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Radford STA15 Restoration.

Bad Parameter said:
Are these active production resistors? If so what type or Mouser, Digikey, Farnell etc link? They look nice in vintage gear. Very nice work, though for myself I'd have gone for a do no harm shabby chic restoration as mention in post #9.

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The resistors are the original ones I stripped from the old runners and sockets.
They are German made by Rosenthal (they have an excellent reputation and were used in Radford and Beam Echo amplifiers etc.) who were better known as a porcelain manufacturer. https://en.wikipedia.org/wiki/Rosenthal_(company).
They used the crossed sword logo a lot before going to the colour band identification maybe in the mid 60s? Picture from an earlier Radford.



I would have liked the shabby chic look, see my other restos..., but this was just too rusty and scabby to respond to a gentle clean and rub down. There was no undercoat originally so the rust creeps every where and once 'cured' and rubbed off I had no choice but to repaint it. It sort of chose its own route.

I think you are correct with the above post, with so many USA amp makes all with more power and many cheaper, why would anyone choose a Radford in the States? Though they do exist, most I guess are still n the UK and parts of the old Empire...
 
Well she is alive, well and in stereo.
Yesterday evening finished off channel 1 and quickly checked all was well, popped in a new GZ34 and then went for its first full stereo test...


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Very nice until there is a quiet gap between tracks and I hear a very slight fizz and pop from the channel 1 speaker? Not very loud, but just enough to be heard from a foot or so from the speaker. Begin to wish I had replaced the anode resistors for the EF86s. I know it is not a poor connection on the socket...
So I pull out the EF86 and the popping is still there, ummm. Now the ECF82 I used is new out of the box so I find an old one and put that in and the popping and fizzing has stopped. Bu**er, a noisy NOS ECF82, but at least I do not have to take the board(s) out again.
Let her play for an hour or so and rechecked the voltages. Even the original Mullard EL34s cathode voltages are within half a volt of each other, result.

She's been on from 6.00 to 8.00am today and all is well.
The mains transformer is very quiet (for a Radford) even when hot, I cannot hear it from my listening position. You have to be closer than a couple of feet to start to notice a slight hum.

So I've put the original badges in a polly bag and then tucked it into the wiring for future reference. And screwed the bottom cover back on to make it easier to place on the unit.



Whole sequence pictures so far here:
https://photos.app.goo.gl/VpUCFZjwhYME1N6K8

Time to start listening...
 
Fabulous job there Al! Love these restorations. Beautiful work Sir. How’s she sound compared to your Stereo 20?
 
Thanks for the comments everyone.
I've had little time this week, but have got the cage on and had a few hours listening. I've put some Winged 'C' EL34s in as I will probably store the original Mullards away for special days.




Here are the parts replaced. Some of the high voltage electrolytics actually measure okay, but as you need to get the boards out to replace them, I decided to go for a complete replacement. The impedance switches were very stiff and were suffering from the green wire problem so they got replaced.




And for fun got the SC22, that came with the amp, going. It actually sounds better than I had expected/remembered.



As it needs a good tidy up and I already have a very good example I've put it on ebay...
( https://www.ebay.co.uk/itm/Radford-...d=link&campid=5338728743&toolid=20001&mkevt=1 )

Alan
 
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Amazing result Alan! that will be electrically good for decades to come and bring many many hours of listening pleasure I have no doubt! It also looks as good or better than new :)

My amp somehow has a bottom cover that does not align well, so I decided to have new one laser cut with extra slots cut for airflow over the EL34 cathode resistors in particular.

20210720_161419[1] by Jonathan Chiles, on Flickr

Re the hookup wire I didnt actually know what was original as my amp already had been redone with Woodside boards and some horrible van den hul stranded wire which was way too thick and also poorly soldered. The stuff I was actually after was solid core of around 20AWG which is 0.81mm dia or 0.032in dia. This I think is close to the wire that I always find in Leak amps and it always seems to be the right thing to use as it stays put so well. (The only downside on the Leak wire is always the crappy PVC insulation shrink-back)

Also I am interested in getting the best out of the ECF82 for my amp and had also read that "matching the triode and pentode" would result in the lowest distortion. Assuming this can be measured and in some cases heard? Maybe Arkless can comment.. but I have eventually settled on some made in Canada Mullards that both give specs very close to the Av manual for Ia and gm. Not sure how one matches the triode and pentode but assume this cannot be done just using a valve tester?
 
Maybe consider some US-made 6U8A replacements for the ECF82? They can still be found for a fair price NOS, including some JAN types.
 
Raoul Duke said:
(The only downside on the Leak wire is always the crappy PVC insulation shrink-back)
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The thing that amazes me with the Leak wire is it is still serviceable and hasn’t turned to sticky goo the way wire in so many similar vintage guitar amps and radios has by now! It melts irritatingly easily, but at least you don’t need to remake the looms etc! It would be good to find some stock of it.
 
Raoul Duke said:
Amazing result Alan! that will be electrically good for decades to come and bring many many hours of listening pleasure I have no doubt! It also looks as good or better than new :)

My amp somehow has a bottom cover that does not align well, so I decided to have new one laser cut with extra slots cut for airflow over the EL34 cathode resistors in particular.

20210720_161419[1] by Jonathan Chiles, on Flickr

Re the hookup wire I didnt actually know what was original as my amp already had been redone with Woodside boards and some horrible van den hul stranded wire which was way too thick and also poorly soldered. The stuff I was actually after was solid core of around 20AWG which is 0.81mm dia or 0.032in dia. This I think is close to the wire that I always find in Leak amps and it always seems to be the right thing to use as it stays put so well. (The only downside on the Leak wire is always the crappy PVC insulation shrink-back)

Also I am interested in getting the best out of the ECF82 for my amp and had also read that "matching the triode and pentode" would result in the lowest distortion. Assuming this can be measured and in some cases heard? Maybe Arkless can comment.. but I have eventually settled on some made in Canada Mullards that both give specs very close to the Av manual for Ia and gm. Not sure how one matches the triode and pentode but assume this cannot be done just using a valve tester?
Click to expand...

Yank said:
Maybe consider some US-made 6U8A replacements for the ECF82? They can still be found for a fair price NOS, including some JAN types.
Click to expand...

I had best results from USA version 6U8A but I'm pretty sure that was simply because the ECF82's were part worn and the 6U8A's new.
Unfortunately you need a full distortion measuring set up to measure the distortion. I found THD could vary from 0.03% to 0.4% from just changing the phase splitter valve! (the 0.4% ECF82 was likely from an old TV set and well worn).
I'm an analogue guy and have all the required traditional test gear but I guess a suitable sound card or USB audio interface could be used in conjunction with suitable software to do the same thing.
 
Raoul Duke said:
Damn gorgeous amplifiers, what a collection! My label is knackered but reads s/n B0045, so early?
Mine is running every evening now and extra happy as I found some XF2 Mullard EL34 to stick in there.
20210726_100335[1] by Jonathan Chiles, on Flickr
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Raoul, each to his own of course, but I don't think I'd be comfortable with that shallow a gap below the next shelf. Have you felt the underside when the Radford has been on for a while?
 
Marchbanks said:
Raoul, each to his own of course, but I don't think I'd be comfortable with that shallow a gap below the next shelf. Have you felt the underside when the Radford has been on for a while?
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Hey Marchbanks, you are correct of course. That shelf gets quite toasty indeed, mainly above the mains transformer, but no scorch marks yet after a week of evenings like this.. keeping an eye though! This shelf spacing was designed for Stereo 20.
 
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