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Old 13th March 2010, 08:48 AM   #21
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If you are keeping the original circuit, then 5751 are a good idea - anything to cut down the gain.

I have a stereo 20 which has had about 15 different circuits in it during its life. I took the view that the original circuit is a total waste of time and almost anything will sound better. So the first thing to do is strip out all the insides and rewire all the essentials with new wire - proper twisted pairs for the heaters and kept away from the signal runs. It can be a good idea to replace the tube sockets, which can deteriorate. I then rewired the circuit board for a newer and simpler circuit, taking out most of the yellow wire underneath the board so I could wire directly between components on the top of the board.

I personally hate global feedback on amps, so getting rid of that was the first design consideration. I continued to run the outputs in UL at first, but then rewired them in triode. Without global feedback you need very little gain so you can use any of the really good sounding lower gain 9 pin tubes - ECC99, 6N30, 6N1P, E80CC... Just use any simple Mullard type circuit,

I like to keep the look of the amp original - no hacking away at the metalwork. But it's a simple matter to put in two small 10H 100mA chokes in parallel underneath the mains transformer - the ones I used fitted perfectly on the end of the 4 bolts attaching the mains transformer, which I lengthened.

All the revised versions of the Leak sounded a lot better than the original - as well they should!

If I had a really tatty amp that I would not mind changing the shape of I'd use 807 tubes in triode - perfect for the OPTs. Morgan Jones has a neat mod for re-wiring the OPTs which is a worthwhile mod. I'd also then use DHTs in the input, like 3a5 which can be used simply with filament bias and then only needs a small transformer to feed three of them with a LM1084 as voltage reg or CCS. Maybe the existing heater circuit could even be used to produce the DC if it didn't hum.

andy
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Old 13th March 2010, 11:24 AM   #22
Gudmund is offline Gudmund  Iceland
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Quote:
Originally Posted by andyjevans View Post
If you are keeping the original circuit, then 5751 are a good idea - anything to cut down the gain.

I have a stereo 20 which has had about 15 different circuits in it during its life. I took the view that the original circuit is a total waste of time and almost anything will sound better. So the first thing to do is strip out all the insides and rewire all the essentials with new wire - proper twisted pairs for the heaters and kept away from the signal runs. It can be a good idea to replace the tube sockets, which can deteriorate. I then rewired the circuit board for a newer and simpler circuit, taking out most of the yellow wire underneath the board so I could wire directly between components on the top of the board.

I personally hate global feedback on amps, so getting rid of that was the first design consideration. I continued to run the outputs in UL at first, but then rewired them in triode. Without global feedback you need very little gain so you can use any of the really good sounding lower gain 9 pin tubes - ECC99, 6N30, 6N1P, E80CC... Just use any simple Mullard type circuit,

I like to keep the look of the amp original - no hacking away at the metalwork. But it's a simple matter to put in two small 10H 100mA chokes in parallel underneath the mains transformer - the ones I used fitted perfectly on the end of the 4 bolts attaching the mains transformer, which I lengthened.

All the revised versions of the Leak sounded a lot better than the original - as well they should!

If I had a really tatty amp that I would not mind changing the shape of I'd use 807 tubes in triode - perfect for the OPTs. Morgan Jones has a neat mod for re-wiring the OPTs which is a worthwhile mod. I'd also then use DHTs in the input, like 3a5 which can be used simply with filament bias and then only needs a small transformer to feed three of them with a LM1084 as voltage reg or CCS. Maybe the existing heater circuit could even be used to produce the DC if it didn't hum.

andy
Andy.

You are way beyond me as for as the circuit is concerned. Don't know that much.

Tried to make a new precise PCB copy of the original without all the bottom wiring. The idea was: outer holes for wires - inner holes for silverpins.

What is best - hardwiring or PCB with cobberlines? Would like to refresh one of two amps - one original - one with new brown paint and Leak "marks" like "L" and "R" , "Input", ECC83 a.s.o.. Do you know the brown paint code? How do we make the "silkprint" on the paint?

Want to use EL84 - the type I can afford with best sonic sound! Is it possible for you to show the circuit you prefer for the Stereo 20?

Which Morgan Jones edition talks about Stereo 20 mod. ?

Many questions.

Gudmund
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Old 13th March 2010, 12:37 PM   #23
SY is offline SY  United States
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3rd Edition. Which anyone even half-serious about tubes ought to own.
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Old 13th March 2010, 12:54 PM   #24
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If you are keeping the original circuit, then 5751 are a good idea - anything to cut down the gain.

You can cut gain more by using the 12AY7, while retaining the high RP and low gm.
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Old 13th March 2010, 12:58 PM   #25
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If you want to restore Leak equipment, best to join the Leak users group - hjleak at Yahoo Groups.

I don't think I have any record of all the circuits I dreamed up. They were pretty simple - some kind of voltage amp first stage direct coupled to a differential pair with the second grid grounded. I then used russian teflon caps to couple to the outputs.

The Morgan Jones OPT mod came from some thread or other - can't remember as it's long ago. I have a pdf of it somewhere. Maybe it's possible to do a search here or it may have been the Audio Asylum. May have been the Leak users group. It direct wired the transformer for 4 ohms so you no longer had the switchable impedences on the output.

Andy
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Old 13th March 2010, 01:04 PM   #26
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Here's a well thought out circuit to use with a proven track record. Would go nicely in a stereo 20. Should also be plenty of posts here on it if you search under Poinz, Eric Kingsbury and Musical Machine. Plus Eric is an absolutely lovely guy and it's a pleasure corresponding with him.

http://www.audiotropic.net/Products/machine.html

andy
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Old 13th March 2010, 01:08 PM   #27
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Here's Poinz's circuit

6V6 Amp Audio Circuit; Parts Expo
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Old 13th March 2010, 03:58 PM   #28
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Hi is this the opt mod you refer too?

[IMG]file:///C:/DOCUME%7E1/DEANE/LOCALS%7E1/Temp/moz-screenshot-1.jpg[/IMG]One of the best things you can do with a Leak is to improve the output transformer. Most loudspeakers these days are nearer to 4 Ohm than 8 - especially those using a pair of bass drivers. But if you set a Leak to match to a 4 Ohm load, you only use half of the secondary.

If you have the courage to take the shells off the output transformer, you will find that the individual windings are terminated on cardboard strips glued to the coil. You can then reconnect them as shown in the right hand diagram so that you use both windings - which makes for a transformer with reduced leakage inductance - a better transformer, and it cost only a little time. You will probably go one step further and connect new wires directly to the transformer and throw away the phenolic tagboard at the base of the transformer and its associated old wires.

Having tweaked the secondary, you'll probably look at the primary and see that it's possible to replace the wires there too. The PVC-covered wires solder to little solder tags inside the paper windings, and it's perfectly possible to desolder them and replace them with longer wires that go directly to the valve sockets


Click the image to open in full size.

here are three output windings. Two are already in parallel. For the third, EC8010 is right. The third winding - looking at EC8010's useful diagram - starts at the 4 ohm tap and goes up to the 15 ohm tap. this winding should be re-connected, the 4 ohm side to negative and the 15 ohm to positive. I hope this is clear
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Old 13th March 2010, 04:25 PM   #29
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I run a cd player direct into my amp (no pre amp) player has a vol control. To bring the players output down i just use a simple voltage divider built into the interconnects rca jacks at the amp end -20db. it has allowed me to use the it without having to set about the amp with a soldering iron and keeping it totally original
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Old 13th March 2010, 07:14 PM   #30
Gudmund is offline Gudmund  Iceland
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Quote:
Originally Posted by Eli Duttman View Post
You can cut gain more by using the 12AY7, while retaining the high RP and low gm.

Eli
Looked at a datasheet for the 12AY7 before answering you. Found Mu=44 at Duncan's page. Could this type be used in the original circuit without changes? - a short explanation of Rp and low GM ? Rp = Plate resistance? - GM = Gain?

Sy
Well - this is a hobby - need to buy the 3. edition.

Andy
Untill the experience with the Stereo 20 comes up, I have to work with a circuit close to the original. - saw the note about teflon capasitors.

The AudioTopic amp has high built quality - could not find the circuit - Shall remember Poinz and Eric. Have checked the Leak user group.

Stereo 20 may have insufficient circuits compared to newer designs - have though heard a standard Stereo 20 which played well at my friends setup.

Gudmund
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