With the exception of the Type 15, all Leak valve power amplifiers use the same basic circuit toplogy as the TL/12. This comprises a first stage of voltage gain with (either pentode or triode), a triode long-tailed pair phase-splitter/driver stage, followed by either a triode, tetrode, or ultra-linear output stage with cathode bias using separate cathode resistors.
Leak Type 15
15 Watts, four stage circuit of 1945, 0.1% distortion.
Valves: EF36/6J7, ECC33/6SN7, 2xKT66 in triode. 12 Watts for 0.1% distortion. GZ32 rectifier.
Valves: EF37/6J7, ECC33/6SN7, 2xKT66 in tetrode, with 1 x KT61 configured as a cathode follower to regulate the screen supply voltage (also know as the "Partridge Control Valve"). 25 Watts for 0.1% distortion. GZ32 rectifier. The output transformer has a third winding on it for supplying feedback to the input stage, in contrast to the other circuits where the feedback is taken from the loudspeaker terminals.
Valves: EF86, 6SN7, 2xKT61 in ultra-linear. Leak state that the Brimar 6AG6G is an exact equivalent to the KT61. 6L6 valves may be used in place of the KT61 if various circuit modifications are made (cathode bias resistor changed from 100 Ohms to 300 Ohms/3Watt, the associated bypass capacitor omitted, the first stage plate resistor increased to 330kOhm, and the resistor feeding the first stage screen changed from 470kOhm to 1MOhm). 10 Watts output. 5Z4 rectifier. Gold enamel finish.
Leak TL/12 Plus
Valves: EF86, ECC81, EL84/6BQ5 in ultra-linear for 12Watts output / 0.1% distortion. 5AR4 rectifier. Gold or dark metallic grey finish.
Leak TL/25 Plus
Valves: EF86, ECC81, 2 x EL34/6CA7 (or 2 x KT66) in ultra-linear for 25 Watts output / 0.1% distortion. 5AR4 Rectifier. Gold or dark metallic grey finish.
Leak TL/50 Plus
Valves: EF86, ECC81, 2 x KT88 in ultra-linear for 50 Watts output / 0.1% distortion. 5AR4 rectifier. Early units were issued in gold, while later units were metallic dark grey, similar to the illustration.
Leak Stereo 20
Valves: 3xECC83, 4 x EL84/6BQ5 in ultra-linear for 12 Watts output per channel / 0.1% distortion. 5AR4 rectifier. The first stage in the stereo amplifiers is a triode (one-half of an ECC83 per channel), where as in the mono amplifiers the first stage is a pentode. Gold or dark metallic grey finish.
Leak Stereo 50
Valves: 3xECC83, 4 x EL84/6CA7 in ultra-linear for 25 Watts output per channel / 0.1% distortion. 5AR4 rectifier. The power supply has 390V on the output valves, and C-R-C filtering. First stage is an ECC83 triode shared between channels. Gold or dark metallic grey finish.
Leak Stereo 60
Valves: 3xECC83, 4 x EL34/6CA7 in ultra-linear for 30 Watts output / 0.1% distortion. 5AR4 rectifier. The power supply has 420 V on the output valves, and C-L-C filtering. Otherwise the circuit is the same as the Stereo 50 above. First stage is an ECC83 triode shared between channels. Gold or dark metallic grey finish.
All the Leak valve pre-amplifiers listed here have the same basic circuit toplogy (details not known for the RC/PA however). All have two stages of amplification, and all use "shunt-feedback" on both stages (voltage sense the output, and feedback a current to the input terminal - or in op-amp terminology, the grid is a virtual earth). The first stage determines the overall sensitivity, and corrects for disc or tape head equalisation. The second stage implements tone control and treble filter circuits.
Features in advertisements up to about Sept 1950, for use with the TL/12. Circuit details unknown - but advertisements quote a feedback tone control circuit for Bass and Treble boost and cut. Three position Input selector switch, with no provision for selecting disc equalisation from the control panel. Mono only.
This pre-amp was supplied for use with the TL/12. There is one dual triode in the pre-amp, an ECC40. Shunt feedback is used on the first stage to provide various sensitivities and equalisation curves (LP, 78A, and 78B are the curves provided for disc. Mic and Radio inputs also provided). The second stage provides bass and treble in three boost and cut positions for each. A shunt feedback circuit is used again. 15mV sensitivity on disc input. Mono only.
This appears to be a revised and improved version of the RC/PA/U. The valve used is an ECC81/12AT7. The same input and tone controls are provided, but a central "Slope" control is added, and becomes active when using treble cut. The rate of treble cut-off can be varied between 5dB and 50dB per octave. The circuit uses a twin-T R/C feedback network with variable Q. Mono only.
Leak "Varislope II" (August 1954)
This pre-amp features higher input sensitivities than previous pre-amps, and has improved appearance. Two EF86 valves are used, with a similar shunt feedback circuits as used in previous pre-amps. The disc equalisations provided are: BRIT-78, BRIT-LP, COL-LP, RCA-Ortho, AES and NARTB. Tuner, Tape, Mic and Auxilliary inputs are also provided. Bass and treble are switch boost and cut, with the variable slope facility available for treble-cut. Steel front panel finished in gold enamel. Mono only.
Leak "Point One"
Issued for use with the cheaper TL/10 power amp. The unit has two EF86 valves. Four equalisation settings are available for disc (AES/RCA-Ortho, Col-LP/FFRR-LP, NARTB/HMV-LP, and BRIT-78), plus Tape and Tuner inputs. Baxandall tone control circuit used. Two EF86 valves. Steel front panel finished in gold enamel. Mono only. Varislope filter is not included.
Leak "Varislope III"(circa 1956)
Designed for use with the then new TL/12 Plus, TL/25 Plus and TL/50 Plus power amplifiers. Valves are two EF86 pentodes using the familiar shunt feedback circuits of all Leak pre-amplifiers. The bass and treble circuits now use the Baxandall shunt feedback circuit (see Wireless World, Oct 1952) and potentiometers are used now instead of switches. The variable slope circuit was modified so that normal treble boost could be selected at the same time that treble cut at 9kc/s, 6kc/s and 4kc/s could be selected and with any slope. Issued in brown and gold livery with a "diakon" transparent plastic front panel, styled by Richard Lonsdale-Hands Associates. Mono only.
Leak "Point One Stereo" (circa 1958)
Effectively a stereo version of the Varislope III, but with the Varislope circuit removed. Bass and treble circuit is the Baxandall circuit. Four EF86 valves are used - two for each channel.
Leak "Point One Plus"(circa 1958)
Advertised for use with the new "TL/12 Plus" power amp. Finished in gold and brown, with diakon front panel. The treble filter circuit has a fixed-slope roll-off for three turn-over frequencies. Mono only.
Leak "Varislope Stereo" (1958 - early
Effectively a stereo version of the Varislope III. Original issued in the same gold and brown livery as the Varislope III. Later the name was updated to "Varislope 2 Stereo", and the finish was revised to the 1960's "silver and grey" to match the new transistor amplifiers from Leak. The circuit uses four EF86 valves shared between channels, with facilities similar to the Varislope III. 3.5mV sensitivity on Pickup input.
Leak "Varislope Mono"
The Varislope Mono was issued in the the 1960's "silver and grey" finish that matched the new range of transistor amplifiers being released by Leak. The circuit is similar to the Varislope III, but the choice of slope is limited to two positions - steep and gradual. (Possibly also issued in gold and brown livery.) 3.5mV sensitivity on Pickup input.
Leak "Varislope 2 Stereo" (early/mid
Electrically similar to the "Varislope Stereo" but with only two "Slope" positions. Finished in "silver and grey" to match the new transistor amplifiers from Leak. The circuit uses four EF86 valves shared between channels, with facilities similar to the Varislope III. 3.5mV sensitivity on Pickup input.
Trough-Line" (Prior to 1958)
88-100Mhz, Foster-Seeley discriminator circuit, magic eye tuning indicator, 2kHz accuracy. 10lbs weight. This unit was advertised in the January 1958 Wireless World, and appears to match the "Varislope II" and "Point One" pre-amps (steel front panel finished in gold enamel) even though it appeared in the advertisement with the then new Varislope III pre-amp.
Leak "Trough-Line II" FM Tuner
Click here for further details on the Trough-Line.
Leak "Trough-Line 3" FM Tuner, and
"Trough Line Stereo"
Similar to the Trough-Line II, with revised silver and grey finish. The "Trough Line Stereo" came with a stereo decoder (which can be improved on), and some difference in the valves used compared to the "Trough-Line II". The stereo decoder could be factory retro-fitted to earlier models (in which case the original Volume control is replaced with a switch to select AC-On/Off and Mono or Stereo mode).
Leak "550" Cabinet Loudspeaker (circa 1950) Two way system. Other details unknown. Advertised in Sept 1950 Wirless World.
Leak Electrostatic Speaker (mid 1950's Prototype only) Discussed in "The Gramophone", May 1955.
Leak "Sandwich" Loudspeaker (circa 1960) Two way system with many design innovations and advanced use of new materials. So named because the cone material comprised a 10mm thick cone of expanded polystyrene, laminated with skins of aluminium foil back and front, giving a cone stiffness 200 times greater than conventional paper cones, giving true piston action over the working frequency range. Designed by D.A. Barlow, previously of the Aluminium Development Laboratory.
Early systems apparently had a paper cone tweeter,
but was later revised to the sandwich diaghram as well. (The tweeter bears
a physical resemblance to the Wharfedale super 3.) Box volume is approx
2 cu feet, and sealed. Woofer is approx 13" diameter.
Leak "Dynamic Pickup and Arm" (prior to 1950) Advertised in December 1949 Wirless World. Frequency response: +/- 1dB 40 Hz to 14kHz on Decca test record. Ruby or diamond stylus. A unit examined in Australia had an associated transformer with the arm - this is most likley a moving coil unit. No counterweight, but a spring loaded ball attached to the rear gimbal determines the tracking weight. Any further information appreciated.
Leak "Dynamic Pickup and Arm" (early 1950's) A moving coil unit, with resistance of 6 Ohms and frequency response: +/- 1dB 40 Hz to 20kHz. Various options for LP and 78rpm styli. On LP, 2-3 grams tracking weight is recommended, with the diamond stylus. 8mV output after the transformer. Counterweighted straight arm, with plug-in heads (in contrast to the first Leak Dynamic pickup above).
Leak "Stereo Pickup" (early 1960's) 1mg tip mass, elliptical stylus, variable reluctance, 2 grams playing weight, 1.2mv/cm/sec output. An unusual feature of the arm is that the pivot is at the same height as the playing surface to reduce the effect of warped records. To achieve this the arm is curved in the vertical direction.