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A DIY Radford STA25 restoration

Mike P

Mike P

Trade: Pickwell Audio
Like the title says, I have just bought a Radford STA25. Eeek!

The seller had powered it up slowly on a variac and briefly demonstrated it to me with some small cheap bookshelf speakers but other than that it hasn't been tested or properly assessed.

I'm new to Radfords, and still quite new to valve amps in general if I'm honest, so any advice on assessing the amp would be gratefully received.

So, here it is:

P1110580 by Michael Pickwell, on Flickr

All looks to be Kosher to me under the cover.

P1110572 by Michael Pickwell, on Flickr

And here's the circuit boards:
P1110571 by Michael Pickwell, on Flickr

The spark supressor has obviously gone pop at some point.

P1110568 by Michael Pickwell, on Flickr

And it looks like someone has emptied their collection of ferrites into it! :confused:

P1110569 by Michael Pickwell, on Flickr

P1110570 by Michael Pickwell, on Flickr

The small valves are all Mullards

P1110575 by Michael Pickwell, on Flickr

P1110576 by Michael Pickwell, on Flickr

But the four EL34's are a mixed bunch.

P1110574 by Michael Pickwell, on Flickr
Second from left is obviously branded Chelmer. The one on the far left looks to be the same construction but is slightly shorter. The third from left is a Mullard XF2. I've no idea what the one on the far right is.

So, there we have it. Peach or lemon?
 
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Mike P said:
And it looks like someone has emptied their collection of ferrites into it! :confused:
Click to expand...

I’m obviously no expert, but the four things in a row look like a solid state bridge rectifier, i.e. four fairly ancient diodes, and three of them look to be cracked!

PS It looks in very decent cosmetic condition and as long as the transformers are fine it can be sympathetically rebuilt to as-new condition. That is exactly what I’d do.
 
Tony L said:
Obviously no expert, but the four things in a row look like a solid state bridge rectifier, i.e. four fairly ancient diodes, and three of them look to be cracked!
Click to expand...

That was my first reaction too but I'm pretty sure the diodes are intact and that someone has added bits of ferrite bead to the legs.
 
I’m pretty sure the ferrite beads aren’t stock so I’d be ripping all those out for a start. I googled a few images and they certainly don’t seem to be part of it. If it was mine I’d just want to put it back exactly as it should be, which shouldn’t be that hard to do. As its been tampered with I’d check every single thing though, i.e. don’t assume the resistor and cap values are correct. Get the schematic and go over it with a fine tooth comb.

Straightening the transformer lug out shouldn’t be an issue, you’ll have to take it off, but you may well be stripping and cleaning anyway.
 
The diodes don’t seem to be in A1 nick, do they? DD058 according to my schematic, if that helps. Look on the bright side, it’ll cost less than a GZ34.
 
I think the o/p stage is similar to the STA 15 ,in which case the green w/w resistors should be 390ohm and not 39 ohm as they look to be.Most were shipped with 470ohm ,can not see the EL 34,s lasting long if they are '39.I would also fit WIMA o/p caps to both channels
 
My hunch is that everything but the ferrites and the yellow RS branded poly cap is correct but we'll see.
 
Most of the wiring to the OPT's is missing! I see some are tied up with tape....
All the ferrite beads need to come out. Check all resistors and capacitors are both correct and in tolerance etc.
Obviously you'll need a decent set of output valves and the ECF82/6U8's are pretty critical to getting low THD.
They are a common valve used in TV sets and any S/H ones will prob have come from a TV and the section's will be unevenly worn. Get new ones.
 
Just checked on the circuit diagram and the STA25 does have a 39ohm resistor where the STA15 is a 470ohm.My mistake ,apologies.
 
A word of caution,the large white 2.2uf/250v cap on my diagram should be a 2uf (2.2 should be fine) 350v.More important,the 22uf/250v should be an 8uf /450v.The original caps (8uf/450v) were dual section to go with the cap adjacent.F&T still make them .Hope this helps.
 
For referebce here are some clear pictures of an amplifier fully rebuilt by myself with all original wire colours and components etc.

DSC_1500.NEF_.jpg

DSC_1501.NEF_.jpg

DSC_1499.NEF_.jpg

DSC_1498.NEF_.jpg

DSC_1497.NEF_.jpg

DSC_1485.NEF_.jpg
 
mik_rik said:
A word of caution,the large white 2.2uf/250v cap on my diagram should be a 2uf (2.2 should be fine) 350v.More important,the 22uf/250v should be an 8uf /450v.The original caps (8uf/450v) were dual section to go with the cap adjacent.F&T still make them .Hope this helps.
Click to expand...
It's all helpful, thank you.

I've found the circuit diagrams. Does anyone know if a diagram exists which shows the positions of all the parts, for quick reference, to save me having to trace everything? I found such a thing very helpful when doing my Leak Stereo 20's.
 
Mike P said:
It's all helpful, thank you.

I've found the circuit diagrams. Does anyone know if a diagram exists which shows the positions of all the parts, for quick reference, to save me having to trace everything? I found such a thing very helpful when doing my Leak Stereo 20's.
Click to expand...
There’s one on the RR website. Google STA25 pcb layout and you will find the image. (Not sure which version it is, though. It’s also pretty clear in the images above.)
 
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