Leak Stero 20 Renovation - component values

[coylum]

Hello,

About a month ago I started a thread regarding coupling capacitor size in the subject amplifier prior to commencing renovation: http://audiokarma.org/forums/index.php?threads/leak-stereo-20-coupling-capacitor-value.794955/

The renovation is now almost complete but I remain a little confused as to the ideal values for some components. It seems that Leak changed certain values over a period and also changed the OPT (8778) and mains transformer as a later development.

My amplifier is of the older age group and uses 3921 OPT's and a 300-0-300v mains transformer type 8162.
I have attached the circuit diagram that came with the amplifier but the actual component values fitted vary as follows: C5 is 0.1uF rather than the 0.02uF shown and R12 and R13 are 470K (1M shown).

I am aware that these modifications were included by Leak at the time of the introduction of the new OPT but were they introduced prior to the new OPT and are they suitable for the 3921 OPT?. There is also some question as to whether R5 should be reduced to 47K (from 100K) but I don't understand the reason.

For interest (?) my renovation has included a complete strip down, respraying of all metalwork in Rose Gold; replacement of resistors with metal film; coupling capacitors with metalised polypropylene and restuffing of original smoothing caps with 47uF Panasonic EE's. R21 has been relocated to a seperate tagstrip to prevent further damage to the main tagboard (3W and soldered so that it can fall off!) - also a HT fuse of 250mA has been fitted. I have attached a couple of pictures. Still need to reapply white labelling and Leak logo!

 

[Dandy]

Great paintwork!

 

[multichamp12]

A question about the paint, is that spray paint? If so what brand?

 

[coylum]

Great paintwork!

Thanks - the result was probably better than I expected.....

A question about the paint, is that spray paint? If so what brand?

Yes,this was spray paint in a can. The brand is Rust-Oleum, 2xUltra Cover, Metallic Rose Gold, purchased, in Australia, from Bunnings.

https://www.rustoleum.com.au/product-catalog/consumer-brands/ultra-cover-2x/metallic

 

[multichamp12]

Awesome, thank you!

 

[Jock Strap]

I am just starting a refurb of a Stereo 20 so, if you are still there, I can trade some information. I have nearly completed a parts list to identify original parts and what replacements I plan to use. It's not yet finished but so far it looks like this (upload). Let me know if looks anything like yours? My supplier is HiFi Collective, hence the Japanese Kiwame metal film resistors that appear to offer good value for money. The challenge of the smoothing caps has me thinking of two solutions, both retaining the original appearance on top. The notes on the parts list doc tell the rest of the story so far. Using a downloaded fuzzy schematic and having to contend with variations in the design with no serial numbers or even dates has made the job tough. Finding suitable components has been a long tedious job searching for values to match the originals has been hard work, too. HiFi Cllective have a good stock of Allen Bradley resistors that tempted me, hence another table in development on my upload.
I also have a Varislope Stereo pre-amp and a Varisplope 2 Stereo pre-amp to choose from so I plan to refurb one of those to go with the S20, despite the many comments that say they are not great.

 

[Jock Strap]

I would like to add this comment made by bikerhifinut on vintage-radio.net. I do not know if it is accurate but should provide a lead on the differences between models if it is:

The differences are in a lower HT and the output transformers UL tap moved to 25% from 43% (As far as I can remember without taking to the text books).
The other differences are a lower anode load resistor on the first Valve and 470k grid leaks on the EL84s instead of 1 meg.
The amp runs the EL84s very hard and these mods help prevent them getting burnt out.​

 

[coylum]

I am just starting a refurb of a Stereo 20 so, if you are still there, I can trade some information. I have nearly completed a parts list to identify original parts and what replacements I plan to use. It's not yet finished but so far it looks like this (upload). Let me know if looks anything like yours? My supplier is HiFi Collective, hence the Japanese Kiwame metal film resistors that appear to offer good value for money. The challenge of the smoothing caps has me thinking of two solutions, both retaining the original appearance on top. The notes on the parts list doc tell the rest of the story so far. Using a downloaded fuzzy schematic and having to contend with variations in the design with no serial numbers or even dates has made the job tough. Finding suitable components has been a long tedious job searching for values to match the originals has been hard work, too. HiFi Cllective have a good stock of Allen Bradley resistors that tempted me, hence another table in development on my upload.
I also have a Varislope Stereo pre-amp and a Varisplope 2 Stereo pre-amp to choose from so I plan to refurb one of those to go with the S20, despite the many comments that say they are not great.

As you can see from my original post my renoveation was completed some time ago - and the amplifier "sings". All values within spec - frequency response, distortion and a complete lack of hum! Overall very pleasing.

I used Vishay metal fim resistors (mainly 2W as you need the length for the tagboard!); Illinois metalised Polyester capacitors as well as a couple of silver mica and polystyrene; The original can electrolytics were re-stuffed with 2 x 47uF Panasonic EE types which are slim, 105 degree rated and high ripple current capacity. They will fit side by side in the original cans.

I have now restored several vintage valve amplifiers and have never found it necessary to use what I will call "boutique" components to achieve a good result. All that is necessary are high quality standard components. The odd "difficult" value can be achieved by series or parallel connection of two components as relevant eg 0.25uF was approximated with 0.22 and 0.022uF - well within the tolerance level!

Out of interest the total cost of my replacement capacitors and resistors was $AUD 74.50 from Mouser Australia - approximately 41 pounds sterling. I will send a copy of the invoice via private message for your information.

As far as the Varislopes are concerned - I personally wouldn't bother. I renovated one previously and it detracted from the performance of the Stereo 20 - noise and distortion particularly. I am currently using my Stereo 20 with just a passive volume control/CD player or the front end of a Leak Stereo 30 Plus (solid state) if I need to use a MM cartridge - which is rare as I tend to listen to vinyl via my main Luxman 505uX.

Cheers, Malcolm

 

[Jock Strap]

Malcolm, this is the most comprehensive input I have had since starting this project. I like Vishay components so I will re-evaluate my parts list using your invoice. Maybe I can cut my total cost in half?
What effect/benefit does replacing the 32uf smoothing caps with 47uf have?

So, what did you do with the Varislopes? There must be a minimal renovation that would put them into original working condition, ready to put on eBay as "serviced and working" instead of the usual "not tested" sales? I like the idea of building a custom pre-amp having a MM phono input and Aux (for my Blue Tooth receiver); and a stepped attenuator, perhaps?

Thank you, Roger

 

[coylum]

Hi Roger,

It is quite normal when renovating valve gear to increase the size of the smoothing caps, within reason, to get better smoothing performance. It is also much easier these days due to the smaller size of the capacitors. I have found Panasonic EE and FC electrolytics to be highly performant and yet reasonably priced.

I completely renovated one Varislope in that I replaced all resistors and capacitors and as mentioned the performance still detracted from the amplifier alone. Personally if the physical condition is quite good and they do actually work, I would just sell them "as is" - they always seem to fetch a good price on ebay and most would expect to renovate further anyway (?). My difficulty with the one I renovated, when I came to sell, was how to truthfully describe its performance!

For a moving magnet preamp you couldn't do much better than a Muffsy kit or similar: https://www.muffsy.com/ . As far as Aux is concerned you have such a high sensitivity on the Leak that an attenuator plus a passive volume control will be all you need. Yoi can add a selector switch choose the MM or Blue Tooth input. I have never felt that tone controls were necessary with my Leak Stereo 20 and CD or MM inputs. Channel balance is another matter - in my case I carefully reduced and matched the input sensitivity of the Leak to 250mV (from 70mV) and then used a 23 step log attenuator as the volume control. Conventional log volume controls can exhibit quite a major discrepancy between tracks.

Cheers, Malcolm

 

[Jock Strap]

I found this link to s site in the UK: http://www.classiquesounds.co.uk/leak.htm
Strip out the Varislope box and replace the contents with a passive pre-amp, using high-quality components would probably cost no more than refurbishing the thing? Adding a Phono stage for MM and MC inputs and straight through stages for the digital inputs could produce a very nice piece of kit that looks like it built in 1960? Could just stick a Muffsy inside the box? Might have a go at that?

Malcolm, you have got me thinking again, and I like that!