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Old 7th January 2005, 07:01 AM   #1
alanr is offline alanr  Canada
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Default Leak Stereo 20 rebuild/restore question

Well, after years away from tubes (having modified many and built from scratch many more), I finally got around to finishing off my Dyna ST70, starting from a bare chassis and using blank boards purchased from Curcio Audio.

Frankly, it sounds great, so good I'm kicking myself for not doing it years earlier .. so good I was afraid that I was suffering from "built-it-myself" syndrome. But two weeks later, I'm sure that it really does sound that good.

I'm now spoiled and so obviously need great sound at work as well; an old Leak Stereo 20 is the candidate.

The amp was working and sounding fine when the power transformer failed years and years ago. An adequate replacement is surely in my dungeon somewhere. But the chassis has enough scratches and whatnot that a new paint job is required for a "proper" restoration.

Therefore, my mind runs to a "rebuild" as opposed to a "restore". I have enough tools to make short work of a nice, from scratch chassis, and I likely have all the other electronics (polypropylene caps, etc) "in stock".

I started to lay out a PCB (what I do as a part-time job), duplicating the original Leak audio circuitry, but adding some solid state regulation to the various stages, when it occurred to me that if a person was going to that amount of work, it would be foolish not to think about it all before plunging in too quickly.

So here's the question for you, the experienced experts.

Is there a better "triodes early 6BQ5s later" design than the one used originally by Leak ? The efficiency of the speakers is unknown at this time, so a triode-wired output is maybe an option, maybe not.

Opinions, and better still, pointers to schematics, gratefully accepted.

Thanks in advance.

Alan
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Old 7th January 2005, 12:45 PM   #2
SY is offline SY  United States
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A couple of notes:

The Leak opts are pretty poor in the bass. If you'll be using the amp in a multiamp system where they're not called on to supply power at 30 Hz, they'll sing.

The Leak input stage is a pretty good one, IIRC. One could certainly use Morgan Jones's Bevois Valley design for a rebuild, but I don't know how much better it will be. Joe Curcio's driver design is an excellent one, too, as you know, and could do very well in this application.
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Old 7th January 2005, 04:17 PM   #3
alanr is offline alanr  Canada
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Thanks for the reply !

I'm aware of the weakness in the Leak's output, and have read about opening them up and paralleling the output windings.

But the future use for this amp is to drive a pair smallish bookshelf speakers on my desk in the office. Low stuff isn't really a problem.

But here's the thing .. I've been out of this so long that I don't know what a "Morgan Jones Bevois Valley" is, and Google doesn't turn up anything, nor does DIY here have much.

Can you elaborate a bit ?

Thanks again.

Alan
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Old 7th January 2005, 04:37 PM   #4
SY is offline SY  United States
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Ah, sorry. The book is "Valve Amplifiers," by Morgan Jones. The amp project is called Bevois Valley and uses Leak iron to make a well-engineered EL84 amp.
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Old 8th January 2005, 03:05 PM   #5
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I've rewired the OPTs on my ST20, and they do indeed improve. It's a fairly simple job and I could supply a diagram of which wires to connect if you email me. Need to scan it. As for circuit.... well, now there's a question. My design decisions would be:
1) Take out the circuit board and make room for parts
2) Totally upgrade the power supply - I have two chokes a channel, one for the output stage and a further one for the input stage. I have also managed to get in five polypopylene PSU caps. I've been juggling things around in the ST20 for years and years, so I know most of the tricks. One is to raise the chassis on feet to make room for chokes bolted facing downwards to the holes provided by the mains transformer.
3) Use some valves in triode - EL84s OK but I've been wondering about 6005, a rugged 6AQ5, which gets good press. Joe Rosen used to sing its praises.
4) Don't use any global NFB
5) Driver stage can be a diff pair with CCS or similar on the bottom. Take your choice, really. 6N1P, E80CC, 6N30, 5687, whatever. I used a 6SN7 in an octal socket in the hole provided by removing the cap on the front end of the amp.

So many different ways to go!
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Old 9th January 2005, 05:03 AM   #6
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Default the Leak st20'sOPT's are NOT weak in the bass!!!

we measured 20 hz at 20 watts < 2% in pentode mode, so it can't be true. And I love organ music.

NO affordable OPT is good at infrasonics, filter the inputs or yr pre's output below 15 or 10 hz.

The circuit is weak, esp. the input stage, and

Too many high gain small signal tubes, precisely 3 too many IMO&E.

Unnecessarily high NFB around the input stage, which is not as 'sweet' as it could be.

Go here and read, if you want to know more, I can send more.
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Old 9th January 2005, 10:22 AM   #7
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Hello Timbo - no I don't think the bass is weak on the transformers, just that when you use all three of the secondaries instead of two, it seems to sound better. You could try it (Enclosed)
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