Pre/Power Amplifiers

Radford Electronics was set up in Bristol by Arthur Radford back in 1959. In some ways Radford was a late starter in the world of high fi delity, especially compared to Peter Walker of Quad or Harold Leak, and the electronics refl ect this. Indeed, Radford’s designs are often described as being the most ‘modern’ of vintage amplifiers.
   It was the Series Two amplifiers, soon changed to Series Three, that put Radford’s designs on the map, the Series 3 range comprising two monoblocks – the MA 15 and MA 25 – plus two stereo versions, the STA 15 and STA 25. The matching SC 22 preamplifier was available for £32 10s. Common to most vintage amplifier sets, this unit ‘scavenged’ power from the power amplifiers, but Radford also offered a selfpowered version for an extra £5.
   Specification-wise Arthur Radford made no outlandish claims for the Series 3. Input sensitivity is a rated 0.5V for full output. However, what is striking is Radford’s interest in the amplifier’s ‘rise time’ and square wave response – something that was pretty unique at the time.
   Technically the amplifiers employ a variant of the classic Philips/ Mullard ‘5-20’ circuit with the first stage utilising a high gain EF86 pentode valve. In the case of the Radford circuit, a 6U8 (ECF82) dual tube triode/pentode valve is used as a phase-splitter to drive the push-pull EL34 pentode output stage. The output transformers use ‘ultra linear’ connections.
   The difference between the 15 and the 25 models come down to power supply and output valve bias arrangements. On the MA and STA 15, high tension power supply rectification is provided by a GZ34 rectifier, providing around 330V high tension. Output valves on the 15s are automatically biased via cathode resistors; on the 25 the output stage is operated in fi xed bias with a negative grid voltage applied to the output valve, which must be adjusted to suit the output valves with the aid of test points located on the amplifi ers chassis (2.0V across the test point to chassis earth).

MAGICAL MIDBAND
Bass-wise, we felt the Radford to lack a little accuracy, the tones of the double bass and Hammond organ footwork on Jimmy Smith’s The Cat CD becoming subtly blended, making it diffi cult to separate them completely in the mix. Similarly, the limited sound pressure level down low meant Alison Balsom’s trumpet recording of Mozart’s Rondo alla Turca soon ran the amp out of steam.
   However, the midrange is simply delicious. Smooth and liquid, it boasts a freedom from harshness and compression that stands out even by valve amplifier standards. A vinyl cut of Grace Jones’ ‘Slave To The Rhythm’ was truly infectious, Trevor Horn’s immaculate production being revealed cleanly and clearly. Switching to digital and some recordings from Ella Fitzgerald and Dinah Washington, the magnifi cent midband was further underlined. There is something unique about it; one might even describe it as 3D-sounding.

VERDICT
Already well established as a true classic, the STA 15 III really does bridge and blur the vintage/modern line. Given that this is a 45-yearold amplifi er, it sounds remarkably fresh, modern and sophisticated and clearly lives up to the hype of being one of the best of the British vintage brigade. With its liquid midband, reliability and sturdy build – even by 2010 standards – it’s hard not to be impressed.

Originally published in the Yearbook 2010