RADFORD: STA100: A very, very happy owner

After months of waiting (and wondering if it was worth the wait), it's finally arrived, and sounding pretty wonderful.

This STA100 project kicked off because I had been concerned that my Tannoy Canterbury cab/ HPD combo needed more welly than I had been throwing at it with my old KT88 300b interstage amp (30w and highish output impedance was giving me floppy bass on the HPDs where it had been magical with the Golds), and while various solid state and Class D options I tried produced an audible improvement in things like bass depth and quality, lack of something intangible led me to feel that what I really needed was a chunky valve amp. I got the amp from Finland anticipating little more than some capacitor re-forming as it had lain idle for a while. But it turned out to have a problem with one of the output transformers, so sorting things out took a little longer than anticipated. It's been with Graeme Hirst (Valvebloke) since June, mainly awaiting fresh output transformers after a bit of diagnostic detective work.

It's a bit of kit without much of an online profile, Radford's late 60s take on the KT88 PP amp, using Radford's characteristic phase-splitting technology on the input side. Very period in concept and with gobs of negative feedback (as do the much more common STA12, STA15 and STA25), it was intended as a studio workhorse and the original amp has 8-16 ohm windings and 100v line for professional use - this one is ex-BBC from 1967-1973 at which point it came into the possession of the owner I got it from, a professional studio engineer (ex-BBC himself) who had it at the heart of his personal setup for some years. The main input is single ended but balanced is possible, as balanced input transformers are fitted. I've only tried single ended so far but I will have a crack with balanced as my Music First MkII TVC has balanced input and output option while my AN 4.1x(-ish) DAC has balanced output into the MFA.

The few online references to it seem to be very polarised - on the one hand it's regarded by some as the best KT88 amp ever made (being a Radford and all); but for a couple of others of the view that pro gear like this should never be allowed in a living room, it's spawn of the devil - cold, harsh and unforgiving. I decided to give it the benefit of the doubt. Having come to the conclusion that properly sorted and with the right speakers (Quads or Tannoy Golds for example) the baby Radfords are streets ahead of any 60s or 70s competition and still better sounding than the majority of valve amps around today, I had to have a crack at their big brother. I didn't even know before I got into this that Radford had even made a KT88 amp - go to be worth a shot?

It was a bit of a nuisance when it arrived needing work, but in for a penny ... I had the option to trade it in to Will at Radford Revival for something more 'normal', but by this stage curiosity wouldn't be satisfied till I'd heard the thing. And talking to Will, it seemed that while it would be costly to sort, the transformer problem was actually something of an opportunity. Will pointed out that Radford's TT100 had a near-identical KT88 output stage (Radford went to solid state designs for some years after the STA100, then BACK to valves, at least for the output stage, for the later TT100 - though this was a hybrid amp with a FET input stage). But the output transformers in the T100 were wound for 4-8 ohms - much more sensible for modern speaker loads than the 8-16 ohm/100v setup of the original STA100.

Will had Arthur Radford's original spec sheets and was able to have new transformers wound to be essentially indistinguishable to originals, with even the original laminations still available. The output transformers are pretty special, bigger than most mains transformers, while the amp does actually sport two separate mains transformers for the power and signal sections. It is a real beast, the iron on it is quite staggering when you take the lid off. At the same time Graeme and Will worked out a method of getting rid of a slight measured instability in the power supply, by adding a sixth transformer - an additional choke - which (fingers crossed) together with substituting KT90s - unavailable back then - with higher voltage tolerances than KT88s, would hopefully eliminate any problems associated with the reputation it had way back when of throwing the occasional power valve when cranked up. (All mods - minor really - are easily reversible should full originality be required at some future date. But unless it's destined for a museum I doubt that will be required).

So how does it sound? Deathly quiet for starters, in large measure to Graeme's care in pulling it all together, I'm sure. Quite a surprise for such a big vintage beast. And then - it disappears. Massive, expansive sound stage. Great detail. Wonderful timbre with a clean, smooth top end. Not a hint of strain when cranked. Extremely natural, with the juiciness and bite of instruments and vocals coming across superbly. But most of all it has that wonderful snap and boogie that the STA15 and STA25 have on more sensible speakers (such as the Tannoy Golds I used to use). The STA100 is far from clinical but it's still at the opposite end of the spectrum from typical single ended amps which not only add (not necessarily wanted!) 'character' to the mix, I've usually found they also fail to light up the bigger Tannoys. The closest modern counterpart might be to some of the bigger EAR amps, but with perhaps a transparency-without-coldness that I've found my EARs (and I've had a few) never quite managed to deliver.

According to Graeme the measured specs are very good for a valve amp. Damping factor (he tells me) is around 24 which is probably about ideal for Tannoy DCs - too low you get floppy bass (as in any single ended amp I've tried with them), too high (as in most solid state amps) and the life is sucked out. (There are exceptions of course in both cases but this is my typical experience from a lot of trial and error).

Anyway I'm absolutely delighted with the STA100 - to be honest (and I've been around the block on valve amps and Tannoys over many years) I doubt there are many amps even in the same league for this purpose - Anthony's Copper Amps for sure, but not that many others I can think of. Over the moon with it and while I appreciate that this one has taken quite a bit of faffing and fettling to get 'just right', it was well worth the effort in the end.

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