advertisement


Mike's Stereo 20 refurbishment thread.

Tony,
R5, the anode load resistor, in the later (8778 transformer) models is 47k not 100k as described previously. See 2nd schematic.
Then later still that 47k was shunted by a 4.7k and 300pF capacitor (the bizarre thing) in series. That is another aid to stability. Ref. Jez's 'kettle of fish' remark in post #56...

Jez,
I was quoting the remarks in ''Firsts in Hi Fi'' page 143 about the purpose of C1 used in the majority of Leak's amplifiers. I know there is some ongoing debate about it...

Alan
 
snowman_al said:
Tony,
R5, the anode load resistor, in the later (8778 transformer) models is 47k not 100k as described previously. See 2nd schematic.
Then later still that 47k was shunted by a 4.7k and 300pF capacitor (the bizarre thing) in series. That is another aid to stability. Ref. Jez's 'kettle of fish' remark in post #56...

Jez,
I was quoting the remarks in ''Firsts in Hi Fi'' page 143 about the purpose of C1 used in the majority of Leak's amplifiers. I know there is some ongoing debate about it...

Alan
Click to expand...

Very unlikely that it's to do with RF interference and much more likely that it's to help maximise the NFB at audio frequencies whilst rolling it off above the audio range fast enough to ensure stability. The ECC83's (and way they are used) will be crap at RF amplification!

The NFB values and step network are a compromise on these as the NFB is taken from the 16R tap and not directly from each tap, switched with impedance selection. The Radford STA25 for example switches in different compensation caps and NFB resistors for 4, 8 and 16R loads.
On my own St20 I've optimised the anode resistor value for min distortion (Leak mis-calculated it) and taken the NFB directly from the 4R tap (mines set for 4R only use) then changed the NFB resistor and the compensation caps plus the step network values to optimise it for 4R in step response, settling time and min THD. Pity Leak didn't fit a switch and do this in the first place!
Must get round to redoing it for 8R now that my beloved KEF R105.3's (4R) have had to be pensioned off and replaced by 8R Spendors... Gonna need a bulk order of them round tuit's!
 
Interesting stuff. So this was another very distinct change and should really be viewed as V3 of the circuit. I just had a look at an old pic of mine before my rebuild where it looks to have a full set of original resistors (that measured way off spec when we pulled them) and it just had the single 47k there. I’m happy to have it rebuilt to its original spec so have no desire to emulate the later mod, though those with grey ones may well be tempted. I also use it on the 16R tap (JR149s) so I guess the NFB is pretty optimal there, not that I understand such things yet!
 
Today's activity has been ordering all the parts.

The components I've chosen are pretty much the same as on Tony's rebuilt bronze example, i.e. K40Y-9 PIO caps and Takman carbon film resistors and F&T for the two big electrolytics.

The two remaining things to sort are:

1) I'm still struggling with the paint. There are too many different metallic VW colours with names similar to 'Sand' and without the paint code I'm not confident which to order. A search on the Halfords website for the colour Tony used proven futile. I think I'll take the amp to an automotive paint supplier and get them to colour match it if they can't positively identify which colour Tony used.

2) I'd like to fit an On/Off rocker switch but I don't want to have to drill the hole out to a larger diameter in order to mount one if at all possible. I'm still looking for a switch that'll drop straight in without drilling the chassis.
 
Odd about the paint, do Halfords no longer do it? Be warned that the F&T caps have an indentation on the sides, so if you are as OCD as me you’ll need to use a car body filler to get the right look! It’s actually very easy and only added a few minutes plus a bit of drying time to the process.

Be very wary of little switches like this one:

18548368800_9500997008_b.jpg


This was actually my ‘mystery crackling noise’ and the reason I rebuilt the whole amp (which I’m very pleased I did as it is way better now!). The issue was this under-rated switch randomly arcing as can be seen from the burnt bit! If you can get proper 3Amp 250V ones in that size it fits ok, but that one was not of the correct rating. I’d also fit a spark-suppressor cap as you get a ‘crack’ through the speakers when turning off otherwise. This is detailed somewhere on my thread.

The switches Classique Sounds fit look like ones off old guitar amps etc, so pretty period-correct, but I’ve no idea if they drill the hole out or not. I’m perfectly happy without a switch as I always turn everything on and off from an individually switched MusicWorks mains block anyway. Saves wear on the equipment front panel etc.
 
Tony L said:
Odd about the paint, do Halfords no longer do it? Be warned that the F&T caps have an indentation on the sides, so if you are as OCD as me you’ll need to use a car body filler to get the right look! It’s actually very easy and only added a few minutes plus a bit of drying time to the process.

Be very wary of little switches like this one:

18548368800_9500997008_b.jpg


This was actually my ‘mystery crackling noise’ and the reason I rebuilt the whole amp (which I’m very pleased I did as it is way better now!). The issue was this under-rated switch randomly arcing as can be seen from the burnt bit! If you can get proper 3Amp 250V ones in that size it fits ok, but that one was not of the correct rating. I’d also fit a spark-suppressor cap as you get a ‘crack’ through the speakers when turning off otherwise. This is detailed somewhere on my thread.

The switches Classique Sounds fit look like ones off old guitar amps etc, so pretty period-correct, but I’ve no idea if they drill the hole out or not. I’m perfectly happy without a switch as I always turn everything on and off from an individually switched MusicWorks mains block anyway. Saves wear on the equipment front panel etc.
Click to expand...

I've already ordered a spark suppressor cap, a different manufacturer but the same spec as the one you used.

After some more Google research I've come to the conclusion that the paint I need is Volkswagen code LA1V which is indeed called 'Sand Metallic'. That's what I'm going to order and I'll report back about the colour match. It's a colour that was used on the Mk1/2 Golf etc.
 
The replacement Bulgin mains plug arrived today.
P1110118 by Michael Pickwell, on Flickr

I've also been stripping out the small electrolytics and the 1W resistors from the PCB.

Suffice to say some of the electrolytics were past their best! This one is suppose to be 50uF.
P1110115 by Michael Pickwell, on Flickr

As expected the carbon comp resistors have all drifted high to varying degrees.
 
Mike P said:
The replacement Bulgin mains plug arrived today.
P1110118 by Michael Pickwell, on Flickr

I've also been stripping out the small electrolytics and the 1W resistors from the PCB.

Suffice to say some of the electrolytics were past their best! This one is suppose to be 50uF.
P1110115 by Michael Pickwell, on Flickr

As expected the carbon comp resistors have all drifted high to varying degrees.
Click to expand...

Now that's a knackered electrolytic! As a matter if interest it would be interesting to see what it measured after a few hours with 12V across it...
 
Indeed, the resistor positioning around the tube bases makes me look decidedly sloppy! These amps deserve the neatest work as Leak’s original build quality was so immaculate. Some rebuilds are just disgusting IMO, just heresy!
 
Absolutely so pleasing to see this sort of work done on a worthy patient!

I wish you years of pleasure from it when finished.

Best wishes from George
 
Thanks for the kind comments folks :)

My F&T big electros just arrived in the post but I'm still waiting for the paint to be delivered.
 
Mike,
Going very nicely there.
If I may make a couple of comments?
First, turn the 100 ohm resistor through 45 to 90 degrees away from the 47uF capacitor. It will reduce the heat transfer to the cap and save the paxolin scorching so much in future.
Second, it looks as if you have fitted the suppressor across the mains input? It really should go across the transformer primary (or switch contacts) if you are going to use the 'link' wire to switch the amp on/off.

Alan
 
snowman_al said:
Mike,
Going very nicely there.
If I may make a couple of comments?
First, turn the 100 ohm resistor through 45 to 90 degrees away from the 47uF capacitor. It will reduce the heat transfer to the cap and save the paxolin scorching so much in future.
Second, it looks as if you have fitted the suppressor across the mains input? It really should go across the transformer primary (or switch contacts) if you are going to use the 'link' wire to switch the amp on/off.

Alan
Click to expand...

Thanks Alan, I've just turned the 100R wirewound away from the axial electro 45 degrees, I feel that's the best compromise between it being away from both the board and the capacitor.

Look how close it was fitted by the factory!
P1110096 by Michael Pickwell, on Flickr

Regarding the suppressor, TBH I was just copying Tony's.
 
Worth noting that your magic green resistor hasn’t got cut board eyelets, so couldn’t drop even if it melted the solder. My Grey S20 is just the same, though my bronze one does have the tops cut off the tags.

I just stuck the suppressor where someone advised on my thread. It certainly works (popped loudly on switch-off before, now doesn’t), though I turn the amp on/off at the mains and have no intention of fitting a switch.
 
Mike P said:
Regarding the suppressor, TBH I was just copying Tony's.
Click to expand...
The position Tony's suppressor is fitted is fine as Tony switches on/off at the mains. He did away with the switch he had in post #65 and has a piece of wire in the 'Switch' position.
If you do fit a switch as you mentioned in post #64, then you need to move the suppressor to be across the transformer primary or across the switch. Otherwise it will be 'out of circuit' and in-effective as you switch off...
 
Tony L said:
Worth noting that your magic green resistor hasn’t got cut board eyelets, so couldn’t drop even if it melted the solder. My Grey S20 is just the same, though my bronze one does have the tops cut off the tags.
Click to expand...

I've already cut a 'V' notch into the tags on mine to allow it to drop out. The pic a few posts up was taken before I cut the notches.
 
You must log in or register to reply here.


advertisement


Back
Top