Modifying the Leak Stereo 20

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The Leak Stereo 20 amp is a classic and many survive from the fifties and sixties. In stock form it comes with the ECC83 (12ax7) in both the input and phase-splitter positions. This was and still is a popular valve, and purists will simply recommend finding a good sounding variant of it and leaving the circuit as it is. The output tubes are a pair of EL84 per channel.

Experimenters who have already substituted better resisters, capacitors, valve bases, connectors and the like may wish to try a further modification on the circuit, partly to improve the circuit and partly to allow for the use of different valves.

Possible circuits, with the appropriate change in components include:

Mullard type circuit – Chris found’s variation:

Input stage (1 of) - 6CG7, 6N30, E80CC

Phase Splitter stage (2 of) – ECC99, 6N1P, 5965 (*** See 5965 shootout for recommendations and alternatives).

The first valve base needs to be rewired for 9aj type valves, by taking the 6.3v heater supply to pins 4 and 5. The circuit then needs to be changed to accommodate the different needs of the various valves.

The PSU can be changed to dual mono after the first cap, and can use two chokes of 10mH available from Maplin UK, which fit face down under the mains transformer using the same bolts.

Concertina type circuit:

Input stage (1 of) - 6N1P, 6N30, E80CC

Phase Splitter stage (2 of) – ECC99, 6N1P

Radical All-Triode circuit – Andy Evans ‘final’ version of the Stereo 20:

Input stage – 2 of 5BK7 in differential pairs with a Morgan Jones CCS underneath it. These are heated by the 5v rectifier windings

Output stage – 8 of 6S4A (four per side) in parallel push-pull. Fixed bias. These tubes use up ten holes on the top, including those designed for the GZ34 and the two PSU caps.

The new PSU has four chokes of 10mH as supplied by Maplin UK and five 100uF electrolytics. Rectification is solid state. All this goes – with some ingenuity – under the chassis.

Sound Quality

The Mullard circuit can be improved by using more appropriate tubes as described above. The Concertina is at least as good. These circuits are useable without global negative feedback, though a little helps tidy the sound. There is a definite improvement over the original in all areas.

The big step up comes with using all triodes and using no global NFB. This is a wholly different amplifier, and sounds, well, fantastic. The final touch is to rewire the output transformer so as to use all of its secondary windings, giving the best overall sound that any Stereo 20 has ever produced. Componants in this ‘final’ version (how can you better an all true triode amp?) are Russian ex-military Teflon coupling caps and Holco resisters.

Circuit diagram and tubes

A circuit diagram will be available at a future point. Tubes of various kinds can be purchased from Andy Evans, (see audio page small ads tubes).