Building a new Leak Stereo 20.

[Westo091946]

Split from this thread:-

https://www.vintage-radio.net/forum/...d.php?t=175082

Sorry new to this site I am attempting to build a new Leak 20 I have all the circuits and lots of pictures so have built up a reasonable picture of the main component board with it's wiring on the reverse side the wiring loom has me foxed i have tried every where to get information the heater wiring ? is it split into two 6.3-0-6.3 V giving 6.3 half phase for each set of valves ? I am 75 did city and guilds Radio and TV only up to second year and have not touched any thing in almost 40 years. this has been hard going i am stumped cannot seem to be able to work out the loom.
I know that leak changed the ht transformers because there are two types 300-300 and 280-280 which is the more recent i think the 300-300 running close the the EL84 Max V could you possibly help please. I would be very grateful. Thank you

[Radio Wrangler]

What are you using for output transformers?

The Leak ones were a bit special in order to minimise phase lag at high frequencies. Leak's amplifiers run rather large amounts of feedback to get their advertised distortion figures, and consequently the stability margin at high frequencies is highly critical on the build-up of phase lag.

David

[snowman_al]

Interesting (bold) project there.
As David says, quite a lot of the Stereo 20 'signature' is from the output transformers. Significant changes to them will require the feedback elements changing.

The mains transformer is easier. You need 300-0-300 volts at 200mA. (That gives you a margin over the specified 160mA consumption.) 280-0-280 is fine too. You want to end up with 310 / 320 volts +ve HT at the output transformer centre tap. (I.e. After the 100Ω resistor R21.)
The GZ34 needs 5 volts at 2 Amps.
The EL84s and ECC83s need 6.3 volts at 4 Amps, so get at least 5 Amps. It needs to be centre tapped too, so look for 3.15-0-3.15 volts.
Better to over spec the currents if possible, it leads to a cooler running final result. A 'problem' with the ST20 is the marginal original spec of the mains transformer. They do run warm (hot) after 3 or 4 hours of use.

The heater wiring are the Green and Grey wires and the centre tap wire is Black or Brown (sometimes dark purple). The 0 volt reference to the chassis is at the ring tag on the reservoir cap C12/13 mounting screw. The HT and heater centre taps both go there, as do the speaker ground connections.

Use the later version of the schematics, R5 (ECC83 anode resistor) needs to be 47kΩ. R12 and R13 (EL84 grid leaks) Must be 470KΩ. C5 is 0.1uF 400volt (as is C3).

Good luck, Alan

[GrimJosef]

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

[vidjoman]

Mains transformers are available for Leak amplifiers. I believe they also have copies of the output transformers as well. Found with a Google.

[GrimJosef]

Do you have a link for the output ones ?

Majestic say they'll wind anything you want, but it won't be off-the-shelf.

Sowter say their UA21 will do for the Stereo 20, but it's rated for 40W power and only has a primary (anode-anode) impedance of 6k6, which I think might be a bit low, and I can't see any mention of 25% tappings.

VVT have 10W ones with 20% and 43% tappings, both with 6k6 a-a impedance (VTP18738) and 8k2 (VTP13186). These might be quite close, but they're not spot-on, and the quoted 10H primary inductance is a bit low.

Primary Windings also do a 10W 6k6 primary one with 20% tappings. Now the primary inductance is 35H, which is better. They do have an 8k one, which might be closer to the Stereo 20 value, but now the tappings are at 40% and the thing is rated for 25W.

Cheers,

GJ

[snowman_al]

Sowter do their U064 optx for PP EL84s. 8k, 20% or 43% taps and 4, 8 and 16Ω output match and 10 Watts.
Problem I see is the cost of the iron. Nearly cheaper to buy a 'doer upper' ST20...

[GrimJosef]

Oops, missed that one. They just list 43% for it here https://www.sowter.co.uk/push-pull-o...ansformers.php but it looks like they can do 20% instead. Maybe they could do 25% ? That might then be quite close. As you say though, not cheap. Over £400 for a pair once the VAT's added.

Cheers,

GJ

[Westo091946]

I have started a project:-Build a leak 20 from the ground up
I did radio and tv some 55 years ago as hobby only finished year two as work got more demanding. I built the mullard amps fine bit of kit.
So though I would get my hands dirty and take on this project on in my day it was mono only one channel giving my age away, now we have 2. 2.1,5.1 and 7.1 I collected as much as I could in by way of circuit diagrams pictures and some schematics Leak 20.

I have also discovered the mullard Book (publication HiFi amps in Adobe format) and my son wanted a valve amp so I will build the 10+10 for him I have manage to source the Pot's for this amp at only £7-50 each exact match (2Meg) from a company who used to manufacture these for mullard and are still producing pot's switches (Wafer) and other items.

To continue:- thought looking at the circuit's of the leak 20 and picture I had seen on the net including the underside PCB Component layout it would be fairly easy process , but it has proved to be not so simple.

I have the underside wiring of the component pcb if you could call it that, have good photographs and can see the connections to pcb and valve bases, but my problem is under pcb wiring loom I think that this is predominately the valve heater circuit wiring split into two ? with the 6.5-0-6.5 volt split ? between the two channels given the mix of valves and seeing the configuration on the valve bases the ECC83 heaters are pins 4 &5 Linked and 9, 6.3 Volt if pins4,5,6 are connected this would need 12.6V heater voltage, El84 are 4 & 5. 6.3 V and so I am assuming that there are two runs for the valve heaters, and are parallel with centre tap to earth ? also the wiring of the input and output of the output transformers as well. I know that the EZ34 has it own 5 volt supply

I have been trying to pickup an old 20 but they are fetching quite a high price.

I am wondering if anyone could help me, with any information about the wiring harness, I have managed to get some info from another forum member which was very much appreciated , if any one could contribute
to this I would appreciated any input.

I can get off the self mains transformer but it's the 300-300 which is the very original which they switched for a 280-280 volts the higher HT value is limiting value of the ELE4, from my research mullard confirmed to Leak that this was OK (so I read) but leak anyway changed to the lower HT and changed R12 and R13 from 1Meg (300-300)V to R13 and R13 to 470K (280-280)V I do not know the the mA rating for the Mains Transformer so if some one could help here please.

We were always told, and it's in the Mullard book of amps to twist the heater wires together as this would have a cancelling effect on any emission's from the heater wiring affecting the high gain audio circuitry.

I have started to put together a schematic of the wiring with information I have I hope that I can complete this with some wider input from members.

I have the parts list for the Leak 20, Resistors and Capacitors only

Thanks

[qualityten]

I have just seen that you are already asking the same questions in another thread: https://www.vintage-radio.net/forum/...d.php?t=175082, so I'm deleting my reply here.

[Radio Wrangler]

Threads merged.

It's confusing when the same discussions get split over multiple threads. People lose track of what they said in which one

David

[Westo091946]

Sorry about the double post the downer avenue as you say might be the answer but you would have to pay anything from £700 to £900 the, a company Variable voltage Tec do an EL84 ultra linear grid of 20% and 43% with a rated 10 watts this is £80 off the shelf they also do the HT 300-0-300 with both heater tapping and is rated at 200 mA for £95

[snowman_al]

Originally Posted by Westo091946

...I have the underside wiring of the component pcb if you could call it that, have good photographs and can see the connections to pcb and valve bases, but my problem is under pcb wiring loom I think that this is predominately the valve heater circuit wiring split into two ? with the 6.5-0-6.5 volt split ? between the two channels given the mix of valves and seeing the configuration on the valve bases the ECC83 heaters are pins 4 &5 Linked and 9, 6.3 Volt if pins4,5,6 are connected this would need 12.6V heater voltage, El84 are 4 & 5. 6.3 V and so I am assuming that there are two runs for the valve heaters, and are parallel with centre tap to earth ? also the wiring of the input and output of the output transformers as well. I know that the GZ34 has it own 5 volt supply
...
We were always told, and it's in the Mullard book of amps to twist the heater wires together as this would have a cancelling effect on any emission's from the heater wiring affecting the high gain audio circuitry...

OK
Let's clear up the ECC83 connections and 6.3 volt heater wiring conundrum.
The ECC83 can be used at 12.6 volt if you use pins 4 and 5. (The filaments are then in series and pin 9 is not used.)
It can also be used on 6.3 volts if you link pins 4 and 5 together, for one connection, and then use pin 9 for the other connection. The 2 filaments are now in parallel. (Have a look at the valve data http://www.r-type.org/pdfs/ecc83.pdf.)
Leak use the Parallel method so pins 4 and 5 are linked and pin 9 is used. That way only 6.3 volts is needed.
Also on the Leak the 6.3 volt heater supply is referenced to ground / 0 volts by a centre tap on the transformer, that is one of the hum reduction parts. The heater winding is effectively 3.15-0-3.15 volts.

I've drawn the heater layout in the diagram enclosed, you can match it to the picture. Solid lines are Green wires, dotted lines are Grey wires.

Follow this method and hum (from the wiring) is not a problem.

Originally Posted by Westo091946

... I can get off the self mains transformer but it's the 300-300 which is the very original which they switched for a 280-280 volts the higher HT value is limiting value of the EL84, from my research mullard confirmed to Leak that this was OK (so I read) but leak anyway changed to the lower HT and changed R12 and R13 from 1Meg (300-300)V to R13 and R13 to 470K (280-280)V I do not know the the mA rating for the Mains Transformer so if some one could help here please...

The 300-0-300 will be fine, later build ones have a 300-0-300 volts transformer from the factory. The late build charcoal one in the picture does.
The ratings were mentioned in post #3.

Alan

[Radio Wrangler]

For the output transformers it isn't just a matter of the impedance ratios and the ultralinear tap percentage. They also need to be carefully wound to get the internal capacitances right and to reduce the leakage flux. These things influence the stability of the whole amplifier. Leak were quite bold in their design and definitely were pushing things a bit.
You'd need to find someone who has had to dismantle a damaged transformer and has recorded the winding scheme for re-winding it.

David

[Ed_Dinning]

With this type of output transformer you pays your money ..............
A simple OPT has all the pri wound, then all the secondary. OK and cheap for a Dansette record player but not Hi Fi.
Going up the scale : wind 1/2 pri, sec, 1/2 pri; much better coupling

Wind A-UL tap 1/s sec, Wind UL to HT, 1/3 sec, wind HT to UL, 1/3 sec, wind UL to A

Much more complex and time consuming and it can go on from there. I believe the Williamson was wound in about 14 sections.
If you put more iron in the core you will get better bass response. It is also possible to wind "side by side", which can reduce inter winding capacitance, you can alter the sequence of windings. It can go on and on as the costs go up and up.
A reasonable definition of a Hi Fi output transformer should be that it is at least as big as the mains transformer.

Ed

[Trevor]

Just remember at the time a Stereo Twenty with its preamp was a lot of money.
I was a young lad working in a small hifi business the sale of a Leak Stereo Twenty was no small thing.
Trev

[Radio Wrangler]

And a lion's share of that money went into the fancy winding of the output transformers.

Not all of the flux from every bit of a winding gets to go directly past every other bit of every winding. If you start looking at what goes on in the winding stack in detail, it gets messy and you start to learn what imperfections there are and what compromises have to be made.

The game isn't just to make the amplifier stable across the whole audio band, you have to go beyond that and make it stable at ALL frequencies so that it doesn't oscillate at any frequency. On top of that, you want enough stability margin in the audio region that it has good damping. You can test this by looking at the square wave response with a pulse generator and oscilloscope. This really sorts out the proper designers from the charlatans, Leak was no charlatan. In choosing to build a Leak design, you have taken on quite a challenge. If you don't do the full elaboration of the original design, it isn't going to work like the original design, and you can't expect whatever special attributes in the sound that are attributed to these amplifiers.

I'm sorry, but those good output transformers are the key to the amplifier and the amount of work involved in them is expensive. Even the amount and size of the lams costs extra.

David

[Valvepower]

Hello,

I’ve always had a soft spot the ST20 and felt it to be good example of synergistic industrial design where electronic, production and commercial design all meet to make good product.

Sadly, the ST20 has been a victim of ad-hock modifications, which don’t add or do anything for the original design, but that’s my own personal opinion. If your OK with a 125mV input sensitivity restore to stock and use as-is.

Using commercial parts like Welwyn metal film resistors, ICEL or KEMET (Was Arco) polypropylene capacitors and F&T can electrolytics I’ve witnessed the ST20 amplifier kick "high-end" valve gear into touch.

I’ve pair of those Primary Windings “5-10” EL84 output transformers, which look a rather tidy job, on the “round-tuit” list to try out in the bit-a-this and bits-a-that EF184 amplifier projects to replace the RS “EL84” transformer. From memory the Primary Winding is 6.6K Ohm a-to-a with 20% UL taps, whereas the RS has 43% UL taps.

In the dim and distant past, I’ve spoken with Steve Spicer (of the Leak Book fame) about the constriction of the Leak output transformer, the information of which is on the Leak site http://44bx.com/leak/8615tx.html however, to echo what RW said, if I were to get serous, I’d strip down a Leak transformer to find out exactly what’s what.

I’ve recently used a pair of output transformers in a Cadet 2 rebuild using PCL86’s (don't want to spend £££'s on ECL's), which were both rewound by Mike and copied from the originals, which are absolutely bang-on.

That’s enough waffle for now….

Terry

[Radio Wrangler]

If you look at the page Terry's pointed to you'll see how complex the primary structure is, and on top of that, notice the use of two wires wound in parallel and connected in parallel to reduce the height of the layers taken by the secondary.

Those are some very artful transformers, and needed in this case.

If you ever look at any Radford amplifiers, you'll discover that Arthur Radford was essentially a transformer designer.

David

[murphymad]

Hello,

If you are serious about having the Leak Stereo 20 Output and Mains transformers to the original specification and exact winding copies then I can manufacture them for you.

No, it is not cheap and there is a lot of work involved.

I may even have some of the transformer shrouds of the original pattern here as well but the original speaker tap selectors may be a problem.

I have been rewinding Leak amplifier transformers for a long time and have pretty much done every transformer for every Leak amplifier.
The largest part of these were originally made by Hinchley transformers of Devizes.

I would suggest that you go with the 280-0-280 volt HT and the 25% screen taps.
This arrangement is kinder to the EL84 valves and does not affect the sound quality or the output power of the original design.

Leak also lowered the HT voltage on other amplifiers of the time due to the problems encountered with running the EL84's at maximum stated operating HT.

Best regards,

Mike...