Radford STA15 Restoration

[foxwelljsly]

And yes, the pattering noise is my mains ethernet homeplug. Thank you for the pointers. I wonder if there is a way to filter it on the mains input to the amp?

No. Drove me nuts for ages. In the end I just ran ethernet round the outside of the house. Better all round.

 

[chiily]

Maybe I'm a bit deaf...

I bought, as spares, a quad of EL34s RECs from Watford Valves a couple of days ago. Fitted them last night and spent about 3 hours listening to music. And I was quite impressed by how they sounded, really even, nothing harsh or unpleasant, for the price (£14 a valve) you can't say no. I would go as far to say that they weren't that different from the Mullard XF2s I have been using, and some JJs.

Unsure how long they will last, but I'm more and more convinced that the ECF82 and EF86 have much more influence on the sound quality than the power valves.

I found this post which backs up Jez's and Will's point about the ECF82 driving distortion.

It's a lovely amplifier, everyone should have one.

 

[Mike P]

Postman delivered me these today...

2022-06-01_09-10-00 by Garf Arf, on Flickr

I've done a little reading around on ECF82s/6U8A in Radfords and a stumbled across this post from Will at RR and I thought I'd repost it here...

One foible with the STA15/25 is that they are quite sensitive to the choice of 6U8/ECF82. This has been noticed by a few people. Because of the dissimilar sections forming a long tailed pair phase splitter, it's quite possible for an otherwise perfectly good 6U8/ECF82 to bias up "oddly" in these amps, where one side draws too much current and throws the phase splitter off balance. This causes far more even order harmonic residual hence the wildly differing THD readings. One way I've found to quickly check for a "goodun" is to measure the anode voltages at the phase splitter, if they're within around 20v of each other it generally means it'll perform on the lower THD end of things. This is certainly easier if you don't have a distortion analyser at hand!I select for this in new and rebuilt amps of course. Generally a good one will stay balanced in the circuit and won't drift (thankfully!)A Radford with carefully selected valves will often beat the 0.1% distortion spec.

I assume the difference is between these two voltages I've drawn a very crude and particularly non-artistic ring around.

InkedST15 Series III Anodes_LI by Garf Arf, on Flickr

I did wonder what the 15V marking meant, I understand now.

Yesterday I had a play with this on my STA25.

The amp came with a pair of Mullard ECF82 and with these fitted I got the following:

Chan 1 - pin 1 287v, pin 6 287v
Chan 2 - pin 1 298v, pin 6 280v

So channel 1 was looking very well balanced but an 18v difference on channel 2.

I then swapped in a pair of brand new G.E. USA made 6U8

Chan 1 - pin 1 288v, pin 6 283v
Chan 2 - pin 1 278v, pin 6 300v

Then I swapped the valve on channel 2 (only) to another brand new G.E. 6u8 and I got 291v on pin 1 and 294v on pin 6.

I haven't had a chance to give it a listen yet to see f I can tell any difference.

 

[nmtjb]

I've had this sat in a cupboard for a few years now, only powering it up every couple of months, waiting for some time, space (to run ESL57's ie.) and a kids free zone to get it running in a system. I bought it years ago from the Wam, it being the subject of a restoration thread, and using what was decribed at the time as good quality components in lieu of the old failed items.
Seeing all the great, more authentic restorations on here I'm wondering if its worth getting this reworked or just use it as it is? The valves all have very low hours being installed as NOS only months before I purchased it, (or so I was told). It sounds fine to me, low noise floor.

STA15 4 by Neil Tully, on Flickr

STA15 3 by Neil Tully, on Flickr

STA15 2 by Neil Tully, on Flickr

STA15 1 by Neil Tully, on Flickr

 

[chiily]

How does it sound Neil?

It has had quite a few changes. Personally the large caps look out of place and the using non axial caps in axial positions get my OCD going

One great plus is that the wiring loom has been replaced, so green rot has been banished. Edit, I'll take that back, some of the loom has been replaced. I spy some original wiring up by the speaker impedance switches.

And that's quite a collection of different resistors. The original ones rarely wander out of spec, I wonder why they were changed.

It does look neat.

Anyway, how does it sound?

 

[Darmok]

I can't guage how many STA15s or STA25s were built. I assume much less than Quad IIs and Leaks.

Radford STA numbers = 12.000 in total, estimated.

STA 7s, STA12s, STA15s, STA20s, STA25s, STA60s and STA100s

Quad 11 valve power amps production = 100,000 estimated.

Leak Stereo valve power amps production numbers, unknown.

Leak TL valve power amps production numbers, unknown.

Edit: Original 1960s, "Radford Electronics" made in Bristol above production numbers.

It would be nice if "Will" of "Radford Revival" could give us a vague indication of his Radford modern recent production numbers?

 

[Dan K]

I've had this sat in a cupboard for a few years now, only powering it up every couple of months, waiting for some time, space (to run ESL57's ie.) and a kids free zone to get it running in a system. I bought it years ago from the Wam, it being the subject of a restoration thread, and using what was decribed at the time as good quality components in lieu of the old failed items.
Seeing all the great, more authentic restorations on here I'm wondering if its worth getting this reworked or just use it as it is? The valves all have very low hours being installed as NOS only months before I purchased it, (or so I was told). It sounds fine to me, low noise floor.

STA15 4 by Neil Tully, on Flickr

I wonder who did the upgrades? Some odd choices and a lot of VERY expensive resistors

The blue ones are Shinkoh at £10+ a piece (although almost impossible to source), the others are mostly Audio Note Tants again £10 or so each.

Sliver in Ptfe is very bad. Jangle, jangle, jangle

That Silmic 2 as the front end cathode bypass needs to go in the bin, although the soft romantic mush of the Silmic is likely mitigation against the nasty hard jangle from running all the silver wire in teflon

 

[nmtjb]

Thanks for replies. Sounds pretty good into efficient speakers, and decent with the esls, balanced presentation overall, I wouldn’t say wooly or jangling at either end, nice mid range, similar to other vintage valve amps I’ve heard. However, it didn’t persuade me away from my Croft OTLs with the quads, or modern 300bs/ Class A with conventional speakers. Originally bought to put together a system that reflects my age, i.e TD124, Radford, esls. This will happen again now I have more space.

 

[Dan K]

I wouldn’t say wooly or jangling at either end

In my experience this is entirely true:
https://www.decoaudio.com/deco_audio_silver_wire.html

You could likely strip out at least half the mega money resistors plus the red caps and sell them all for £300 or so, it will sound no different

 

[nmtjb]

Interesting article, I like the guys at Deco Audio. Stripping it out probably won't happen in my ownership but useful for when it gets passed on. I suppose in the long term I should get a Radford Revival loner unit for a direct comparison.

 

[Mike P]

If you need any, I have a large stock of new unused vintage Wima 0.47uF of the type used originally in the STA15. You can use new ones of course but somehow they don't look period correct.

I have to say I don't like the look of all those big axial film caps in there. Hot melt glue often fails eventually in my experience.

 

[chiily]

If you need any, I have a large stock of new unused vintage Wima 0.47uF of the type used originally in the STA15. You can use new ones of course but somehow they don't look period correct.

Something more period correct, with a healthy dose of "only change it if it is broken/out of spec/leaking/etc" is for me.

Though someone has taken a great deal of effort and time making the changes and they are done reasonably tidily.

For me, it looses that "Radford" look, but some people may be absolutely content with that and I'm not going to argue against their choice, and because it still sounds great.

Enjoy the amp, change it back if it bothers you.

Long shot, but if you know who did the changes ask them if they still have the removed parts in a bag somewhere.

 

[Darmok]

The Radford STA25/3, Radford SC22P and Radford FMT3 tuner, I bought on behalf of my Brother in Law in 1984 for £175:00.

Twas located in Maidenhead Berkshire, my Sister, BiL and myself travelled to Maidenhead via intercity rail departed from BR Paddington.

Seller demonstrated the Radfords in GWO, driving his pair of Quad ELS63s.

He was alarmed that we were going to return with our acquisitions to East London via public transport, gave us all a lift back to the East End of London in his car, a lovely Gentleman.

The Radford STA25/3 only required the reservoir capacitor replaced as it was beginning to bulge out of the base of the can.

A very nice looking package, seller was first owner since he bought it new in the late 1960s.

A very nice Sunday afternoon trip out of London.

 

[Yank]

STA15 4 by Neil Tully, on Flickr

I see one small capacitor where the two channels have different types - the one on the left has what appears to be the original brown polyester film cap, and the one on the right is a polystyrene. As a small pF cap it must be compensation on the feedback loop.

 

[GT]

Sliver in Ptfe is very bad. Jangle, jangle, jangle

That Silmic 2 as the front end cathode bypass needs to go in the bin, although the soft romantic mush of the Silmic is likely mitigation against the nasty hard jangle from running all the silver wire in teflon

Utter rubbish!!!

Not that this is any of my work, but if you bother to look you will see the PTFE tube has a significantly larger internal diameter than the wire, meaning that the silver wire touches the PTFE tube only in a couple of places, so the wire is effectively in an air dielectric. The Silmics are decoupling the cathode bias resistors so will have a very small influence on the sound produced. If they had been used in the signal path then that would have been a different story, but no one would ever use them in the signal path when the original coupling capacitors fitted were polyester.

FWIW, if I was restoring an STA15, I did one about 3 weeks ago, I would keep everything to look as original as possible as that was how the amplifier was intended to be. I would only make very small and subtle changes to certain components to maintain originality but to aid minor improvements to the sonic performance. You can do this easily and still maintain the look of the original amplifier. If you want to make a better sounding amplifier then choose a better circuit/better design, build it yourself and fill it full of boutique parts...

 

[Dan K]

Utter rubbish!!!

Not that this is any of my work, but if you bother to look you will see the PTFE tube has a significantly larger internal diameter than the wire, meaning that the silver wire touches the PTFE tube only in a couple of places, so the wire is effectively in an air dielectric. The Silmics are decoupling the cathode bias resistors so will have a very small influence on the sound produced. If they had been used in the signal path then that would have been a different story, but no one would ever use them in the signal path when the original coupling capacitors fitted were polyester.

FWIW, if I was restoring an STA15, I did one about 3 weeks ago, I would keep everything to look as original as possible as that was how the amplifier was intended to be. I would only make very small and subtle changes to certain components to maintain originality but to aid minor improvements to the sonic performance. You can do this easily and still maintain the look of the original amplifier. If you want to make a better sounding amplifier then choose a better circuit/better design, build it yourself and fill it full of boutique parts...

Morning Graham,

This is the first time we have ever disagreed, and yet when I read your first paragraph it looks as though you actually agree with what I wrote. Puzzled

 

[GT]

Morning Dan,

Firstly, there is silver wire, and then there his silver wire. High purity soft annealed silver is not bright sounding. Poor quality silver wire, which the vast majority available is, can sound bright and hard. If you put a high quality silver wire in a large diameter PTFE tube, or are able to use an insulator of natural materials then this solution for internal wiring is hard to beat. If you have a push/pull amplifier like a Radford which uses a lot of global feedback, then some components including poor quality silver wire will give a bright sound as you indicate in your post.

 

[Raoul Duke]

Chiily, Mike P and others... Just opening up my STA15 and thought I'd check on how my ECF82 are performing. I have a question regarding measuring these anode voltages, just to make sure I dont do anything stupid. If I'm correct the requirements are a 1kHz tone at 400mV on the input and then I'm assuming a dummy load should be connected on the output? Not sure I have more than a 10W resistor here

 

[chiily]

Chiily, Mike P and others... Just opening up my STA15 and thought I'd check on how my ECF82 are performing. I have a question regarding measuring these anode voltages, just to make sure I dont do anything stupid. If I'm correct the requirements are a 1kHz tone at 400mV on the input and then I'm assuming a dummy load should be connected on the output? Not sure I have more than a 10W resistor here

That's right, you should always have a dummy load connected to any valve amp when turned on.

 

[Raoul Duke]

I just heard from Will at RR that these voltage measurements are taken at idle so no input signal is needed apparently, I guess this means I can just have my speakers connected then and wont have to go hunt down a dummy load.