Leak Stereo 20 modifications

[Jock Strap]

I have had a Leak Stereo 20 on my shelf for the last 18 months, waiting for me to repair/restore it. Today, I took a look at and it has been modified by someone who appears to know what they doing. The whole Rectifier stage has been replaced. The GZ20 has been replaced by a 6080 Twin Power Triode and there is a whole new circuit board with two tubes on it: 12AX7 and a CV449.

Does anyone recognise their work and provide a schematic?

 

[gadget73]

obvious question, what is a GZ20 ? The Leak Stereo 20 schematics I'm finding use a GZ34 rectifier, and I find no reference to a GZ20 tube. Number implies some sort of rectifier though.

6080 is usually used as a pass element in a regulated power supply, usually driven by a small tube and with a reference tube. That sounds like the 12ax7 and CV449. Something roughly like this for example



best guess without a full schematic, someone was trying to improve it by converting to a regulated power supply. No idea if thats for the full output of the amp or just the driver circuitry.

 

[Jock Strap]

That makes sense. Thank you for the schematic.
GZ20 is my typo... should be GZ34... WTF was I thinking?

 

[maxhifi]

This looks like a well done modification to me. The person doing the work employed quality parts and good workmanship. Regulating the power supply should also improve the quality of the amplifier. My only real concern would be ensuring the heater windings of the power transformer are not overloaded. Well, that and the one burned looking resistor on the terminal board.

 

[Tunedguy57]

I have a Stereo20 and can say that at max current absorption the drop at HT is negligible.
Probably that Stereo20 has silicon rectifiers instead GZ34, so current availability on 5 volt windings remains high enough for the job.
Sincerely the mod is not so attractive, since makes a venerable old amp a sort of Frankenstein, with sound performance, surely dry and less sweet than original.
This amp has fame to be one of the best Leak ever made...too bad modifying it.
IMHO

 

[kyle]

I have a Stereo 20 as well. I think that the best approach is to replace out of spec parts like electrolytic caps and carbon comp resistors which were all off value by quite a bit on mine due to age. I also replaced the RCAs and speaker connectors. I've had them for about 30 years and they still sound great. The output transformers are very good.

I've seen mods on these amps ranging from changing the input tube circuits to adding volume controls to complete circuit redesigns. These are great sounding amps with the stock circuit and don't require boutique parts to get that sound.

I got mine fro free in a console with a Garrard changer, a Leak Troughline tuner and Point One preamp. My parents neighbour died and their kids were going move it to the dump. It still had the original tubes and was playable. If I remember correctly, the tag inside the console was dated 1962. I got all the brochures with it as well.

 

[gadget73]

my usual contrary stance, as a big fan of power supplies with very low sag and low ripple, I'd be real tempted to leave this in place if its performing correctly. The best 6bq5 amps I've had have also been the ones that had the stiffest power supplies. The mushier the supply, the mushier the bass tends to be.

 

[kyle]

Unless you are planning on keeping the amp for a longer period or know people that will buy it modified it's not always a good idea to make big changes.
The Leak is a pretty good amp as is and modifying it changes it from a $1000 amp to a $500 amp.

 

[Jock Strap]

I have a Stereo20 and can say that at max current absorption the drop at HT is negligible.
Probably that Stereo20 has silicon rectifiers instead GZ34, so current availability on 5 volt windings remains high enough for the job.
Sincerely the mod is not so attractive, since makes a venerable old amp a sort of Frankenstein, with sound performance, surely dry and less sweet than original.
This amp has fame to be one of the best Leak ever made...too bad modifying it.
IMHO

Unless you are planning on keeping the amp for a longer period or know people that will buy it modified it's not always a good idea to make big changes.
The Leak is a pretty good amp as is and modifying it changes it from a $1000 amp to a $500 amp.

I am certain that modifying these classics will reduce their value.
I restored a 1963 Stereo20 a few years ago and sold it to a guy in Austria. He enhanced it slightly with some "boutique" caps and better tubes and he said it was the best amp he ever had and he was an 'audiophile'. I bought this one to "restore" rather than repair so that's what I am going to do. I have already stripped out the additions and preparing to plan the restoration. In my last amp I re-stuffed the exposed caps, each with a pair of Panasonic radial caps but this amp only has one original can - maybe I can find one or make one? The Power Tx has been getting too hot but no damage done. The AC Outlets socket is missing and one of the bakelite speaker terminal mounts is broken... hmmm!

 

[Jock Strap]

I am restoring this amp to original spec from 1960 so all the passives have now been removed and I have put together an order for replacements. I like to use Kiwame (2W and 5W) resistors as most values are available and their dimensions are about right for the job. I avoid NOS Carbon resistors as their values are not stable and cooking them in the oven fo 8 hours does not appeal to me, anyhow, not all the values I want are available. I decided to keep the big silver Russian caps as they look OK.
I am unable to understand what the previous owner was doing as all the passives are the wrong values, there are wires cut and shorted all over the place and the additional wiring is messy. The PreAmp Input Socket was not compatible with a Bulgin plug so NOS one is being used to replace it. Two leads on the OP Tx were disconnected?

If anyone can see anything that makes sense to them then please let me know. I will publish the parts list next time.

 

[gadget73]

converted from ultralinear to pentode or triode operation perhaps? Have to see where the screens were tied. Possibly thats what the regulator was actually feeding.

 

[Chops54]

There used to be a very active Leak forum a few years ago. I don’t know what happened to it after Yahoo was hacked but there were several guys there that would know what was done to this Stereo 20. I do remember seeing a 20 modified like yours Jock. Might even be the same one

 

[kyle]

The schematic and layout diagram are pretty easy to follow:

 

[Jock Strap]

The schematic and layout diagram are pretty easy to follow:

View attachment 2732044 View attachment 2732045

I have these diagrams and used them a few years ago on my last S20 restoration but as I bought the amp to "restore" I could not be bothered with tracing through the circuit to try and document the results of the conversion.

 

[Catmanboo]

Interesting thread on the Leak 20. I've one for years, project in the works. Finally tested the rectifier tube recently, it's stone dead but lights up. No wonder it barely squeaked. Found 2 boxed mullard & amperex nos in the stashes, I replicated bad can caps underneath & kept cans for looks. I'm at a bit of a loss though at exactly the back panel is supposed to look like, as the funky proprietary brit oddball connectors are long gone.

 

[Jock Strap]

I replicated bad can caps underneath & kept cans for looks. I'm at a bit of a loss though at exactly the back panel is supposed to look like, as the funky proprietary brit oddball connectors are long gone.

Here is a picture of one I restored a few years ago. The Switch Cable hole is often used to insert a switch where the preamp has no power outlet, like the Varislopes do. The Power Inlet and Outlets are "dangerous" so they should be disconnected. However, I have kept the Inlet and I make sure the plug lead is in perfect condition and the plug cap cannot be unscrewed... the cap can be unscrewed exposing the live terminals! I think a 'Schurter C14 Panel Mount IEC Connector Male, 15A, 250 V' might just fit in the Power Outlets hole but I am still working on that idea?
I do not like to change the appearance of the amp on the outside so restuffing the caps is my preference. Can't help changing the appearance from below as all of the passive components need to be replaced to make it technically "good": the caps will be "bad" after 60 years and the carbon resistors will be "off" their original values as they absorb moisture over time (same problem with NOS resistors so they are not a good replacement, either).

 

[Jock Strap]

Catmanboo, take a look at another similar thread of restoration/repair/renovaton of Leak Stereo 20...
Leak Stero 20 Renovation - component values

 

[Catmanboo]

Here is a picture of one I restored a few years ago. The Switch Cable hole is often used to insert a switch where the preamp has no power outlet, like the Varislopes do. The Power Inlet and Outlets are "dangerous" so they should be disconnected. However, I have kept the Inlet and I make sure the plug lead is in perfect condition and the plug cap cannot be unscrewed... the cap can be unscrewed exposing the live terminals! I think a 'Schurter C14 Panel Mount IEC Connector Male, 15A, 250 V' might just fit in the Power Outlets hole but I am still working on that idea?
I do not like to change the appearance of the amp on the outside so restuffing the caps is my preference. Can't help changing the appearance from below as all of the passive components need to be replaced to make it technically "good": the caps will be "bad" after 60 years and the carbon resistors will be "off" their original values as they absorb moisture over time (same problem with NOS resistors so they are not a good replacement, either).

Where I'm at. Project was sidelined a few years back due to lack of a serviceable rectifier tube. I recently found 2 n.o.s. boxed GZ34s in one of my tube stashes, so here we go! Power receptacle was a drop-in standard U.S. period-correct unit common to most gear of the day, courtesy of my parts stash. Unstuffed the shorted C1, replicated at its connection points on the board. The newly-emptied can looked like a good place for a power switch in lieu of the grommeted switch cable hole in back. I'm holding in hand what was found hanging out the back of this thing, i believe was external power source for the tuner or preamp? Since I dug this out for pics, might as well reinstall tubes with a new rectifier & variable it . Stay tuned...

 

[Jock Strap]

I'm holding in hand what was found hanging out the back of this thing, i believe was external power source for the tuner or preamp?

That cable is the Switch Cable. The Varislope Preamp has an on/off knob that makes and breaks accross the Switch Cable. Not a power supply cable!

 

[Catmanboo]

Today, I sorted through my tube stashes for good ones to wake this thing up. Variac/DBTed it over many hours. Got sound/ noise! Distorted audio no doubt all the original caps & resistors aside from what I replaced decades ago. But- nothing overheating but r39, 100 ohm 1 watt wirewound , already have a replacement much larger watt wirewound to relocate away from replacement caps i subbed for C1 right on the board. No red plates, no smoke. I'll call that a big win & plan the next step, check & replace resistors that need it & start rounding up replacement caps.