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Old 16th Mar 2021, 11:54 pm   #21
bikerhifinut
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Default Re: Building a new Leak Stereo 20.

Mikes words on transformers , and Ed D's are sage.

I will say this, Either would wind you a proper job that will absolutely work well.
So in that context you will most definitely get what you pay for.

Now I wouldn't go down the route of attempting to build a perfect "clone".
There are a few reasons in my way of thinking why it's not really practical to attempt, aside from the eye watering cost.

I have had the sometimes almost tear inducing experience of building a valve power amplifier based heavily on the leak TL25/stereo60 designs.
Mike Barker actually has inspected my original monobloc attempts and gave me some very good advice, and informed opinion.
This was used to good advantage when I rebuilt the monos as a stereo amplifier with the very generous help of another forum member who supplied a very nice and well over specified hand wound toroid mains transformer.
I used the Danbury 50W output transformers that I got for the original project, bearing in mind Mikes advice on those as well. (Danbury are no longer in business)
The final amplifier has some commonality in terms of the basic circuit, in particular the leak design of the Long Tail Pair phase splitter. I also reduced the overall gain and with it the feedback ratio to give it a more useable sensitivity in todays world of Half a Volt, later I reduced it further to a Volt to suit my sources. I would refer you at this point to David RWs observations on amount of overall feedback and stability.
I also departed from the use of a GZ34 Valve rectifier, and opted for silicon diodes.
So you can see by now I no longer had a leak copy in terms of following the circuit blindly but I did end up with a very nice sounding amplifier that measures very well indeed and is my everyday amplifier in the system.
I didn't use the leak circuit board system, it's really got no place in DIY. It was designed to be a sub assembly that was built by workers outside the main factory that would be QC checked and then sent to the production team in the factory. But for a one off DIY amp, I'd recommend one of the traditional methods where you can follow your wiring easily and ensure the routing and layout is as good as possible to avoid issues later. I personally favour the tagboard technique of sub assemblies of parts that are then wired up as it makes component changing and substitution easier. Others like the bus bar style of hard wiring. There's no right or wrong method, but there is a wrong way in all methods of wiring and earthing.
I hope I haven't put you off the idea of building your own take on what is a real classic, but it isn't going to be an easy job at all. I got a good result in the end but it was a long journey and a steep learning curve.

The best way to get a stereo20 in this way is to find a "basket case" with good transformers at a reasonable price and then spend a lot of midnight oil rebuilding it.
And that's one heck of an ask.

Andy.
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Old 18th Mar 2021, 12:08 am   #22
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Default Re: Building a new Leak Stereo 20.

I recall reading sometime in the distant past that Leak OT's were wound with bifilar windings can anyone confirm this ?
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Old 18th Mar 2021, 12:40 am   #23
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Default Re: Building a new Leak Stereo 20.

Early versions of the original Leak TL12 output transformers are found with Bifilar windings in each of the two sections.
This may have been dropped for more standard techniques quite quickly because a lot of the TL12 'Partridge' output transformers that I have rewound are standard.

The original TL12 'Partridge' open framed output transformers are excellent giving test results that would put modern £800 - £1000 'Special' output transformers to shame!

However, You just cannot beat the Williamson output transformer in my opinion.
The original design on modern high quality iron really is the most wonderful thing.
A big job to make and a big transformer when finished.
I think only beaten by the McIntosh 'Unity Coupled' output transformers.
Now that is a real beast to wind!!!

Mike...
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Old 18th Mar 2021, 12:40 am   #24
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Default Re: Building a new Leak Stereo 20.

Quote:
Originally Posted by retailer View Post
I recall reading sometime in the distant past that Leak OT's were wound with bifilar windings can anyone confirm this ?
Sort of bifilar. Two smaller diameter wires wound side by side, but connected together at the ends. Simulating one winding of flatter wire. Follow the link in post 18.

David
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Old 18th Mar 2021, 7:15 pm   #25
Westo091946
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Default Re: Building a new Leak Stereo 20.

Quote:
Originally Posted by GrimJosef View Post
One notable feature of the later type of output transformers (model 8778) as opposed to the original ones (model 3921) was that the ultralinear tappings were changed to 25% compared with the original 49%. This allowed the nominal output power to be reached at the lower HT voltage that the later design used. That, in turn, had been necessitated by a tendency for the output valves in the earlier design to go into thermal runaway ('burning up' as the Leak engineers described it). I'm not sure if anyone sells output transformers with 25% UL tappings off-the-shelf.

Cheers,

GJ
Primary Windings:-

http://primarywindings.com/

and Variable Voltage Technology Ltd

https://vvttransformers.co.uk/

sell universal output transformers for the EL84 with 20% and 43% Ultra Linear tappings. Push Pull transformers are available off the shelf, but they also will wind to order.

Last edited by Station X; 19th Mar 2021 at 10:37 am. Reason: Readability. Links added.
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Old 22nd Mar 2021, 11:42 am   #26
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Default Re: Building a new Leak Stereo 20.

I've just done a pair of 6BQ5 PP output transformers - UL primary, to the owners specs, the secondaries are bifilar, a bit of a PIA to do. Once I had the length worked out and cut, I just left each length to float around the workshop - one on the left and other on the right and wound by hand, to be sure I overestimated and only wasted around 15cm or so of each length - not something I want to try with primaries.
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Old 22nd Mar 2021, 9:18 pm   #27
bikerhifinut
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Default Re: Building a new Leak Stereo 20.

Quote:
Originally Posted by murphymad View Post
Early versions of the original Leak TL12 output transformers are found with Bifilar windings in each of the two sections.
This may have been dropped for more standard techniques quite quickly because a lot of the TL12 'Partridge' output transformers that I have rewound are standard.

The original TL12 'Partridge' open framed output transformers are excellent giving test results that would put modern £800 - £1000 'Special' output transformers to shame!

However, You just cannot beat the Williamson output transformer in my opinion.
The original design on modern high quality iron really is the most wonderful thing.
A big job to make and a big transformer when finished.
I think only beaten by the McIntosh 'Unity Coupled' output transformers.
Now that is a real beast to wind!!!

Mike...
I hope this isn't cheeky Mike, but i was reading Steve Spicers book again for some background on the transformers and he states that the Technical manager at Hinchley in Devizes was a Mr K W Barker.
I wondered if it was just coincidence?

Very Best Regards

Andy.
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Old 6th May 2021, 3:26 am   #28
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Default Re: Building a new Leak Stereo 20.

I'm planning a number of stereo amp builds, one of them was the a copy of the Leak 20, around 18 months ago I started on the Leak but things got in the way and I only completed an under chassis wiring diagram, not having access to the real thing I worked from pictures I found on the internet so there MAY be errors - however I feel it is 99% correct, except that I forgot I was looking at the chassis underside and marked the right and left components incorrectly they are in their correct positions however. There may be other forum members or people just browsing the internet that would find it useful so I'm posting it here as a PDF, feel free to point out errors, I have tried to make it clear so wire placement may not be exactly in the right place, but if one also uses underchassis pictures, a reasonable copy of the amp should be possible .

There are around 7 pages in all, filament wiring, wiring harness, tag board and components, links on the bottom side of the tag board completed chassis under with voltages at various points taken from the circuit.

Last edited by Station X; 7th May 2021 at 8:03 am. Reason: Attachment removed at OP's request.
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Old 6th May 2021, 9:54 am   #29
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Default Re: Building a new Leak Stereo 20.

Looks good ! I thought I couldn't see a wire to C11 on the diagram, but it's just pale-coloured and at the limit of my eyesight !

The L/R labelling in Leak amps is unusual. The convention that other manufacturers follow is that the speaker terminals on the rear panel are labelled to correspond to the speakers as viewed from the front (the listening position). So when you look at the rear of the amp the R terminals are on your left and the L ones on your right. Leak do the opposite, which means that in their case the speaker leads have to cross one another as they go from the terminals to the speakers.

What this means as far as your diagram is concerned is that your labelling under the chassis is consistent with Leak's labelling. So the L channel EL84s are on the same side of the amp as the GZ34 (which Leak call V5, rather than V5L) and the R channel ones are on the other side, with the HT smoothing capacitors. Your labels above the speaker terminals are reversed though.

There's also some ambiguity in the case of the Stereo 20 about where the front of the amp is. Normally we know where the rear is because it's where all the terminals are, but in the Stereo 20 the power and speaker terminals are at one end and the input terminals and the pre-amp link are at the other. Leak put their logo which, presumably, you're meant to stare at while listening, on what I'd call one side of the amp. Ho hum.

The only other thing I noticed is that you show R7 in each channel as 100k. In fact Leak put 91k components in that position. The reason is that a long-tailed pair phase-splitter with equal anode loads provides slightly unbalanced outputs. This can be corrected by tweaking one of the loads. Curiously, when I did the calculation for the ECC83 running under the Stereo 20 conditions I found that the optimum value for this resistor would be about half way between 91k and 100k. So modifying the load from 100k to 91k hardly improves the balance at all. I can't be certain my sums were correct, of course ...

Cheers,

GJ
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Last edited by GrimJosef; 6th May 2021 at 10:01 am.
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Old 6th May 2021, 2:02 pm   #30
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Default Re: Building a new Leak Stereo 20.

Yes you are 100% correct GJ, I worked from a pic of the underside of the amp and then later noticed it on the circuit and meant to go back and change it but it slipped my mind - I can see now that the pic I worked from has in the R7 position a 100K resistor with another underneath in parallel and most probably the total would be 91K, my bad.
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Old 6th May 2021, 4:38 pm   #31
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Default Re: Building a new Leak Stereo 20.

Just found another error I omitted R6 and R11 from both channels both 3M3 resistors

Last edited by Station X; 7th May 2021 at 8:05 am. Reason: Attachment removed at OP's request.
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Old 6th May 2021, 4:49 pm   #32
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Default Re: Building a new Leak Stereo 20.

2 things,

Yes 100kΩ in parallel with a 1MΩ gets you 91kΩ (if you were looking at my pictures).

Then there is a tiny problem with the output connections.
On the Leaks there are 3 tags on the output transformer secondary, not two.
The 'one' is grounded to the chassis earth and a wire to the -ve output tag.
The 'two' is the output to the +ve output tag. It connects to the impedance selector plug inside the transformer. It does not go to the feedback tag on the board.
The 'three' is the feedback (straight off the 16Ω winding) to the board feedback connection.

You already spotted the missing R6 and R11.

But brilliant work! Alan
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Last edited by snowman_al; 6th May 2021 at 4:56 pm. Reason: R6 and R11 correction
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Old 7th May 2021, 1:40 am   #33
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Default Re: Building a new Leak Stereo 20.

Hello snowman - it probably was your pic I was looking at, thanks for posting on the net. With regard to the output transformers I did see that there were 3 connections, I reasoned that it was highly unlikely that a person could acquire an original pair of Leak output transformers and replacements would be highly unlikely to have an impedance selector mounted on top with the 3 terminals on the bottom so I decided to leave that part up to the end constructor, however now that you mention it it might be better to stick to the Leak layout - builders attempting it can then work out the connections that suit the transformers they are using. I've asked moderators to delete my previous attachments and I'll post an amended pdf file in due course, it is probably just as well as I had a thought that it would be nice to have some info/descriptive text on each page.
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Old 7th May 2021, 8:06 am   #34
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Default Re: Building a new Leak Stereo 20.

Quote:
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I've asked moderators to delete my previous attachments and I'll post an amended pdf file in due course, it is probably just as well as I had a thought that it would be nice to have some info/descriptive text on each page.
Attachments deleted as requested.
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Old 8th May 2021, 4:22 am   #35
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Default Re: Building a new Leak Stereo 20.

Revised TL20 layout/under chassis wiring, I had to print and scan and make a concession and reduce the DPI to 300 as at 600DPI the file was over the 4.5M limit. My trusty and elderly version of Corel turned unreliable when I upgraded to Win10 so I now use Inkscape which does not have a multipage capability or tiled printing, which are minor irritations and can be worked around.
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Old 8th Feb 2024, 12:59 pm   #36
Westo091946
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Default Leak 20 New build.

It's been some time since my original post but have taken some time to get started with this project, mainly due to available time and sourcing components and fittings.
The first issue was transformers but I have now sourced From Majestic Transformers a direct equivalent for the HT and output units one issue was the changed HT 280-0-280 and circuit changes with R12 and R13 from 1 meg to 470K I looked at the 1/2 watt resistors which were extremely small so opted for 2W carbon film 5% question, is it OK to use these for the build instead of the lower wattage value.
Should I also use higher Value Electrolytic Capacitors and Higher Voltage for C2,C7 and C9.
The component board I have just completed the problem with this was double sided tags finding a source I have only used them in selected locations and used single tag else ware, the rivets I have used to secure the tags to a 3mm epoxy board, are hole through copper brass plated 4mm by 5mm which were punched into place, used an image of the component board as template for drilling. see attached photographs.
The heater wiring I intend to use twisted pair running that around the edges of the chassis keeping it well away from the rest of the wiring loom. So that is as far as I have got. I have also got all the valves and basses.
I would be regretful for any input for my questions please.
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Old 9th Feb 2024, 8:19 am   #37
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Default Re: Building a new Leak Stereo 20.

If you're not married to the leak stereo 20, primarywindings will be releasing the 5-10 in stereo DIY kits next month,so the transformer problem is solved for you. All you need to do is to assemble and solder.

I recently substituted the Chinese output transformers in the Armstrong 220 clone I built with the primarywindings 8k 43% mullard 5-10 ones and I can't complain. No changes to the stock lead and lag compensation circuit values required.
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Old 9th Feb 2024, 2:57 pm   #38
Westo091946
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Default Re: Building a new Leak Stereo 20.

Thank for that I have two ongoing the leak 20 and the mullard 10 stereo using the ECL86 output, I also have the components including the twin ganged pots to original spec made by a Welsh company that used to supply mullard they still produce pot's I have the volume, balance and tone controls, as this amp has a one valve pre-amp in each channel.
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Old 9th Feb 2024, 4:54 pm   #39
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Default Re: Building a new Leak Stereo 20.

Hello,

Interesting primary windings will be releasing stereo 5-10 DIY kit next month.

I still have a pair of primary windings 5-10 output transformers to plumb into a 5-10 alike project. I currently have a pair of Gilson transformers borrowed from a Sterns amplifier in these amps.

Terry.
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Old 9th Feb 2024, 5:30 pm   #40
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Default Re: Building a new Leak Stereo 20.

Quote:
Originally Posted by Westo091946 View Post
"...original spec made by a Welsh company that used to supply mullard they still produce pot's

Interesting. What is the name of the "Welsh company", and where are they?


Mike
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