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Old 1st Feb 2023, 11:32 am   #1
Denis G4DWC
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Default Leak Stereo 20 Refurbishment

I'm in the process of giving some TLC to my Son's Grey painted ST20 mostly just replacing any out of tolerance components and sympathetically adding some HV ceramic fuses to protect the Transformers.

I bought it a Rally about twenty years ago and it's done really good service but it was time to give it the once over. A number of components had been changed by a previous owner and after searching the Web I have two drawings.

There are some slight differences. One looks like an original with the input stage Anode Load (R5) at 100k and the Mains Transformer giving 300 - 0 - 300V.

Another drawing shows the Mains TX as 280 - 0 - 280 and R5 at 47k with a series connected 4k7 and 300pF in parallel with R5 presumably to add an additional pole in the overall response. It's clear that these components have been added on an original drawing as the font is different and there are no component numbers. The amplifier here has these components fitted.

I'm sure there must have been changes over the years as I understand there was a change in Output transformers. The ones in this unit are marked 8778 and the Mains TX 8162

Can anyone tell me if these changes to the input stage Anode load components were official Leak changes or the result of someone tinkering.

Incidentally I had planned to re-stuff the chassis mounted 32+32uF Electrolytics but they're still in really good shape with Capacity correct, ESR at just on 0.5R and leakage at around 220uA at 350V on the Red Reservoir side and around 15uA on the other. The New Nichia High Temp ones I bought to replace them have ESRs of well over an Ohm. I think I'll leave the originals in place for the time being.

Thanks

Denis
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Old 1st Feb 2023, 11:59 am   #2
GrimJosef
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Default Re: Leak Stereo 20 Refurbishment

Your output transformers are the 'later' type, used after a circuit re-design to stop the EL84s going into thermal runaway. The lower HT voltage was also part of that re-design. I have seen other amps with the modified first stage anode load, although that doesn't absolutely mean that Leak did it. There's a re-designed circuit shown in post #3 here https://www.vintage-radio.net/forum/...d.php?t=177747 which has the load resistor dropped to 47k but no HF roll-off bypass (4k7 + 300pF).

The other change you should see is to the EL84 grid leaks. These were originally 1M but were reduced to 470k, which is a major help in keeping the valves out of harm's way.

Cheers,

GJ
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Old 1st Feb 2023, 1:06 pm   #3
Gabe001
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Default Re: Leak Stereo 20 Refurbishment

Upon reducing the grid leak, did they also increase the coupling cap value to keep the LF response the same?
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Old 1st Feb 2023, 1:09 pm   #4
Valvepower
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Default Re: Leak Stereo 20 Refurbishment

Hello,

As far as I know the series 4K7 and 300pF in parallel to the 47K anode load resistor was a Leak modification. It was done very late in the production of the ST20 (and TL12+) to help with stability. These are usually the later Grey units using the Erie type 8, 9 & 10 ceramic resistors.

I normally use the F&T dual can 32+32uF for driver decoupling and the 50+50uF for reservoir/smoothing and filter duties.

Terry
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Old 1st Feb 2023, 1:13 pm   #5
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Default Re: Leak Stereo 20 Refurbishment

Leak kept the coupling capacitor value the same at 0.25uF (0.22uF)

Quote:
Originally Posted by Gabe001 View Post
Upon reducing the grid leak, did they also increase the coupling cap value to keep the LF response the same?
Terry
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Old 1st Feb 2023, 1:17 pm   #6
Gabe001
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Default Re: Leak Stereo 20 Refurbishment

Quote:
Originally Posted by Valvepower View Post
Leak kept the coupling capacitor value the same at 0.25uF (0.22uF)

Quote:
Originally Posted by Gabe001 View Post
Upon reducing the grid leak, did they also increase the coupling cap value to keep the LF response the same?
Terry
Thanks Terry. I suppose it only makes a couple of Hz difference with a cap that size
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Old 1st Feb 2023, 1:17 pm   #7
GrimJosef
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Default Re: Leak Stereo 20 Refurbishment

Despite the grid leak change the coupling capacitors stayed the same, at 0.25uF. The RC time would still be 0.12s which they must have felt was long enough. Maybe it wasn't what was limiting the open-loop LF response. I'm guessing that was probably the output transformer.

Cheers,

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Old 1st Feb 2023, 1:40 pm   #8
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Default Re: Leak Stereo 20 Refurbishment

Hello,

GJ is quite right.

The changing of the 1M to 470K gets explained in the Steve Spicer book.

This modification was affected when EL84 valves were burning up out in the field, and the change of value would have been done when the faulty units came back for repair. I figure to keep incurred costs down – seeing that the 0.25uF would have been a costly part, plus it’s another part to change – they kept with 0.25uF

Terry
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Old 1st Feb 2023, 3:38 pm   #9
Denis G4DWC
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Default Re: Leak Stereo 20 Refurbishment

Thanks GJ for the very prompt reply. I've been out this morning and just got back home.

I was hoping you'd know the answer to this question. The link to the circuit is the same one that I downloaded from another site.

Thanks also for the additional info and yes, you are correct, the EL84 Grid Leaks are 470k. I'll retain the added components across R5.

The other comments from Valve power are also appreciated.

I'll crack on with getting it all back together now.

One other question, did these amplifiers come with a bottom cover plate? If not I'll get some perforated metal and make one.

I'm tempted to not refurbish the Pre Amp and instead just make a simple passive Tone control unit as the Main amp has too much gain already for modern kit.

Thanks

Denis
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Old 1st Feb 2023, 6:05 pm   #10
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Default Re: Leak Stereo 20 Refurbishment

Hello,

As far as I know all (standard) UK specification Stereo 20 amplifiers didn’t have a base, but I think they could be supplied if required.

I think the CSA approved units for the Canadian market had bases as standard. These also had changes to the mains wiring, connectors, and fuses to meet CSA compliance. This may also apply to American units to comply with UL standards which are not dissimilar to CSA requirements - I have to be honest I'm not that sure about the American amplifiers.

Terry.
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Old 1st Feb 2023, 6:45 pm   #11
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Default Re: Leak Stereo 20 Refurbishment

I think Terry's right in that I've never seen a Leak baseplate for a UK Stereo 20. They came as standard with four little kinked metal 'feet' which screwed onto the shorter edges of the chassis. If you turned the feet in then you could stand the amp on them, say on a shelf. If you turned them out then each one had a hole in it through which you could put a screw, say into the wooden base of a cabinet. This way you could move the cabinet without the amp sliding around inside. In either case the feet lifted the chassis up a few millimetres above whatever surface it was standing on. This was enough for air to circulate inside the chassis and help cool the warm resistors (specifically the 100R in the HT supply, which was sometimes configured to melt its solder and drop loose in the event of gross overheating).

Cheers,

GJ
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Old 2nd Feb 2023, 8:04 pm   #12
Denis G4DWC
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Default Re: Leak Stereo 20 Refurbishment

Thanks both for the additional Information. I'll make a perforated bottom cover for it.

Yes, I was aware of the "drop off" 100R WW resistor. The one on this unit has been quite warm and discoloured the paxolin panel a bit. It looks original but there's no way this one will drop off if it gets too hot as the leads are bent to fit into the solder tags.
The plan is to fit some 400mA HV ceramic fuses in series with the Rectifier anodes to protect the HT transformer and maybe a lower current 200mA device in series with each Output transformer centre tap. 200mA fuses are quite fragile and I'm just a little concerned about the rapid heating and cooling in sympathy with Music that may cause annoying premature failure. Won't know until I try it.

Thanks once again for the helpful Info'

Denis
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Old 2nd Feb 2023, 8:50 pm   #13
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Default Re: Leak Stereo 20 Refurbishment

Fuse protection of output stages is notoriously difficult.

In the Stereo 20 each EL84 should draw a nominal 41.5mA. A 200mA fuse will last for ever at 200mA. If we look at, say, a Littelfuse 216 series 200mA fuse (ceramic bodied, quick-blow, 250VAC rated) we find from the datasheet https://www.mouser.co.uk/datasheet/2...pdf-310019.pdf that it will survive 350mA for more than 15 minutes, albeit with quite big tolerances on those numbers.

So if just one of the valves fails then it will need to pass well over 300mA to blow the fuse in a quarter of an hour. That's more than 7 times the valve's nominal operating current. I fear it might burn out the 270R cathode resistor (which will be dissipating 24W with 300mA flowing through it) or explode the cathode bypass capacitor (which will have north of 80V across it) before the fuse goes. I'd worry about the output transformer primary winding and the HT supply series components too.

Of course in the case of a high-current arc in one of the EL84s the fuse might save everything else. But that's not the usual failure mode.

Cheers,

GJ
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Old 2nd Feb 2023, 9:13 pm   #14
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Default Re: Leak Stereo 20 Refurbishment

My two pennorth.
I concur with GJ and Terry, Terry knows my St20 quite well over the years, a later bronze one with the later transformers and as original I can confirm the 47k anode resistor on V1. But no correction network on V1, it being older I guess.
Terry's advice on the F&T Psu Caps is bang on the money, although I used 50 + 50 in the second half also, it probably makes not a jot of difference here as they mainly perform a decoupling function with a degree of extra smoothing from the RC filter made by the dropper resistors.
Now the controversial bit. I dont think your addition of extra fuses, especially one on the CT of the output transformer will make that much difference to overall reliability they seem tough units to me and mines had the odd bit of unintentional abuse over the last nearly 20 years of ownership.
I didn't see any mention of coupling capacitor construction, and you obviously have a good degree of technical knowledge, but for the sake of form and perhaps a less aware reader, I assume you are using decent (not necessarily "Audio grade" whatever that means) film capacitors here rather than potentially unreliable Paper in oil jobs.

Your idea of a passive control unit ticks my personal boxes, I'm not convinced about a passive tone control though and I'd knock out a baxandall type bass and treble control using a couple of standard audio Op amps TL072 5532 etc etc. Plenty of circuits out there. It's not so much about attenuation due to a passive control so much as it presenting a non variable input impedance for me, and I'd use a buffer stage in front. It's a cracking 10W amp that I think still cuts it today and I happily let mine drive a pair of Quad ESL989 which it does admirably and without fuss and to easily enough volume to annoy the station manager downstairs!

As to the original preamplifiers, they are pretty things and I feel well worth preserving in their own right, my personal favourite being the Point One stereo over any of the varislope variants in terms of outright sound quality, but they are a little bit compromised and the phono stage is definitely way off the mark, I don't know if thats important to you but I've had very good results from a couple of relatively simple op amp designs based around the ubiquitous 5532 and also an obsolete Hitachi chip that was supposedly developed way back for feedback based EQ preamps such as cartridges and tape heads etc.
The essence I feel was close attention to the values for the EQ. I had huge help on this from Valvepower of this parish and the result was an astonishingly flat response. But if Terry cant work out the right values, ain't no one can!
Best regards.

Andy
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Old 3rd Feb 2023, 2:28 pm   #15
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Default Re: Leak Stereo 20 Refurbishment

I think secondary fusing of the power transformer is worthwhile. Protects against various failure conditions including runaway tubes ...
Valve Wizard - fusing
I have used inline fuse holders so no added complexity.
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Old 3rd Feb 2023, 7:23 pm   #16
Denis G4DWC
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Default Re: Leak Stereo 20 Refurbishment

Thanks as always for the helpful replies.

I've been giving a bit more thought to separately fusing the HT to the OP Transformer Centre taps and, on balance, decided to give it a miss with conventional fuses. I have a few ideas that are reversible and will doubtless upset the purists by using some modified bits of sand.

It's a job to know where to stop to try and make things safe and reliable for another 40 years!

As regards the Coupling capacitors, I'm able to see through the Audiophoolery and Snake oil benefits of certain capacitors and other components. I've just replaced the existing coupling capacitors with some CDE/Illinois 630V PolyProp types from Mouser. Annoyingly the outer foil isn't marked on these capacitors but was a simple matter to identify and mark each end. Not always the same end as the makers details and Logo!

It's pretty much back together now so will be testing and measuring it next week.

The Existing Leak Pre-Amplifier is used with a separate input switching box for the various inputs and, as there's no need for a Phono input at the moment, a smaller unit is an attractive option. I've built a few small Tone control Pre amps in the past using NE5534s of which I have many. Snag is there are just so many projects in progress most of which are not radio or audio related.

Thanks again for all the replies.

Denis
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Old 4th Feb 2023, 11:18 am   #17
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Default Re: Leak Stereo 20 Refurbishment

Sand-based protection of the output valves ought to be fairly straightforward. Use a quad comparator to check for the (smoothed to DC) cathode voltages going too high, and trip a relay in the mains supply if any of them does.

Cheers,

GJ
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Old 4th Feb 2023, 8:38 pm   #18
Denis G4DWC
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Default Re: Leak Stereo 20 Refurbishment

Thanks for the note GJ. I'd already thought about measuring the Cathode voltages on a New Amplifier build that I've been tinkering with for sometime. All the parts are here it's just time that is in short supply. I have a feeling we discussed it when you dropped in here a few years ago with some shiny KT66s.
Using a small micro-controller makes things simpler than individual comparators and another benefit being that it can record and store which stage caused the problem in the few uS before power is lost completely. On this ST20 I'll attach it to the new base cover.

Thanks as always for the helpful comments they are appreciated.

Denis
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Old 4th Feb 2023, 8:48 pm   #19
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Default Re: Leak Stereo 20 Refurbishment

That might depend on what you mean by 'simpler' . But if you're already comfortable with uC programming then you might be quicker doing it that way than hard-wiring it.

Cheers,

GJ
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