A Power Amp for Paul

[Radford Revival]

Paul of Reference Fidelity Components has commissioned me to build him an STA25! Clearly a man of exquisite taste in amplification

I thought it would be nice to document the process of building it to showcase the parts used and effort taken.

I didn't start taking photos from the very beginning but the amp starts out as a chassis surround, and I begin bolting various bits on. The transformers and most of the amp circuitry sit on a sub chassis made from brush stainless steel.

As you can see the main surround has a few basic bits mounted.


Some of the wiring beneath has been done and the tails come up through grommeted holes ready for the transformers.

[Radford Revival]

Here is the underside, where the basic PSU and mains wiring has been completed.


This is another sub-assembly the Radfords use. The valve bases sit on their own "runners". These are made from bright annealed stainless steel.

[brian2957]

High quality parts and beautiful workmanship Will . Paul's a lucky man

[Macca]

Is there any deviation from the original design or is it an exact copy?

[Radford Revival]

As has been sad ad infinitum, the output transformers are the most important component of a valve amplifier. The famous T2075 (a Radford designation) is used in the STA25. It is a heavily interleaved design (primary and secondary windings are wound in sections on top of each other to minimize leakage inductance). This allows circa 26dB of feedback to be used without stability issues. This results in very low distortion and a high damping factor.



Of course one needs to power the thing. The mains transformers we use are particularly large, for a low temperature rise. You could get away with a transformer approximately 3/4 the size, but we choose not to. A nicety of the mains transformer is that it employs an electrostatic shield, to reduce any interference from primary to secondary windings.

[Radford Revival]

Is there any deviation from the original design or is it an exact copy?

The audio circuitry is exactly as original, the only deviation is the addition of some resistors in the bias circuitry to protect against the wipers in the bias pots going open!

In addition, a few of the resistors have been up-rated in power as we of course have samples of originals that have run for many years, and you can clearly see on some where a bigger resistor would be of benefit.

As a circuit it's more than the sum of its parts so tweaking it will not yield much if any improvement.

[Radford Revival]

Speaking of circuitry, here are the set of completed PCBs (sans a few bits) destined for the amp. More pics to come, the amp has progressed from this, I just need to upload and take more pictures!

[Radford Revival]

Oops, here's one I forgot to upload earlier. Just shows the output transformers and runners mounted. Things have progressed much further than this, more pics to come soon.

[Reffc]

Looking great Will. Your attention to detail and neatness are a credit to you which is one of the reasons I decided on a Radford Revival STA25. Looking forward to the updates!

[Radford Revival]

Looking great Will. Your attention to detail and neatness are a credit to you which is one of the reasons I decided on a Radford Revival STA25. Looking forward to the updates!

I thought it was the bribery