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Leak stereo 20 modification

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SY

SY

Ex-Moderator
Joined 2002
SYclotron.com
Depends on your goal.

Restoration? Change out the power supply caps and any other electrolytics. You don't need fancy "audiophile" components, just well-rated 105 degree C electrolytics and decent film-and-foil polypropylene caps.

Performance? Gut and rebuild. Morgan Jones shows a lovely example in his book "Valve Amplifiers." His Bevois Valley circuit drops into that chassis with some minimal metalwork.
 
G

Gudmund

Member
Joined 2010
SY said:
Depends on your goal.

Restoration? Change out the power supply caps and any other electrolytics. You don't need fancy "audiophile" components, just well-rated 105 degree C electrolytics and decent film-and-foil polypropylene caps.

Performance? Gut and rebuild. Morgan Jones shows a lovely example in his book "Valve Amplifiers." His Bevois Valley circuit drops into that chassis with some minimal metalwork.
Click to expand...

Restoration is the way for me. I would like to hear the amplifier as it was. I will stay with the standard components as you suggested.

Have looked for Morgan Jones. There are more than one edition. - Will have to learn more before I try to change the circuit.

Have looked for replacement tubes - EL84 /ECC83. Value for money - which type shall I buy? Russian?

Thanks
Gudmund
 
Shoog

Shoog

Member
Joined 2002
I had a Leak Stereo 20 as my first valve amp. Lovely sweet hang in the air sound. Definately coloured, but in a very nice way.
Replace all the caps with some reasonable MKP or better types - they can go leaky and if they do they might take down the output trannys which would be very difficult to replace. There is a big ballast resistor in the early stage power supply - do not replace it as it acts as a drop out fuse. Most of the resistors will probably have drifted quite a bit in value. This is probably OK but its small money to replaced them all with good stable carbon comp.

Dump the Varislope preamp as it is rubbish and only worth gutting. The valves inside can be sold on as spares. The Leak has massive gain an d matches well with a passive preamp- anything with gain will be to much. One modification worth considering is replacing the input stage on the Stereo 20.

Shoog
 
G

Gudmund

Member
Joined 2010
pointy said:
as my dad did 50% of the test design work on the st20 transformers in the 50s
(when he worked for leak}.

hjleak : "H. J. Leak & Co. Ltd."

will be able to help you...........!
Click to expand...

Hi Pointy - I had to answer you before I went to sleep. I would like to say thank you for your offer. Your dad has been a very lucky man, - working with Mr. Harold J. Leak in the 50s!!
I would very much like to work with the Stereo 20 and make it play once again. I hobe that I can return to you with questions related to the Leak amplifier.

Thanks
Gudmund
 
SHiFTY

SHiFTY

Member
Joined 2002
I had one for a while. The large gold filter caps are always bad, suggest you disconnect and replace with modern equivalents under the chassis if possible- you can make them slightly larger too if you like- the first cap after the rectifier can be up to 47uF with a GZ34.

All the other capacitors and many resistors will need changing if you want to use this as a reliable, everyday amp. The resistors will work but are usually wildly out of spec and possibly noisy.

Gain is way too high, a switched attenuator is probably necessary to get hum and noise down.

Also I found that a telefunken ECC83 in the first position really makes the amp sing. Use a Mullard GZ34 if you can get it, they are reliable and rugged and last for ever. There are a lot of good new production EL84 around that work just fine, but I like vintage ones.

Also note the power transformer can get quite hot!
 
G

Gudmund

Member
Joined 2010
Shoog said:
I had a Leak Stereo 20 as my first valve amp. Lovely sweet hang in the air sound. Definately coloured, but in a very nice way.
Replace all the caps with some reasonable MKP or better types - they can go leaky and if they do they might take down the output trannys which would be very difficult to replace. There is a big ballast resistor in the early stage power supply - do not replace it as it acts as a drop out fuse. Most of the resistors will probably have drifted quite a bit in value. This is probably OK but its small money to replaced them all with good stable carbon comp.

Dump the Varislope preamp as it is rubbish and only worth gutting. The valves inside can be sold on as spares. The Leak has massive gain an d matches well with a passive preamp- anything with gain will be to much. One modification worth considering is replacing the input stage on the Stereo 20.

Shoog
Click to expand...

The note about the ballast resistor shall be kept in mind. I found a green 100R wirewound 5W R - and a black spot below on the print.

As for the Varislope I do think it looks very old - but nice. I would like to keep it - even if the sound quality is poor. The gain problem may be an issue later. It does not work.

Gudmund

Ps. very little use of the Varislope - 4 pcs. - on the print it looks like - Mullard EF86?
 
G

Gudmund

Member
Joined 2010
SHiFTY said:
I had one for a while. The large gold filter caps are always bad, suggest you disconnect and replace with modern equivalents under the chassis if possible- you can make them slightly larger too if you like- the first cap after the rectifier can be up to 47uF with a GZ34.

All the other capacitors and many resistors will need changing if you want to use this as a reliable, everyday amp. The resistors will work but are usually wildly out of spec and possibly noisy.

Gain is way too high, a switched attenuator is probably necessary to get hum and noise down.

Also I found that a telefunken ECC83 in the first position really makes the amp sing. Use a Mullard GZ34 if you can get it, they are reliable and rugged and last for ever. There are a lot of good new production EL84 around that work just fine, but I like vintage ones.

Also note the power transformer can get quite hot!
Click to expand...

Ok - found circuit on Stereo 20. two 32u+32u was standard capasitors. Have bought two FJW(German) 50u+50u 500V electrolytic capasitors - same diameter as original. Do the old have the can brown painted - or is it a shell?

Have one GZ34 miniwatt , one 5AR4 and one U52 bought erlier on Ebay.
 
Shoog

Shoog

Member
Joined 2002
I am fairly certain that the caps are located in a gold can which can be seperated from the cap itself. It would be nice to re-stuff the can and keep the visuals. If they will not seperate I would be a bit wary of gutting the can as they may contain PCB oils which are highly toxic.

Overhauling the Varislope is a much bigger job than overhauling the Stereo 20. If you like the visuals think about putting in a different but far better circuit.

Shoog
 
I

ifsixwasnine

Member
Joined 2010
Hi

i too have one of these little amps that i am going to restore. Whilst looking for advise, mods etc on the net came across this mod,


[FONT=times new roman, times]This involves essentially changing the biasing of the phase splitter stage to allow direct coupling from the primary stage, reducing gain and fitting the more linear ECC82 valve for the original ECC83 type. I also added the resistors to isolate the transformer windings from the output valve screens.[/FONT]
[FONT=times new roman, times]Take R6 and R11 (the only 3.3M resistors in the circuit) and join them together in the air, isolating them from R8 & R9. Now remove R9 (22k) and replace with a link, remove R8 (1k) and replace with one of 100k. This last one is between the valve socket and circuit board. Remove C3 (0.1u) and replace with a link. Add the 100R resistors to the transformer connections (on my unit, the white and purple wires should be removed from the transformer solder tags and the 100R resistors inserted in line). Replace V2R and V2L with ECC82 valves.[/FONT]
[FONT=times new roman, times]It looks a simple little break your cherry mod.
[/FONT]
[FONT=times new roman, times]I have no idea what if any benefits from this mod. someone on here might care to explain..[/FONT]
[FONT=times new roman, times]
[/FONT]
 
G

Gudmund

Member
Joined 2010
pointy said:
i will try to help you out with the amp but having said that on that site there is many people who know far more about leak kit than i can ever hope to know.

steve spencer, hifi shack, mr j leak (son of hj leak) etc.........................
Click to expand...

Have tried to look at the site you mentioned. Waiting for reply.

Shoog - I will try to separate the psu capasitors , if there are any shell covers. Would like to keep the original look. Takes action on the Varislope later.

Ifsixwasnine - A circuit would help me. - Think I would like to hear the original first.

Pointy - could 5751 be used - have looked at mu= approx 70 / ECC83 mu= 100 ..... Wrong or misunderstood? If right -- could 5751 be used without change in the original circuit?

Gudmund
 
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