Radford STA100 Restoration Blog

[Radford Revival]

A long time ago I was given four STA100s to restore by a family friend. They were completely and utterly worn out, kept in a loft for decades, covered in soot, rust, dirt and goodness knows what else.

I began the project but sadly difficult family matters slowed down progress to almost a halt for a long time. Recently I have been working on these full time aside from other side work.

Recently, montesque's amp was added to the restoration queue, giving me five to do.

All of the original four were in similar looking condition:

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2013/02/SAM_0047.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2013/02/SAM_0048.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2013/02/SAM_0050.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2013/01/SAM_1998.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2013/01/SAM_1999.jpg

[Radford Revival]

The amps were stripped down. They were completely toast.

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2013/02/SAM_0051.jpg
http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2013/02/SAM_0052.jpg
http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2013/02/SAM_0053.jpg

[Arkless Electronics]

What rusty old knackered amps... so badly past their best that I'll do you a favour and dispose of them for you :lol:

[Radford Revival]

The output transformers in these are beasts. Large core with many turns for high flux headroom and inductance!

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2013/02/SAM_0055.jpg

Boards de-populated be cloned (long since done!):
http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2013/02/SAM_0056.jpg

This was also re-engineered to fit standard 35mm caps and remade in stainless (also long since done, pictures to follow):
http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2013/02/SAM_0054.jpg

[Radford Revival]

What rusty old knackered amps... so badly past their best that I'll do you a favour and dispose of them for you :lol:

Thank you for the kind offer, they are clearly fit for the skip :D

[Arkless Electronics]

Thank you for the kind offer, they are clearly fit for the skip :D

Especially those heavy old output transformers.... Drool....

You've got your work cut out there! I'm envious of their new owners before they're even finished! :)

[walpurgis]

Really looking forward to seeing how those turn out.

[Firebottle]

Given the age I'd say they are still in good condition.
Admittedly the old components are ripe for replacement but the important bits, perhaps irreplaceable, are still there.

Interesting to see Will.

:)

[Radford Revival]

Thanks for the comments everyone. There has been a lot of interesting progress since those pics were taken, many more pics to come!

[Radford Revival]

Especially those heavy old output transformers.... Drool....

You've got your work cut out there! I'm envious of their new owners before they're even finished! :)

It's a big job that's for sure - a LOT of planning and effort has gone into these so far - in some ways they are being improved on quite significantly. This will become apparent with the coming updates!

[Radford Revival]

Skipping forward a bit.

We got the boards cloned onto modern FR4, with the inclusion of a solder resist mask. They are a true mirror image pair for each channel.

Some of the resistors have been specified much higher in terms of power rating compared to the originals - they run hot, specifically in the phase splitter circuitry. I believe the original resistors were 2W rated, whereas now I used 7W vitreous enamel wire-wound types instead. All non-electrolytic capacitors are polypropylene types. All resistors are metal film aside from aforementioned wire wound types.

Perhaps shockingly the smallest resistors in the picture are actually 1W rated, despite looking like 1/4w or 1/2w types. They are minuscule for their supposed power rating but they are used where 1/4W resistors were specified and aren't dissipating any power. I would struggle to treat these as true 1W resistors, they are so small!

All values are as originals - except for one exception in order to run the output valves a bit cooler. The original configuration runs the output valves right on the edge of their dissipation limits. This addresses this.

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/07/freshsta100boards.jpg

[walpurgis]

Looks very neat. Will you be getting the transformers rewound?

[Radford Revival]

Looks very neat. Will you be getting the transformers rewound?

If it's necessary, we have all the specs to wind any of the transformers in the amp, including the output transformers. No anxiety about irreplaceable transformers here! :D

[Radford Revival]

The STA100 in its original form contains numerous minor design flaws. One of which is a zener in the power supply section that holds the bias rail steady. Sounds like a good idea in theory, but it actually makes the bias stability worse because you forgo the equal-but-opposite percent change in bias voltage to HT voltage as the mains fluctuates - meaning the bias currents are more at the whim of the mains voltage fluctuations than without it.

Here is an original bodged power supply board (mentioned zeners not visible, some time in the distant past they were replaced and relocated to the underside of the board as a completely un-insulated zener string that could touch something else at any time, fantastic!)

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/07/oldpsuboard.jpg


Here is my re-engineered board. The bias supply has been made entirely conventional with no zener regulation, high voltage fast recovery diodes used for the main supplies and fuses also accommodated. The layout has also been made much more logical. The original STA100 has many unintentional internal earth loops and they are known to hum and buzz themselves. I have since gotten one of these amps working and they are now completely quiet with my improved layout.

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/07/newpsuboard.jpg

[Radford Revival]

The valve holder assemblies were of course re-zinced and populated with fresh valve sockets

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/07/runners1.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/07/runners2.jpg

[Radford Revival]

Here is the first amp nearly complete. There are still numerous bodge wires, namely the input, feedback and output transformer wiring (they are temporary and are being swapped for new ones). The mains switch wiring is also missing as the legend panels (front and rear) are yet to come. No point in wiring it up and having to undo it again to get the panels on. I am getting some new ones made as they originals were visually a mess.

As I said this NOT the finished article, there are things to finish and tidy up, but it powers up and works incredibly well. It sounds fantastic.

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/07/mostlydone1.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/07/mostlydone2.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/07/mostlydone3.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/07/mostlydone4.jpg

[hifi_dave]

Great work as always Will.

When will you be making commercially available amplifier(s) ?

[Firebottle]

It sounds fantastic.

I bet! Great work Will.

[walpurgis]

Grand job Will. I'd love to hear it. :thumbsup:

[JohnJo]

Ooh, that looks right proper Will.

Nice work :cool:

[montesquieu]

The STA100 in its original form contains numerous minor design flaws. One of which is a zener in the power supply section that holds the bias rail steady. Sounds like a good idea in theory, but it actually makes the bias stability worse because you forgo the equal-but-opposite percent change in bias voltage to HT voltage as the mains fluctuates - meaning the bias currents are more at the whim of the mains voltage fluctuations than without it.

Here is an original bodged power supply board (mentioned zeners not visible, some time in the distant past they were replaced and relocated to the underside of the board as a completely un-insulated zener string that could touch something else at any time, fantastic!)




Here is my re-engineered board. The bias supply has been made entirely conventional with no zener regulation, high voltage fast recovery diodes used for the main supplies and fuses also accommodated. The layout has also been made much more logical. The original STA100 has many unintentional internal earth loops and they are known to hum and buzz themselves. I have since gotten one of these amps working and they are now completely quiet with my improved layout.




That's fantastic Will. As you know I absolutely love mine (which you have with you of course at the moment) can't wait to hear it with these improvements.

[Edward]

Lovely :drool: :drool:

Put my name down in the hopefully orderly queue that is forming. I already sent you an email on this Will.

Please consider putting the power switch on the side where the bias controls are. The 'front' side. :lol:

Will you be adding the additional protection circuitry you did on my STA25?

E

[Edward]

So we happened to be near Radford HQ last week, Will and Steve (Will's dad) kindly allowed us to visit.

Will kindly played some music through a near completed STA100 to his RFC Raptor speakers.

Absolutely magnificent. I could write lots of words but will confine it to following - massive presence, fluid, ease, dynamic.

The amp itself is huge, as usual the rebuild Will has performed is impeccable. As is his workshop - neat and orderly. :)

I was also very impressed with the RFC Raptors (http://www.referencefidelitycomponents.co.uk/loudspeakers/reference-raptor-loudspeakers/). Wide open sound able to convey music perfectly. The finish is first class.

Overall to both items ... :wow:

[Radford Revival]

The amplifier has been complete for a while, but as the original anodised legend panels were a bit worse for wear, I had to spend a bit of time re-creating them in a vector editing package and have some fresh ones made. I will post a bit about that soon but for now here is the amp completed with the fresh panels! This amplifier also features some special output transformers of my own design.

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/10/STA100-2P.tif.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/10/STA100-3P.tif.jpg


http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/10/STA100-4P.tif.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/10/STA100-5P.tif.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/10/STA100-8P.tif.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/10/STA100-9P.tif.jpg

http://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2017/10/sta100-66p.tif.jpg

[walpurgis]

Wow!

Gorgeous.

[montesquieu]

Beautiful work Will. Having seen the insides of mine this is night and day.

[JohnJo]

Ouch, that is almost painfully nice!

[Firebottle]

Serious iron :eek:

[Radford Revival]

Many thanks for the comments all :)


Serious iron :eek:

Indeed! Lifting these things requires planning, and I'm only half joking!

[struth]

Superb work. Love the way these are put together. Bordering on perfecion

[JohnJo]

Will, at the risk of getting pelted with tomatoes by the traditionalists :D, could this amp support the use of KT120 or KT150 valves instead of the KT88s? They are generally regarded as superior and I'm sure would bring an improvement. Is this something you would consider fitting?

[User211]

Post #4 appears to show a NOS Genelex Gold Lion KT88. A NOS pair is about $1200 (https://www.ebay.com/itm/NOS-MATCH-PAIR-GOLD-LION-GENALEX-KT88-TUBE-UK-TEST-71-71-MIN-44-W-ORIG-BOX-/253155406058?hash=item3af13cf0ea:g:PrEAAOSwIZBZu~T n), a quad double that so be careful!!!:)

[Radford Revival]

Post #4 appears to show a NOS Genelex Gold Lion KT88. A NOS pair is about $1200 (https://www.ebay.com/itm/NOS-MATCH-PAIR-GOLD-LION-GENALEX-KT88-TUBE-UK-TEST-71-71-MIN-44-W-ORIG-BOX-/253155406058?hash=item3af13cf0ea:g:PrEAAOSwIZBZu~T n), a quad double that so be careful!!!:)

Thanks for the headsup :)


Will, at the risk of getting pelted with tomatoes by the traditionalists :D, could this amp support the use of KT120 or KT150 valves instead of the KT88s? They are generally regarded as superior and I'm sure would bring an improvement. Is this something you would consider fitting?

I'm not sure if the heater windings are rated enough to support the extra current demanded by these valves. Having the specs for the transformers, this could be calculated, so it MIGHT be ok, but I wouldn't like to just stick them in and find out. There is generally a bit of headroom designed in when a transformer is specified, but this doesn't seem good practise to depend upon this. As far as I'm aware, apart from that issue they would otherwise work.

[spendorman]

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