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Mike's Stereo 20 refurbishment thread.

Mike P said:
I wouldn't dream of doing that to mine as it's too nice and original but I rather like the idea of making a factory looking example with colour matched painted a choke on a replica chassis or a chassis that someone has already butchered. It's probably what Harold would have done himself without budget constraints.
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Indeed several more expensive and mainly earlier Leak amps did have chokes. I recall seeing comments here and there over the years to the effect of "pity they cheapened the St20 by missing the choke out"... it doesn't suffer too much for the lack of one I guess:)
 
Jez, you've got me curious about the choke set up you've got in your ST20....

I'm new to valve amps and I'd be grateful if you could please answer a few basic questions for me:

The standard power supply is a C-R-C with 32uF->100R->32uF. Have you substituted the 100R for a choke to make C->L->C?

Would I be right in thinking that you've increased the capacitance on the second C only (C12), so you now have something like: 32uF -> 10H? -> 200uF? That way the GZ34 is only seeing directly the original 32uF but there's 200uF on the other side of the choke?
 
Mike P said:
Jez, you've got me curious about the choke set up you've got in your ST20....

I'm new to valve amps and I'd be grateful if you could please answer a few basic questions for me:

The standard power supply is a C-R-C with 32uF->100R->32uF. Have you substituted the 100R for a choke to make C->L->C?

Would I be right in thinking that you've increased the capacitance on the second C only (C12), so you now have something like: 32uF -> 10H? -> 200uF? That way the GZ34 is only seeing directly the original 32uF but there's 200uF on the other side of the choke?
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Spot on yes. The DC resistance of the choke is lower than the 100R of the resistor so I've left it in but with another resistor in parallel to get the HT correct and it is in line with the choke. Now using 440uF and have another 520uF of paralleled caps to add as an experiment... probably no more improvement I'm guessing.
 
Arkless Electronics said:
Spot on yes. The DC resistance of the choke is lower than the 100R of the resistor so I've left it in but with another resistor in parallel to get the HT correct and it is in line with the choke. Now using 440uF and have another 520uF of paralleled caps to add as an experiment... probably no more improvement I'm guessing.
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Thanks Jez.

Going back to the standard setup; given that the GZ34 only sees C13 directly could the capacitance at C12 be increased a little without causing any issues?

I'm thinking that I could add a small modern radial electro as a bypass across the terminals at C12. It would only cost a couple of quid to try and could be removed without trace if I wanted to return back to standard spec.
 
Mike P said:
Thanks Jez.

Going back to the standard setup; given that the GZ34 only sees C13 directly could the capacitance at C12 be increased a little without causing any issues?

I'm thinking that I could add a small modern radial electro as a bypass across the terminals at C12. It would only cost a couple of quid to try and could be removed without trace if I wanted to return back to standard spec.
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You should get away with an extra 33uF no bother. I wouldn't bother with say 1 or 4.7uF polyprop or anything like that as a bypass... you need quantity over quality here! You can bung as much as you like across C10, 11 if you want to try that but 100's+ of uF is where it's at here.
 
Oh for anyone wanting theoretical perfection.... the 91K anode resistor in the phase splitter sims as being optimum at 92.5K... might be worth 0.02% off the THD...
I've designed a FET cascode current source to replace the tail resistor and give perfect symmetry to the phase splitter but haven't tried it in practice.
 
Arkless Electronics said:
You should get away with an extra 33uF no bother. I wouldn't bother with say 1 or 4.7uF polyprop or anything like that as a bypass... you need quantity over quality here! You can bung as much as you like across C10, 11 if you want to try that but 100's+ of uF is where it's at here.
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Great, I'm going to try upping the capacitance on C12 a bit then.
 
Mike P said:
Interesting....
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Remember the TL12+ is the same mains transformer and GZ34 doing half as much work!

I do want to find another immaculate non-botched champagne coloured one to pair mine with as I’d love to hear how it compared to the Stereo 20. It should be rather good from a PSU perspective, though it is a different input and phase splitter (EF86/ECC81 vs. 2x ECC83s).
 
An experiment that beckons but I haven't got the time or inclination ATM would be to rebuild the phase splitter as the Radford circuit... pentode at input, 6U8.
 
I've just finished adding the extra capacitance to C12 as discussed above.

I used a 33uF 450v Nichicon LD series (RS part number 8454613) and simply soldered it to the terminals of the big electrolytic.

P1110155 by Michael Pickwell, on Flickr

It cost a grand total of about £2 to try and can be removed in moments without a trace.

Initial listening impressions are all positive.
 
Also re the TL12+ I believe the first capacitor before the rectifier is 60uF stock from factory, so if the GZ34 is able to handle that it seems the C13 in Stereo 20 could be upped to 47 or 50uF as well without harm?
 
Raoul Duke said:
Also re the TL12+ I believe the first capacitor before the rectifier is 60uF stock from factory, so if the GZ34 is able to handle that it seems the C13 in Stereo 20 could be upped to 47 or 50uF as well without harm?
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It could but there is not much to be gained from this. As the C12 position cap in the TL12+ is 200uF it should be no problem to make it around this in the St20. Nearest common value today is 220uF and with the extant 33uF this makes 253 which should still be OK. Electrolytic cap tolerance is so wide that many of the 200uF caps in TL12+ will no doubt be anything up to 300 or more in practice anyway!
 
Here is a GZ34 data sheet (link). Given good ones are typically well over £100 these days I’d not personally want to run them without some decent headroom available. I know they last very well in a stock S20. Would this sort of mod have any impact on the mains transformer running temperature? That’s another area I’d not want to impact at all.
 
Tony L said:
Here is a GZ34 data sheet (link). Given good ones are typically well over £100 these days I’d not personally want to run them without some decent headroom available. I know they last very well in a stock S20. Would this sort of mod have any impact on the mains transformer running temperature? That’s another area I’d not want to impact at all.
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The ONLY part under any extra stress is the GZ34 and then only for about 5-10 seconds whilst it charges the capacitance as it warms up. The 34 is specced for 60uF (IIRC) max as the first capacitor it see's, the extra here comes after the 100R resistor which limits the current. If 200uF was used with just the 100R and no choke in the TL12+ then IMO there is nothing to worry about. As I said earlier the very wide tolerance on the caps means some TL12+'s will have been working with maybe 300uF anyway. A GZ34 is used in some 100W guitar amps.

I'm using nearly 1000uF after the choke in mine with a Edicron GZ34!
 
I'm listening now with the extra cap on C12 and a brand new quad of 6P14P-EV tubes fitted in place of the Mullards.

Wow this thing is sounding good!
 
Mike P said:
I'm listening now with the extra cap on C12 and a brand new quad of 6P14P-EV tubes fitted in place of the Mullards.

Wow this thing is sounding good!
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If yer think that's good try all the other mods! Was listening to mine loud last night, Rhiannon Giddens was "in the room"... anymore real and I'd of had to offer her some of my pizza! Also using 6P14P-EV here, Reflektor made ones. Easy a match for the Mullards.
 
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