Leak Trough Line II
Written by Bryce Ringwood   

The HB Leak "Trough Line II" tuner was introduced in the early 1960s (1962?) as a high quality FM tuner for high fidelity systems. It is a fairly conventional superheterodyne receiver with a cascode amlifier RF stage (ECC84), A trough-line RF oscillator (ECF80 triode),Oscillator buffer (1/2 of an ECC85), First IF (1/2 of an ECF 80), 2nd IF (EF80), FM Limiter (1/2 ECF 80), audio cathode follower ( Triode of ECF80). The FM Discriminator uses OA79 germanium diodes. The tuning indicator is an EM84 and the power supply rectifier is an EZ80. This gives a total of 7 valves in all. The intermediate frequency is 12.5 MHz - somewhat unusual.Leak Trough Line 2


Cascode Amplifier

At VHF, pentode valves become less favoured because of their high internal noise. If you attempt to use a triode valve as a VHF amplifier, it usually breaks into oscillation because of feedback through the internal electrode capacitances. The ways round the problem are neutralization, grounding the grid, and the cascode arrangement using a double triode. The cascode uses the first triode in grounded cathode, followed by a second triode in a grounded grid configuration. This provides comparatively low internal noise with a reasonable amount of gain. This tuner can therefore be expected to have a reasonable sensitivity (for its time).

Trough Line Oscillator

At progressively higher frequencies,the inductances or coils used in conjunction with a capacitor to tune to a desired frequency have fewer and fewer turns and are made of progressively thicker wire to preserve the "Q" (Quality factor) of the coil. To preserve "Q", a straight peice of wire or rod can be used inside an outer cylinder. This forms a tuned transmission line. The possible benefit is greater oscillator stability, and anyway, it wouldn't be a trough line tuner without it.

Intermediate frequency

The IF is 12.5 MHz, rather than the more usual 10.7 MHz. Since the set has no AGC, it isn't immediately obvious how the tuning indicator works, but it looks as if it simply reacts to the signal strength of the signal across the secondary of the second IF transformer T2. When the automatic frequency control is operating, the tuning indicator is disabled.

Cathode follower

This idea is shared with other FM tuners. It lowers the output impedance. The idea is that the high frequencies won't be lost over the length  of the connecting lead to the main amplifier.

Multiplexer Output

The tuner has an output for an FM stereo adapter. I'm not 100% sure, but I think Leak made one specifically for the tuner. Stereo FM did not appear in the UK until 1972, so a genuine Leak stereo demultiplexer could be something of a rarity. Stereo decoder ICs are available from Mantech and RSE Electronics (if you like truly arcane chips.)

Other Features

The set has automatic frequency control using 1/2 of an ECC85 as a reactance modulator. It also has a switch for local/dx (long distance) reception.

In the 1960's, this was a very desirable FM tuner for the HiFi addict. It had better than average sensitivity and the slightly wider bandwidth plus the cathode follower would have helped enrich the higher musical frequencies. Undoubtedly, the "trough line" gave it the marketing edge. If you had one of these, a "ferrograph" tape recorder and a QUAD amp - you had a "HiFI". Someone tell me what speakers were cool back then - I have forgotten.

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