These amplifiers were, of course, mono, as stereo recordings were not available at the time. It is not commonly known that the pre-amplifiers were designed to be bolted to the end of the main amplifier because most were separated using the umbilical and mounted on a panel. These days owners prefer to expose their 12.1s to view and in any case the pre-amps are seldom used since the amplifiers tend to be arranged in pairs with a simple stereo control unit.
When acquired this unit appeared to be in original condition, with the main components such as transformers and smoothing capacitors undamaged. Many of the smaller components were in poor condition with resistors reading high and capacitors leaky. Of the valves, the 6SN7 needed replacing but the remainder tested GOOD and were retained. The KT66s and rectifier had suffered mechanical damage as their spigots had broken off. Because of the condition of the unit, no attempt was made to power it up at that stage It also has its original 12" Rola speaker, though most would prefer to use the amplifier with more modern units!.
Since owners of these amplifiers expect top performance, the decision was taken to have the unit professionally overhauled rather than just replacing the poor components..
The Vari-slope pre-amp was tested and appeared to work OK, but as it was unlikely to be used, it was not serviced and the on/off switch connections in the amp were by-passed with direct wiring to the mains power socket. The original pre-amp lead had been lost, so a new one was made up and also a separate octal plug containing only a single RCA socket to connect with another control unit.
Safety is a major consideration both to protect the equipment and the user. As a result a solid state protective module, with its own power supply was designed and built. It has two purposes, the first is to identify any major differences in the current drawn by the two KT66s, giving warning of possible valve or component failure. The second function is to operate a relay to disconnect the power in the event of the amplifier being overloaded, bearing in mind that there is no volume control on the unit. These situations are indicated by the two red LEDs showing through the side of the chassis. The additions can be seen in the picture below. A protective cover was also made for the base, as these were an optional extra from Leak, and one was not supplied with this unit.
An illustrated document has been prepared explaining the restoration in full detail, including operational considerations, together with full circuit and performance diagrams. The report includes contact details for Turner Audio who have offered to help in the event of a technical query. The document is quite large but can be emailed on request.
Count since 3/07/05.
Page maintained by
Simon Buxton VK2UA
. Created: 10/02/02 Updated: 20/08/2007