The wonderful Leak Stereo 20 revisited.
 

I've been building balanced amps with directly heated triodes for at
least a year now, so I hadn't heard a Leak for ages. As luck would
have it my pal Ian's Leak needed a service, so I took it home to have
a look.

This was I believe the first of the "Russian Leaks" that we built.
Chris Found and I took several weeks over the revised circuit, and
ended up with a 6N30 direct coupled into a 6N1P. We tested it quite
exhaustively, and I remember the distortion figures were pretty good.

Ian's amp was pretty basic - all I did was replace the phono and
speaker sockets and change all the internal componants - holco
resistors and polystyrene caps, since I had a pile of them. So pretty
non-invasive.

This time I decided to go a bit further. The sockets were being
troublesome so I put teflon ones in for the first three, leaving the
EL84 sockets as is for the while. Changed the mains plug to a modern
one and relocated the fuse where the old power socket was. This
allowed me to do one of my standard mods - put in a couple of chokes.
These are what were Maplins standard chokes - 10H at 100mA - made by
Danbury. Still available and quite cheap. As luck would have it they
fit exactly under the mains transformer in the same bolt holes, so
they go in very fast.

For a full mod, I take out the circuit board and rewire the heaters
with twisted pair etc etc, and split the PSU into dual mono after the
first cap. This means going under the board. The two chokes are then
one in each side.

But this time I decided to go for a non-invasive approach. So I put
the chokes in parallel - 5 henries but only 75 ohms. While I was at it
I upgraded the PSU caps, going 100 choke 120 5k 200 3.3k
100

I took out the polystyrene caps, good as they are, and put in Russian
teflon coupling caps - the FT-3 versions are 0.1uF and fit in nicely.
For the cap to earth from the second grid of the long tail pair (6N1P)
I used a Russian paper in oil K40 - keeping up the Russian theme.

I'll probably go ahead and do the OPT rewire mod next. But first a
listen.

Well - I was amazed. I knew the amp was a good design, since quite a
few people built it and raved about it. But it wasn't far behind the
directly heated triode amps I have been building. Quite a shock.

It's an easy mod, and quite non-invasive - the circuit board stays
exactly as it is, just gets stuffed with different components. From
the top it looks exactly the same.

I jotted down the new circuit, and I'll happily send it out and also a
picture of the underneath to show where everything goes.

Needless to say, it solves all the problems of excess sensitivity - it
acts like a normal amp now! You can still, however, drive it with a
passive pre out of a CD player. I used my nice little gold point one
pre, which I've turned into a passive. Takes me back to what I was
using about 5 years ago! I could almost live with it. Almost but not
quite - I'll still take a DHT amp over it, but surprisingly not by
much. Hmmmm, food for thought.

Andy Evans