The TL/10 was introduced as a lower cost alternative to the famous TL/12, but maintained the distortion rating of 0.1%. This amplifier was the first Leak amplifier to incorporate the "ultra-linear" output stage configuration (popularised by Hafler and Keroes from the USA in the early 1950's, but earlier proposed by Blumlein), and many consider the TL/10 to be much under-rated these days. Leak certainly did not skimp on the ratings or size of the transformers, as they are identical in size to the TL/12 Plus (and Stereo 20 in the case of the output transformer).
This amplifier was also the only Leak circuit to use a single, common cathode resistor in the output stage (all other Leak valve circuits used separate cathode resistors for each output valve, which should reduce the dc-unbalance in the output transformer). However, some TL/10's were issued with separate cathode resistors, individually by-passed as with other Leak circuits. The circuit diagrams for both variants are available at this web site.
The KT61 valve is rather unusual. It has a very high transconductance, and substitutes are hard to find without loosing performance (see below for Leaks recommended replacements) . The Mullard EL33 is a contender as a substitute valve. The author has tried EL34's and 6V6's, but found they do not perform all that well in the TL/10.
Valves: EF86, 6SN7, 2xKT61 in ultra-linear. Leak state that the Brimar 6AG6G is an exact equivalent to the KT61. Leak state that 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.