|Leak TL 12 'Point One' mono power amplifier (1948 - 1955)|
The term "Point One" referred to the distortion level at full power (0.1%) - a remarkable figure for the time. This was achieved by the main feedback loop encompassing all three stages including the output transformer - in effect a 'triple-loop' - hence the designation "TL".
Leak had been making a feedback amplifier (known as the Type 15) with similar low distortion (0.1%) since 1945, but it featured a four-stage circuit and was costly to produce. The TL/12 was a logical and more cost-effective development of the Type 15.
The basic TL/12 circuit was evolved in 1947, initially for the purpose of driving record cutting heads. The output stage utilised a pair of 6V6G (KT66) tetrode valves which were connected in triode mode with cathode bias. Stage one utilised a pentode voltage-gain stage, with the second stage consisting of a long-tailed pair phase splitter and driver. This configuration formed the basis for all of Leak's valve power amplifier circuits for the next 20 years.
Leak's reputation for high performance at reasonable cost and an extremely high standard of manufacturing was soon established. The TL/12 was extremely well constructed and utilised no electrolytic capacitors in the power supply. Oil filled paper capacitors were used instead, these were more expensive but lasted a lot longer.
In January 1949 the advertised price of a TL/12 was £25 15. shillings with the matching preamplifier an additional £6. 15 s. Given that in 1949 the average wage was about £8 a week, this was an expensive piece of gear - about four weeks wages!
In the USA, Allied Radio's 1951 catalog advertised the TL/12 for $153.70 (amp only) and $192 including the preamplifier.
The TL/12 remained in production until about 1955. Its appearance varied over the years, but not the circuit. Early units featured a bronze enamel finish and open-frame transformers. Sometime around 1952 - 1953, a gold finish replaced the bronze, and transformers with pressed-steel shrouds were introduced. Some TL/12 amplifiers around this time had a mix of both types of transformers on the same chassis.
In 1957, the TL/12 was replaced by the TL/12 'Plus' [link] - physically smaller than the TL/12 and using the new "9-pin" power valves in the output stage (configured in "ultra-linear" mode instead of the inefficient but very linear triode-connected KT-66's of the TL/12). Power output was still 12 Watts, and the distortion component remained at 0.1% .
Current value of a single TL/12 in fine fettle is anywhere
between £500 - £1,000; although recently (August 2006) I saw
a matched pair with early serial numbers and minor modifications (RCA
input and input level control) reach £2400 (US$4,500) on eBay. The
immaculate unit with preamplifier (pictured at right) fetched £1480
in March 2006.