Author Topic: Leak Stereo 20 restoration/mods  (Read 9726 times)

Offline afroaudio

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Leak Stereo 20 restoration/mods
« on: June 03, 2015, 04:52:30 PM »
Hi gents,

I know there are a few of these amps around and no doubt some have been restored also by some of you fine folk.

Now that I have one on the table (in not too fantastic state) I would like to know from those more knowledgeable what the best route to take is for component replacement/upgrades and any mods if any that might be advised.

My approach so far with these projects has been to keep all values the same as far as passive components go.  Are there any components that would benefit from upgrade? I have also read that a small power supply choke transformer can be added to help smooth out PS ripple, not sure if anyone has tried?

Then there is the matter of the amps apparently very sensitive input stage.. who has done what here? Attenuated interconnects? Passive pre amp? or switching ECC83 for ECC81 or 5751 valves?

Lastly for the big "can" electrolytic capacitors, can anyone suggest good replacements that fit?




Offline Hi-Phibian

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Re: Leak Stereo 20 restoration/mods
« Reply #1 on: June 03, 2015, 04:58:21 PM »
Hollow out the old can and fit modern caps inside.
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Offline valvesound

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Re: Leak Stereo 20 restoration/mods
« Reply #2 on: June 03, 2015, 05:13:54 PM »
Hi afroaudio....just a quick google search for leak stereo 20 restoration and upgrades give a number of well written and explanations on what guys did with pics. The way the leak has been made with all components on the tag board makes it really quite simple. Usually resistors are replaced as they might have drifted with better 'low noise' parts and then obviously capacitors like electro's should be replaced as the originals might have leaked or dried out and then the coupling caps can do with a upgrade to good quality polyprops. The usual really. If the valves are ok leave them at first and maybe roll with new or NOS later. Here is just an example of a quick search reply I got.
http://audiotweak.net/2013/03/24/leak-stereo-20-rebuild/
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Offline afroaudio

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Re: Leak Stereo 20 restoration/mods
« Reply #3 on: June 03, 2015, 05:49:32 PM »
Hey Valvesound thanks, I can (and have already) pull up any number of articles on the net or threads from other forums, but would like to hear from some local owners/techs, I want to do this thing right :thumbs:

Online charles

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Re: Leak Stereo 20 restoration/mods
« Reply #4 on: June 03, 2015, 10:48:53 PM »
Jonathan,

The first valve amplifier I restored was a ST20.

Keep the reservoir capacitors in their cans and just remove the wiring from it. Replace it with modern electrolytic ones. These are very small ones and will fit under the chassis.

Like valvesound said, replace all the coupling capacitors, except the one in the feedback loop (very low value). Check the resistors and replace  the ones that drifted out of  specification. I usually do not replace all the old resistors.

Keep the old components.

Do not change (modify) the circuit topology of the amplifier.

Do not drill any holes into the chassis.

Keep it as original as possible.

If possible, test the valves with a valve tester to determine the emission/mutual conductance of the valves.



 

Offline skollie

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Re: Leak Stereo 20 restoration/mods
« Reply #5 on: June 03, 2015, 11:44:59 PM »
Jonathan,

The first valve amplifier I restored was a ST20.

Keep the reservoir capacitors in their cans and just remove the wiring from it. Replace it with modern electrolytic ones. These are very small ones and will fit under the chassis.

Like valvesound said, replace all the coupling capacitors, except the one in the feedback loop (very low value). Check the resistors and replace  the ones that drifted out of  specification. I usually do not replace all the old resistors.

Keep the old components.

Do not change (modify) the circuit topology of the amplifier.

Do not drill any holes into the chassis.

Keep it as original as possible.

If possible, test the valves with a valve tester to determine the emission/mutual conductance of the valves.

+100%. As Charles said.

skollie
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Offline Hi-Phibian

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Re: Leak Stereo 20 restoration/mods
« Reply #6 on: June 04, 2015, 12:04:05 AM »
Don't use metal film resistors. It changes its voice.
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Offline Ampdog

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Re: Leak Stereo 20 restoration/mods
« Reply #7 on: June 04, 2015, 06:55:12 AM »
I agree with all comments - as far as maintaining collector value is concerned. This is a valid approach by some, and I respect that. 

But there is another approach,  strangely so even  with some well-known brands. In this case with the Stereo 20; I will refrain from mentioning others here. That bears the decision as to leave certain shortcomings as they were, or improving performance without changing the basic topology (especially with 8 ohm and 4 ohm loudspeaker loading). In all restorations that I have done I have always put this decision to the owners, and I always had the request to go forward with improvements - some regarding different component values, some regarding correction for basic inadequacies, mostly in stability criteria as a result of negative feedback. (Leak stuck to an ideal of 0,1% distortion before overload which was laudable and pioneering, but toughened stability criteria because of a high NFB factor required.)

Summing, for those with access to a schematic, values of an R-C combination later added across R5, plus values of C5, C9, R12, R13 and the use of R21 for purposes of desoldering itself as over-current protection (frown), come under scrutiny. Not sure whether such detail is on topic here; I can provide my alternatives on request. (For the record, consistent results were found with five examples, thus 10 separate amplifiers, so I take such as independant of unit related ideosyncracies. By the way, all models were fitted with the 8778 type output transformer. Stability criteria differ from earlier models with a 3921 OPT for which I have no data.)

Afroaudio,

Regarding your specific questions, I am not aware of problems with the sensitive input stage (are you perhaps confused with such claims for the Mullard 5-20 topology?). I have never been a fan of ECC83 input stages (in fact not the use of that valve anywhere in a main amplifier) but let that ride. Replacement with ECC81 or 5751 will necessitate design changes. (Yes, they will make music; that does not mean they are near ideal for the purpose.) There can be mild effects depending on pre-amp feeding impedance, particularly a 'passive' one to which this amp is not ideally suited depending- but that is getting technical again.

A suitable choke power supply will improve ripple rejection of which there is some audible in the output; there is even space to put one on top, but you will have to do some mechanical work to make it look largely like the other iron - depending how important that is to you. I have done it once.

Folks do inconsiderately use any electrolytic capacitor replacements in the power supply filter. There is a spec there: Such capacitors, particularly the input one (C13) must have minimum ripple specs. I have used the rather expensive JJ  50+50F/500V cans, can fit into the original spaces.
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Online charles

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Re: Leak Stereo 20 restoration/mods
« Reply #8 on: June 04, 2015, 03:12:17 PM »
From Ampdog;

"By the way, all models were fitted with the 8778 type output transformer. Stability criteria differ from earlier models with a 3921 OPT for which I have no data."

I have two ST20's: one is fitted with the 3921 OPT transformer and the other one with a 8778 OPT. One of these days I am going to do a comparison on them regarding total harmonic distortion and spectrum analysis to see how these two models differ from each other. Both are restored and I will use a set of NOS valves for the test.   

Offline skollie

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Re: Leak Stereo 20 restoration/mods
« Reply #9 on: June 04, 2015, 03:56:49 PM »
From Ampdog;

"By the way, all models were fitted with the 8778 type output transformer. Stability criteria differ from earlier models with a 3921 OPT for which I have no data."

I have two ST20's: one is fitted with the 3921 OPT transformer and the other one with a 8778 OPT. One of these days I am going to do a comparison on them regarding total harmonic distortion and spectrum analysis to see how these two models differ from each other. Both are restored and I will use a set of NOS valves for the test.   

OPT's on mine are 8778.

Thanks for the wisdom ;D

skollie
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Offline afroaudio

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Re: Leak Stereo 20 restoration/mods
« Reply #10 on: June 04, 2015, 05:08:39 PM »
Gents and especially Ampdog, thanks for your detailed replies. Johan this is exactly the info I am after, I would like to hear more about your suggested upgrades and will send a PM. I can also understand the folks who would prefer to stick to all original values but I cant help but think that with some small changes things might be improved somewhat, no disrespect to HJL and his team but these amps were designed in the 1950s

My OPTs are numbered 3921.

Online charles

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Re: Leak Stereo 20 restoration/mods
« Reply #11 on: June 04, 2015, 05:27:15 PM »
Another interesting piece of information;

Apparently the ST 20  with the 3921 OPT transformer has a different circuit diagram as the one with the 8778 OPT.

Observed it from the Internet.

I have some original ST 20 manuals. Will check if I can check the differences.

Offline afroaudio

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Re: Leak Stereo 20 restoration/mods
« Reply #12 on: June 04, 2015, 06:57:36 PM »
Thanks for this Charles  :thumbs:

Offline afroaudio

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Re: Leak Stereo 20 restoration/mods
« Reply #13 on: June 04, 2015, 07:44:23 PM »
Here is a useful link for people needing circuit diagrams and noted there are different ones for the 2 versions of the Stereo 20

http://www.44bx.com/leak/leak_ccts.html

Offline Ampdog

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Re: Leak Stereo 20 restoration/mods
« Reply #14 on: June 06, 2015, 04:14:01 PM »
The Stereo 20s mostly under my attention were as per p.3 of the above reference.

I will list my alterations only here, and proceed in a next post with technical discussion, so as not to bore those not interested.

R5 = 100K
R12, R13 = 470K (on some schematics = 1 megohm)
R18 = 15K - R18K
C5 = 470nF/630V
C9 = 150pF
C10,C11 and C12,C13 = 2 x 50 + 50F/500V JJ capacitor

R.C in parallel with R5 (on some amplifiers but not on schematic); R = 4K7, C = 120pF in serie, across R5

Resistor over 15 ohm output terminals: 1K 5W  (not in any versions, see next post)
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