(by Geoff Kremer, 2006)

The modifications explained here dramatically improve the amplifier's performance in all areas.

The input stage has been altered to direct coupling to improve frequency response and phase characteristics whilst the HT supplies to these, and the driver stages, have been separated and individually de-coupled to help prevent crosstalk between the channels and to improve the HT stability.

The effect of the modifications is to open out the sound stage and extend the bass response whilst keeping the sweetness of the mid and top ranges. During development great attention has been paid to keeping the distortion levels as low as possible, with a new set of "run in" valves (tubes), the Total Harmonic Distortion is 0.08% at 8 Watts driven at kHz into a 10 Ohm load while the frequency response is 20Hz to 40KHz.
The amplifier is unconditionally stable under all conditions.

Solid state diodes have been used to replace the GZ34 and can be built into an Octal plug. The reason for this is that the mains transformer is known to be marginal in its ratings, using diodes does away with the need for the 1 Amp heater current drawn by a valve rectifier.

The additional components can be fitted to two small tagboards attached to the main assembly panel. The cost of the components needed for these modifications, carried out on a freshly renovated ST20, should be no more than £20:00 and it must be emphasised that the amplifier must be in perfect working order.

The two "padder" resistors added across the anode loads of the phase splitter are nominal values. These will be OK and can be fine tuned for the lowest distortion figures using a distortion analyser. The 0.5mf coupling capacitors feeding the EL84 valves need to be of good quality Mylor types but you don't have to spend a fortune on 'paper in oil' types, it's a waste of money and will not give any benefits. The 0.1mf grid de-coupling capacitor, connected to pin 7 of the ECC82, must be of a very good, low leakage type.

Valve replacements:
6N14N-EB valves are used in place of the EL84. These are Russian military spec direct replacements.
6N2P in place of the ECC83. This is not a direct replacement as the heater connections are slightly different. Pins 4 and 5 are the heaters with pin 9 being a screen and should be earthed.

These modifications can also be applied to the TL12 and TL12 Plus but the EF86 valve will need to be replaced with an ECC83 using just one half. The other half of the valve can be left unconnected.

The before and after listening tests, by many people, confirm that these modifications can transform the Leak Stereo 20 into a superb amplifier capable bettering the performance of many that are currently made.

Above: Geoff's circuit diagram for his modified Stereo 20

Below: the underside of a modified Stereo 20