|Leak Stereo 50|
In the 1950's, 25 Watts was a lot of power in domestic hi-fi terms, so the Leak Stereo 50 did not sell in large numbers (although in the USA where the less efficient acoustic suspension tyes of speakers introduced by AR were becoming popular, power outputs above 20 Watts were desirable).
The Stereo 50 was introduced by Leak in 1958, alongside the lower powered Stereo 20. The Stereo 50 used the following circuit toplogy:
A single 12AX7 shared between channels as a common cathode gain stage.
A 12AX7 configured as a long-tailed pair for gain and phase splitting. The anode resistors were of unequal value to compensate for the residual un-balance in the phase splitter
The Mullard EL34 (6CA7) were used in push-pull ultralinear with cathode bias from separate cathode resistors. (As each cathode resistor dissapates about 2.5Watts, each pair is positioned above the chassis in a single white tube adjacent to the output valves. The single tube contains two 440Ohm wire wound resistors.) Only 380 Volts was applied to the anodes, so the EL34's are not working very hard at all. Idle current is around 65mA. This enables the amplifier to give 25Watts output, but there is no reserve beyond this (unlike the Leak TL/25 Plus which had about 450V on the EL34's and would give 32 Watts output).
The amplifier on this web page has had the RCA input connectors replaced for gold-plated ones, and the speaker connectors changed to 4mm banana sockets. Late in the production run (around 1961), Leak introduced a Hi/Lo sensitivity switch next to the input stage valve, and the input stage valve socket was mounted from above the chassis (like on the Stereo 60).
A single 5AR4 rectifier in the center of the chassis supplies both channels.
The ripple voltage filtering is done with C-R-C filter, which is inferior
to C-L-C filtering (which was subsequently introduced by Leak on the later
Stereo 60). Presumably this was a cost-cutting measure by Leak, and possibly
a lack of will to get the most power from the circuit as possible. The series
resistance of 100 Ohms has about 25 Volts drop accross it, and so disappates
about 6 or 7 Watts. The resistor is positioned above the chassis to disappate
the heat (see the central white tube in the photos). These resistors were
made by "Standees", and frequently burn-out.
All the inter-component wiring was done using cable looms tied togther with lacing. Capacitors and resistors were mounted on a component board which used lugs rivetted to the substrate, and were interconnected with solid-core hook-up wire (not a PCB).
The cathode resistors and power supply filtering resistor commonly fail in the Stereo 50 (this may be induced by leaky coupling capacitors on the output stage). These resistors are no longer available, and often replacements may have been fitted to the amplifier below the chassis, or shoe-horned into position in an unsatisfactory way above the chassis (this was the case for the amplifier on this web page).
It is possible to fashion replacement resitors that looks vitually identical (as shown on the amplifier on this page). Details will be supplied here soon when sufficient testing has taken place on the technique.