intended for use as a complementary pair in transformerless output stages. All these devices are packaged in metal TO1 cans. Please bear in mind that output transistors must be mounted on a heatsink when run at or near their maximum dissipation. If replacing /K types with their integral heatsink block, the faulty transistor can be pulled from the block and the replacement fitted in its place. Ptot for the AD149 and AD161 AD162 assume they are mounted on a suitable heatsink. Take care when replacing AD161/162, not to damage any mica insulating washers as this size (SO55) is not readily available. A thin smear of heatsink compound should be applied when mounting replacements. The gain (hfe) of germanium transistors is generally lower and far more variable than silicon devices. It is also strongly dependant on collector current and ambient temperature. The base-emitter voltage (Vbe) is also lower at 300mV compared with 600mV for silicon. For this reason you cannot generally substitute a silicon transistor for a germanium device without modifying the biasing arrangements.The old TO7 alloy-drift transistors were specified as follows:
· AF114: RF pre-amplifier in VHF/FM front ends
· AF115: Mixer/oscillator in VHF/FM front ends.
· AF116: IF amplifier for 10.7MHz used in FM receivers.
· AF117: General purpose RF/IF amplifier in LW/MW/SW receivers.
· AF118: High voltage (75v) video amplifier for transistor TVs.
In practice, the AF125 can be used to replace AF114/115/116. There is no listed equivalent to the AF118 and where replacement is needed for an old transistor TV, it may be necessary to re-jig the circuit to take a suitably rated silicon device. Incidentally, alloy-drift transistors tend to be noisy at low frequencies and as such are not generally suitable for audio circuits. Another point to watch is that equivalent books list the AF239S as a replacement for the AF11x range. However the AF239S is a UHF device designed for use in TV UHF tuners and has a low gain (min hfe 10) compared with the AF11x types (min hfe 50) and my experience is that it won't work properly if at all in most radio circuits.
Not even silicon devices are safe from the dead hand of obsolescence. Some familiar but early silicon transistors are unavailable, and with the continuing encroachment of cheap, disposable (and as many would describe it, nasty) surface mounted technology, leaded transistors in general seem to be under threat.
With thanks to Pete Roberts for permission to use the above. Peter also sells vintage transistors as well as other components.
Last updated 3.7.2004