Despite being a “modern” pentode design, does the Radford
STA series of vacuum tube audio amplifiers claim to fame as a great vintage
hi-fi amp is its “classic” vacuum tube sound?
By: Ringo Bones
When I got seriously into hi-fi during the Clinton
Administration, any audio gear older than Chelsea Clinton is already considered
“vintage hi-fi”, even back then, whether original or reissue, Radford’s STA
series of stereo audio hi-fi amplifiers were already considered vintage
classics. When Arthur Hedley Radford established Radford Electronics in 1946
and marketed his first hi-fi amp in 1959, little did he know back then that his
very products will soon become very sought-after vintage hi-fi classics that
they currently are now?
It was the Series Two amplifiers, soon to change into the
Series Three range comprising of two monoblocks – the MA15 and the MA25 plus
two stereo versions – the STA15 and STA25 - and the matching SC22 preamplifier (available
for around UK£32 back in 1964-65)
that put Radford on the map. Famed for its good sound quality and considered a
perfect match for Radford’s STA series of audio amplifiers when it comes to
vinyl LP replay, the preamp’s exemplar vinyl LP replay performance was
primarily due of its avoidance of the use of steep-cut rumble / subsonic
filters. Steep-cut rumble / subsonic filters tend to be disliked by audio
purists because the benefits of steep-cut filters in terms of vinyl record warp
suppression is often easily offset by audible phase and group-delay distortion
they introduced, messing up with the percussive transients of a recorded drum
kit. Group-delay distortion is also the primary reason why older CD players
suck in comparison to good entry level vinyl record replay equipment.
The Radford STA15
and STA25 series of audio amplifiers – as well as their monoblock siblings –
employ a variation of the classic Philips / Mullard 5-20 circuit with the first
stage utilizing a high-gain EF86 RF pentode preamplifier vacuum tube that feeds
into a 6U8 (ECF82) dual triode / dual pentode vacuum tube as a phase splitter
to drive a pair of EL34 power pentode vacuum tubes in push-pull mode. The
output transformers use ultra linear connection. Sound wise, the bass are a bit
unruly by solid-state audio amplifier standards but the midrange sound quality
could give lesser single-ended triode designs a run for their money.
If you are asking
why the second-hand prices of Radford STA15 and STA25 have skyrocketed way
ahead of their Blue-Book inflation adjusted figures can be traced to a crucial
point back in hi-fi history. Back in 1975, hi-fi journalist Martin Colloms set
up a major blind listening test of 18 hi-fi audio power amplifiers for a long
since defunk UK hi-fi magazine called Hi-Fi For Pleasure. Back in 1975, the
transistor amplifier (well, the better designed ones, anyway) was already well
accepted by audiophiles. Prices of the review samples ranged from US$300 to
US$3,000 – about equivalent to US$1,200 to US$12,000 in today’s money. The
auditioning sessions were graced by the presence of many hi-fi industry
leaders, among them Spencer Hughes of Spendor, Julian Vereker of Naim, Philip
Swift of Audiolab, Alan Harris of retailer Audio T., Bob Stuart of Meridian and
John Wright of IMF now TDL in UK.
On the suggestion
of Alan Harris – a serious vacuum tube audio amplifier fan – Martin Colloms
introduced a ringer to those tests: an “ancient” (over 10 years old) 25-watt
per channel vacuum tube audio amplifier, the Radford STA25 Mk. III, worth
perhaps US$100 on the used market back in 1975. Colloms used a selection of
master tapes as the source. When the results of the blind test were analyzed,
the vacuum tube Radford had come on top despite showing the poorest measured
performance in comparison to “more modern” solid-state audio amplifier designs.
Needless to say, the Radford’s second-hand value soared after the review was
published. Based on what we know now, it might be due to the Radford’s use of
very small amounts of negative feedback which its design is around less than 20
decibels worth as opposed to a typical solid-state audio amplifier design which
needed at least 46 decibels of negative feedback or it will burst into
oscillation.
Due to their
recently achieved “cult status” and growing demand from high fidelity audio
enthusiasts, a limited run of the Radford STA25 vacuum tube audio amplifiers
were released to the market in England back in 1984 to 1985. With the high
fidelity vacuum tube audio amplifier revival becoming newsworthy by the early
1980s – and despite of the increasing vacuum tube scarcity in America during
the Reagan Administration – all it took was a lot of nagging to get the then
semi-retired Arthur Radford to permit his former employee, John Widgery of
Woodside Electronics, to oversee the run of 100 individual units of the Radford
STA25s. These amplifiers were designated Mk. IVs and painted gloss black with
gold trim to distinguish them from the battleship gray originals. Each of these
came with a certificate of authenticity signed by Arthur Radford himself. After
his death back in November 21, 1993, the 100 lucky owners of the Limited
Edition Radford STA25 Mk. IVs saw their treasures appreciate in value in high
fidelity audio collector circles.
