foxy

The Leak Stereo 20 Amplifier


Another of my favourite amps, these have very good specs for valve amps of the time and they sound excellent with a pair of efficient speakers. Twenty watts is a little underpowered for many modern less efficient speakers, but they still sound good.


There is the problem of the high voltage gain of course. Keith Snook and many others have worked on ways of reducing the gain by increasing the negative feedback. This requires some crafty and extensive mods to maintain the phase margin so it remains stable, quite a lot of work and brainpower needed. I favour the method of simply padding the input. It only requires a couple of resistors added. These can just as easily be removed to restore the original circuit. There are objections to this approach of course, and you can take a few weeks reading all about it on the interweb, but I have had no problems with it.

A few pictures of a Stereo 20 in original condition.
If you want to embiggen any picture here or on other pages, right-click on it then select view image. Or whatever your browser calls it.
Here you can see the safety resistor R21 (100/3W) as it should be. It is rated so that the solder will melt if overcurrent faults occur. To repair, 60/40 or eutectic will be OK. To replace, you have to judge the thermal characteristics of the replacement. Good luck.
My redrawing of the schematic. The original is a nice drawing, with some very attractive features of the style of the period, but many copies on the web are a little fuzzy and difficult to read, so I thought it might be worthwhile doing this more modern style version. I hope it is useful to somebody.

This image is a png file. To see full size, right-click>view image, then click on image or ctrl-scroll to adjust size. Best printed at A3 size, but A4 will work if you have a good printer and eyesight.

For A4 size pdf click here

For A3 size pdf click here

For zip file of installation and maintenance manual click here

Keith Snook has an interesting discussion of the Leak amplifiers here

Repair

I found this example of a repair done by a bad person on the interweb. Notice how it looks all wrong. Bad cans should be gutted and refilled in my opinion, not replaced by blue things.
Here are a few pics of another specimen, which I am going to repair.
It doesn't look too bad. The paint splatters will clean off.
Some hooligan has drilled a hole in the back panel.
There are some defects in the paintwork.


The most surprising thing is that the two can capacitors are in full working order, and no sign of chemical leakage, and electrical leakage is well within limits. Many of the other components have been replaced with more modern ones. I would guess this work was done over twenty years ago. It doesn't look too good because they seem out of place, but the work has been done carefully and well, and with fairly good but not audiophile components. The faults I found were mostly in some of the unreplaced components. The two remaining green electrolytics are well past it, and probably the main reason for complaints about the sound. The inputs have been padded to 30%, so someone agrees with me about that.
So following my usual conservative approach, I have done little more than what is necessary to get it working properly to within its original spec or better, and check that it is electrically safe as much as it ever was. As you can see, these amps were not designed by someone who was afraid of dead children cluttering up the place. (Yes I know they were usually in cabinets)


There is always some doubt about what approach is best, many would say that (for example) the can electrolytics may be OK now, but their days are numbered so why not replace and possibly improve things with higher spec parts? I'm not totally against that idea and would do it if requested, but I plan to sell this amp and will leave that decision to the next owner. My guess is that they will be good for a few years yet. I could be wrong. They might blow up next week. Who can say? Few of us pass through this vale of tears without losing a much-loved capacitor.
Some things I am going to improve even though it's not strictly necessary are the output terminals. The Leak ones are just crap. I don't know what they were thinking, the sort of combined 4mm socket/binding post that I am using here were available in 1963.


I have used pieces of FR4 board, held on by small bolts through the original screw holes. The bolts are M2.5, which just pass through the holes without damaging the internal thread.
I will keep the original terminals and screws with the amp, someone may one day want to put them back, and they are unobtainium. It might be more appropriate to bury them at a crossroads, but people are funny.
Doesn't look too bad in my opinion. I don't know what to do about the big hole. Maybe put a plastic plug in it. Or leave it for the next owner to worry about.
The capacitors in the output stage. I have decided to leave these alone. They look all wrong but they are fair quality, much better than the originals. Another thing for someone else to worry about.
Here I have made a few changes. The 270 ohm cathode resistors run at a constant 0.5 watts, which makes them very hot, so I have replaced them with 3 watt types. They are still going to get fairly warm.


One of the 100k resistors was well out of spec so I replaced the two of them with these black things which are 0.1% super-secret magic resistors which apparently the MOD captured from a flying saucer which tried to attack Buckingham Palace. Anyway, that's what the bloke at Ye Olde Resistor Shoppe told me.


The capacitors which decouple the dead grid of the second stage I have replaced also, less necessary perhaps, but I had these very high quality poly roll-ups handy.
I have left in place the first to second stage coupling capacitors, again these look wrong but they are good and very much better than the originals. The small capacitors around the negative feedback tackle check out fine. The old green electrolytics have been replaced with some nice blue Philips ones with a higher rating.


Now it's time to stick some volts up it and see what happens.
Works well, mission accomplished. Here you can see it having a go at some Pink Floyd.


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