Leak tl 12 plus service manual

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'TLIIE PLUS
powsn AMPLIFIER

mum . (lent-ms . Aluminum-Io

INSTALLATION ° OPERATION ' MAINTENANCE

NOTES ON THE CHOICE AND PERFORMANCE OF LOUDSDEAKER SYSTEMS

OPERATION

The TL 12 PLUS tnay be fed from the LEAK VARISLOPE MONO' pre-amplilier. or from any other
suitable source. A source impedance higher tltan 25.000 ohms will tend to raise the hum level above the
ttdvcrllsed figure of R2dh below 12 watts. An input of l25mV r.m.s. will give a power output of l2 watts.

INSTALLATION

.

6.

Check that all valves (tubes) are correctly seated in their holders and that the markings on the valves
correspond with those on the chassis adjacent to the holders. The amplifier will work equally well with any
of the alternative valves.

The amplifier should stand on its base in a well-ventilated position. If placed in a case or cabinet. ventilation
must be provided. Four separate fixing feet with screws are provided with the amplifier.

On the British model the mains transformer is tapped for voltages of 205, 225 and 245 and the voltage
selector plug on top of the mains transformer (see TOP (HASSIS' drawing) should be set appropriately.
On the U.S.A. model the mains transformer is tapped for voltages of IOS. HO and 7 and the voltage
selector plug should be set appropriately. The A.C. power supply should be connected to the two terminals
nearest the guide key on the removable plug portion of the mains connector marked A.C. POWER.

In order that the amplifier may be remotely controlled we have provided two terminals marked SWlTCH
ttnderneath the mains transformer (see UNDER CHASSIS drawing). The amplifier will not work unless
these terminals are electrically joined. and when the amplifier leaves our factory a wire link joins the
terminals. A remote switch may be run from the SWITCH terminals. after removing the link. the flex
being passed through the adjacent grommet marked SWITCH CABLE'. Thc flex should be knotted
behind the grommet to avoid strain on the SWITCH' terminals. Most users will wish to make use ofthc
switch incorporated in the volume control of an associated LEAK pre-amplifier. which is supplied with a
plug and twin flex for this purpose. Full details are given in the installation sheets which accompany every
pre-amplifier.

A double socket marked 'A.C. OUTLETS is fitted as a convenient source of power supply for gramophone
motors, self-powered radio tuners. etc. The power taken from this socket should be limited to l00 walls or
thereabouts. This socket is not fused but is controlled by the amplifier switch.

Many British tuner units have no built-in power supplies and require a source for heater and anode currents.
The oetal socket marked SPARE SL'PPLI ES' on the hack ofthe amplifier is provided for this purpose. The
pins are numbered as indicated on the circuit diagram. and connections should be made to the pins similarly
numbered on the removable plug which is supplied. The high tension supply is 300V and on some tuner
units it may be necessary to provide a dropping resistor ofsuilable value. and some units may also require a
condenser for smoothing and or decoupling after the dropping resistor. An earth connection should not be
made directly to the tuner unit. as this is automatically elTected by the above connections via the power
amplifier. The maximum current available from the 6>3V terminals is 2' IA and the maximum high tension
current is 40mA.

A connection to earth (ground) should be taken from the third terminal on the removable plug portion of
the A.C. POWER connector. This terminal is the one furthest away from the guide key and its corres»
ponding terminal on the fixed portion ofthe connector is marked on the chassis by the symbols. It is very
bad practice to omit this connection. which may be made to the water system or to the steel conduit
cncasing the house wiring. providing that these systems themselves are properly grounded. No other carth
connections should be made elsewhere. particularly when a pre-amplifier is also used. if freedom from
eztrth loops' and hum is to be obtained.

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8. The loudspeaker should be connected by a twisted pair ofwires to the terminals marked LOUDSPEAKER.
It will be seen from the circuit drawing that one side ofthe loudspeaker winding is connected to the chassis.
and no part of the loudspeaker wiring should be earthed elsewhere. The DC. resistance of the connecting
wires should be as low as possible. and not more than one-tenth the DC. resistance ofthe loudspeaker. It is
bad practice to operate any power amplifier without a loudspeaker. and if it is desired to mute the loud-
speaker by switching it out of circuit this should be accomplished by use of a change-over switch which
replaces it with a resistor of corresponding value and rating. The selector plug on top of the output trans-
former (see TOP CHASSlS drawing) should be adjusted for the nearest match to the advertised impedance
of the loudspeaker.

GENERAL NOTES ON MAINTENANCE

The circuit diagram provides a qualified engineer with all information required for servicing. However. the
followmg points may be of interest:-

(a) The TL I2 PLUS does not depend upon the output valves (tubes) being a matched pair to give the
stated performance. and if one output valve fails it is not necessary to replace both.

(h) Should it ever be necessary to replace the reservoir capacitor C14 (which is in the same can as Cl3) note
that C14 must be ofa type capable of handling a heavy ripple current. The CM fitted has a very high
margin of safety. being capable of handling 380mA. The values of 60mfd. and lOOmfd. are made high
for the sole purpose of minimising the hum resulting from the unavoidable earth loop which must be
occasioned by the connection of non-powered tuner units.

NOTES ON LDUDBPEAKEH SYSTEMS

Space does not allow for detailed explanations on this vast subject. but the following points should be
noted :-

(a) You cannot get high fidelity results ifyou mount a loudspeaker in the same cabinet as the turntable and
pickup.

(h) The assembly commonly called a loudspcaker' consists of a magnet. a moving coil and a diaphragm
(or cone). This assembly is a motor. You do not listen to a motor: you listen to a loudspeaker
system. which consists ofone or more motors mounted in a housing (baflle. box. cabinet. or born). The
housing plays a profound part in determining the quality of reproduction. The effects of various
housings are discussed by L. L. Beranek in Acoustics. McGraw-Hill Publishing Co. Ltd.. lst Edition.

(c) One good motor. properly housed. is capable of giving fairly good results. but two good motors.
properly housed. will give noticeably better results: in this case one motor is designed to reproduce
bass. and the other, treble. A filter (dividing network) must be used in conjunction with the two
motors.

(1!) Remember that even the best conventional loudspeaker systems are by far the weakest link (i.e.. the
greatest source of distortion) in a high fidelity chain. and that such systems are relatively expensive.
The fundamental weaknesses of conventional loudspeakers are discussed succintly in a paper by H. J.
Leak. High Fidelity Loudspeakers: T/te Per/brmame of Moving-Coil and Electrostatic Transducers.
Journal of the British Institution of Radio Engineers. Vol. 16. No. [2. December, I956. Copies are
available from the Institution. 9 Bedford Square. London. W.C.l. price 7 -. However. such systems
have become out-moded by the revolutionary Leak invention ofthe composite sandwich diaphragm.
The basic principles are described by D. A. Barlow. M.Sc. (Head of the Leak Transducer Research
Department) in Wireless World. December, I958. A further paper describing the complete Leak
Sandwich loudspeaker system was delivered to the British Institution of Radio Engineers by D. A.
Barlow and H. J. Leak on January 24. I962, and will be published in a forthcoming journal of the
Institution.