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Leak Stereo 20

Has anything else in the system changed? Leaks can be very sensitive to hum/earth loops etc upstream. Might be worth unplugging everything from the passive preamp and seeing if anything changes.
 
I tried it with no sources connected, the hum is still there, also tried a different interconnect & swapping the V1 valve between my Mullard & Electro Harmonix, made no difference.
The hum stays at the same level regardless of volume position.
TS
 
Very strange. I assume you are back with the JR149s now, so with your normal reference point for efficiency etc? I'm sure Alan will have a far better idea.
 
Tony S,
Those are good voltages now. Very near Leaks figures.
I assume the slight hum is equal from both channels?
I see you have removed the inputs and it is still there, but really you need to short the inputs to ground to test it properly.
That is a classy rebuild, I don't see much to 'deteriorate' apart from the valves now. You have nothing to loose by replacing the GZ34 and doing the diode mod.

My experience of the ST20 is once restored, there is very little hum or noise. You need to be in closer than a foot to the speakers to notice. That is with the inputs shorted and fairly sensitive speakers (Dynaco A25s).
As Tony L mentions they are sensitive to lead positioning and placement though. One thing I would do is make sure the chassis is earthed properly, mains socket through to the earth tag on the big cap, just in case.
 
Is increasing hum a ‘tell’ a rectifier is on the way out? Annoyingly my little valve tester can’t do them (a real oversight for a device designed for the guitar amp market!), so knowing what to look/listen for would be useful.

I have a very old (1960 IIRC) ‘fat-base’ Mullard that I suspect is the one that originally came with my Stereo 20. It looks a bit tatty, little surviving label print, slightly patchy getter silvering (it is a dual getter). I’ve always assumed it was rather worn, though I can’t recall any increased hum at all.

PS Apparently Mullard ‘improved’ the GZ34 across its production run with each type change (top line of the etched code e.g. F32) having less voltage drop than the preceding. Collectors seem to far prefer the early ones though, very old metal base ones being worth £300+ NOS. I’ve got the aforementioned ‘60 fat-base, a mid-60s four-notch, and three late-60s seven notch (one Tungsram branded). All work, but it is annoying I can’t test them.
 
Will check the chassis earthing & have a play around with cable dressing, see if that makes any difference, the hum is equal through both channels.
There are a few Mullard GZ34s & alternatively branded Blackburn GZ34s currently on fleabay for around £100 which isn't too financially crippling, would something like one of those be suitable?
My instinct would be to try out a new one first, presumably that would cut the risk of accidentally buying a dud one. Would like to make sure I've heard the amp with an absolutely known good GZ34 before proceeding further.
The Watford valves Sovtec GZ34 looks to be good value at £17.50+VAT & seems well liked.
TS
 
Tony L said:
Is increasing hum a ‘tell’ a rectifier is on the way out? Annoyingly my little valve tester can’t do them (a real oversight for a device designed for the guitar amp market!), so knowing what to look/listen for would be useful.
Click to expand...

Not normally, if both halves age at the same rate the rectifier is still balanced, and the voltage drop increases over time. But if one side of the rectifier is significantly different then the voltage output will be lower for one 1/2 cycle and higher for the other. That imposes a ripple on the HT and is audible. (I have had this a couple of times where one side is at 90% and the other 60%...)

Tony L said:
II have a very old (1960 IIRC) ‘fat-base’ Mullard that I suspect is the one that originally came with my Stereo 20. It looks a bit tatty, little surviving label print, slightly patchy getter silvering (it is a dual getter). I’ve always assumed it was rather worn, though I can’t recall any increased hum at all.
Click to expand...

Easy enough to test one GZ34 against another using the amplifier. Measure the voltage at the cathode for each in turn. If you have a new one that is the reference. Has to be said, the Stereo 20 is very easy on the rectifier and I guess many are still using the original one that came with the amplifier.

Tony L said:
PS Apparently Mullard ‘improved’ the GZ34 across its production run with each type change (top line of the etched code e.g. F32) having less voltage drop than the preceding. Collectors seem to far prefer the early ones though, very old metal base ones being worth £300+ NOS. I’ve got the aforementioned ‘60 fat-base, a mid-60s four-notch, and three late-60s seven notch (one Tungsram branded). All work, but it is annoying I can’t test them.
Click to expand...

I believe that is the case. I think some of the kudos is the rarity or matching the date for a specific amplifier...

One thing we do not know about Tony S's hum is is it 50 or 100Hz?
 
TonyScarlett said:
Will check the chassis earthing & have a play around with cable dressing, see if that makes any difference, the hum is equal through both channels.
There are a few Mullard GZ34s & alternatively branded Blackburn GZ34s currently on fleabay for around £100 which isn't too financially crippling, would something like one of those be suitable?
My instinct would be to try out a new one first, presumably that would cut the risk of accidentally buying a dud one. Would like to make sure I've heard the amp with an absolutely known good GZ34 before proceeding further.
The Watford valves Sovtec GZ34 looks to be good value at £17.50+VAT & seems well liked.
TS
Click to expand...

I wouldn't invest in another Mullard until I was 100% sure the original was at fault. It is likely fine.
A Sovtec and a couple of UF4007s will prove it for £20ish. And will let you keep the Mullard in 'retirement' if you want.

There are 50 and 100Hz test tones on You Tube (and variously on the net) listen and see which is a match. Not as easy as it seams...
 
snowman_al said:
I wouldn't invest in another Mullard until I was 100% sure the original was at fault. It is likely fine.
Click to expand...

Personally I would just to have a spare. No matter how crazy this year’s prices are one can guarantee next year’s will be higher! I decided to stockpile valves of the types I use a good few years ago now and I’ve seen the prices of GZ34s double in that time (e.g. I paid £70 each for the two boxed NOS ‘seven notch’ Mullards currently in the TL12 Plus amps, they’d be £150+ now). I plan to keep Leak amps of this type long-term and I’d very much prefer not to be hunting for NOS vintage valves in another 10-15 years time!
 
Just popped down to Halfords to pick up the can of Decuma grey paint I ordered for the caps. In reality the caps don't actually look to incongruous as is. They actually match the black sheathed QED interconnect I'm using quite nicely.
20210515-121458.jpg


I've dressed the cables so that the speaker, interconnect & mains cable are as far from each other as possible. Also checked the earthing, the continuity between the mains earth & earth tags on the caps is fine.
TS
 
Might be worth having a look for any shorts between the resistors that connect the circuit board to the valve sockets. Some of the bent leads are very close to pins and the central earth spigots on the bases. Might also be worth checking the earthing around the input sockets, and that earth connection to the circuit board pin on that nearest corner of the board.
 
John_73 said:
Might be worth having a look for any shorts between the resistors that connect the circuit board to the valve sockets. Some of the bent leads are very close to pins and the central earth spigots on the bases. Might also be worth checking the earthing around the input sockets, and that earth connection to the circuit board pin on that nearest corner of the board.
Click to expand...
I'll take a look at those resistors & earth connections. I've ordered the Sovtec GZ34 & 1N4007 diodes so should have those fitted by next weekend.
Once again thanks for all the help guys.
TS
 
Diode mod completed, Sovtec GZ34 fitted, hum...Still there. Time to talk to Graeme at AmoRegen I think.
At least I now know there's nothing wrong with the Mullard GZ34.
TS
 
My vintage amp love continues.

I've just bought a ST20 and a Varioscope 1 preamp. Neither have been turned on since 1974, but stored in a warm dry house.

There's some fantastic reading and advice in this thread. Really looking forward to bringing these to life again.

Looking forward
 
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