Inspired by the thread in 2 Channel Debate, I decided to tastefully restore a pair of Stereo 20s we’ve had sitting around for a while. I thought it might be interesting to some if I documented my progress.
I started on one yesterday. It still needs a bit of cleaning. It’s in a very original condition.
Needs a new fuse holder. Luckily we have a spare Bulgin that is the same.
Completely untouched underneath! Sadly, a lot of the resistors had drifted off value significantly, as carbon composition resistors tend to do after a long time.
Fresh parts. All measure 0% with my multimeter set to ‘foo’. I have seen so many butchered Stereo 20s with ridiculously (physically) huge coupling caps, bypassed electrolytics, bypassed electrolytics with a bypassed bypass capacitor, and more. All manner of silly modifications that are likely to make things worse.
Progress yesterday after an afternoon’s work. I have made some progress today since then. As pictured the EL84 cathode resistors are not installed, and there are a few original components left. The 100R resistor is a 3W wirewound type (it looks a bit like a metal oxide but is WW). This is supposed to desolder itself and fall out in the event of an HT overload. As original, the leads of this resistor sit through the eyelets of the tags, which you’d think would defeat the purpose. The way I have it installed now is as such that it will fall right off in the event of an overload, but there may have been a reason for the original mounting. I will keep this in mind, it’s not like it’s difficult to change it back to original.
I may also (tastefully) fit an HT fuse. I don’t like modification for the sake of it, but I feel it would be worthwhile. This will be reversible. All components have been (and will be) replaced by equivalents. I have not yet gotten around to replacing the main smoothing capacitors, but I have some ready.
Here is a bit of progress from yesterday and today. Some new passives installed. I can’t seem to get a 91K version of these particular resistors. One of the phase splitter anodes needs this value, but luckily 100K in parallel with 1M happens to make nearly dead on 91K.
I have been preparing the capacitors. They were originally black, nothing a can of ‘Nissan Gold’ can’t fix! It’s not a perfect colour match but it’ll be less visually jarring than black ones.
Here’s the measured frequency response into 8 ohms from the amp. The measurement setup currently isn’t level calibrated. I have a measurements jig I made myself with built in attenuators and signal clamping diodes to protect the sound card. Works well enough for FR and distortion plots.
As you can see the channel matching is excellent, to within around 0.1dB, and this was with absolutely no effort to match any valves. Nice smooth HF rolloff too.
What amazed me was lack of high order harmonic distortion with this amp – this plot was taken at just shy of the start of clipping. The 5th and up harmonics are buried in noise. The spikes you see above 50Khz are my fluorescent lights. Things are even cleaner at lower levels. 50Hz sidebands can be seen which is to be expected due to the PSU ripple. This may be an argument for upping the capacitance.
The amp is now basically complete. I still need to get hold of a Bulgin mains plug, so in the meantime I have temporarily soldered in a mains cable. Here’s how the amp now looks
The sprayed capacitors turned out a lot better than I expected!
Underside shot of the amplifier. All passives have now been renewed. Note the placement of the fuse holder where the HT resistor was, and the relocation of said resistor. The resistor is still in the same place in the circuit. Thanks to the way the Leak comes wired, I was able to re-purpose the original wiring, non-destructively, to place a 250mA HT fuse after the CRC smoothing. You end up with a redundant red wire, which I just capped off with heat shrink and tucked away.
Close up of the relocated HT resistor
And the base-mount fuse holder, which sits neatly on top the original tags for the resistor, soldered in place.
Another shot of underneath
Thank you for reading and please comment.
About author of article:
Will is coming from South West in the UK and currently specialising in hifi servicing and sale.
Please check out his webpage at RADFORD REVIVAL – A resource for Radford & Woodside Hi-Fi