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Radford STA25 Mk III -- a lot of questions.

They do work well and are quite simple. The output transformers are rather good too. Really the most important thing that will likely need changing are the caps. I have serviced one as detailed here: https://theartofsound.net/forum/showthread.php?65086-TT100-Service

Another enhancement that makes life easier is fitting Cliff binding posts which fit in the original keyed cutout, much more convenient than the sprung terminals they came with (I believe also Cliff made hence the same cutout)
 
And of course, finding proper GEC KT88s with good emission so the amp runs reliably. These amps were designed around the GEC KT88 where a pair of them could easily produce 100 watts of power. You won't get that with modern equivalents, you'll just get smoke! Also if you need the power, the STA 100 sounds much better, but then again the STA 25 sounds better than the STA 100, and the STA 15 sounds better than the STA 25. The STA 12 and the STA 15 are much closer together in sound... :)
 
Reliability of modern KT88's is likely to be a major concern with this amp.... I had a HUGE Unison Research KT88 amp in recently which had eaten several sets of crap Chinese valves and I ended up using the modern Russian "Golden Lion" "Genelex" ones which seemed to do the trick.
 
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The old STA100 had a habit of blowing up due to the compounding factors of a few issues, which are quite easily sorted (though the hardest one to sort is the fact that the OPT is optimised for about 10 ohms which most speakers certainly aren't). A few examples of TT100s have slightly different resistor values (lower valued cathode resistors) that run the valves too hard when adjusted for 2.0V as per the settings on the amp, which may lead to problems, but this is easily sorted too. I imagine these were probably early examples.

I have a feeling back in the day they rushed the STA100 design a bit. This is probably why they got the reputation of not sounding as good as the 25/15. They have some silly design and construction oversights. When I've rebuilt them I have to ignore much of how the things were wired originally. Once sorted they can sound very good.

The TT100 isn't so bad in that regard though has a few minor flaws that can be easily corrected. It's certainly an easier proposition that trying to use an original STA100.
 
Arkless Electronics said:
Reliability of modern KT88's is likely to be a major concern with this amp.... I had a HUGE Unison Reserch KT88 amp in recently which had eaten several sets of crap Chinese valves and I ended up using the modern Russian "Golden Lion" "Genelex" ones which seemed to do the trick.
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Back in 1999, I supported Unison Research here in the UK via UKD, we encountered a huge amount of failures of the Simply 2, which used EL34's, one per channel. On investigation, I found the amps had been biassed to run each EL34 at about 4 watts over their rated anode dissipation, so no wonder why the EL34s were always blowing up. I suggested to UKD that we use a KT88 instead of the EL34 and made a prototype amp designed around one KT88 per channel. The intention was to offer this as an alternative to the base model S2, and to S2 owners as a UK upgrade. My prototype was sent back to Italy without me knowing and a few months later the Simply S2K came out and guess what, it was using KT88s. I'll leave you to draw your own conclusions, but needless to say, I stopped supporting those products forthwith...
 
G T Audio said:
Back in 1999, I supported Unison Research here in the UK via UKD, we encountered a huge amount of failures of the Simply 2, which used EL34's, one per channel. On investigation, I found the amps had been biassed to run each EL34 at about 4 watts over their rated anode dissipation, so no wonder why the EL34s were always blowing up. I suggested to UKD that we use a KT88 instead of the EL34 and made a prototype amp designed around one KT88 per channel. The intention was to offer this as an alternative to the base model S2, and to S2 owners as a UK upgrade. My prototype was sent back to Italy without me knowing and a few months later the Simply S2K came out and guess what, it was using KT88s. I'll leave you to draw your own conclusions, but needless to say, I stopped supporting those products forthwith...
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I'm not keen on them due to the policy of both manufacturer and importer of refusing to give any technical information whatsoever to anyone carrying out repairs. It was also very "scrappily" built in the manner of 70's Japanese amps with many boards all over the place and loads of internal wiring... many boards above or beneath other ones! A servicing nightmare. I got it properly sorted in the end... no thanks to Unison or Henley!
Heavy bugger it was! One of the heaviest amps I've ever worked on in fact.... An 80WPC ultralinear KT88 based integrated with a thick "ripple effect" glass front and it seems a rare one in the Unison stable... very little info on it anywhere... about £6500 IIRC.
 
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Arkless Electronics said:
Reliability of modern KT88's is likely to be a major concern with this amp.... I had a HUGE Unison Research KT88 amp in recently which had eaten several sets of crap Chinese valves and I ended up using the modern Russian "Golden Lion" "Genelex" ones which seemed to do the trick.
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yes as far as I can see (I’ll know more on Sunday) the amp has been fitted with new Gold Lions, though I’m not sure of the quality. This was one of the images with the deal

s-l300.jpg
 
mandryka said:
yes as far as I can see (I’ll know more on Sunday) the amp has been fitted with new Gold Lions, though I’m not sure of the quality. This was one of the images with the deal

s-l300.jpg
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Unless you have a stock of original GEC KT88s you will have endless problems and on a very regular basis.
 
JJ KT88s seem to work fine at the operating point used in both the STA100 and TT100 in my experience, provided the cathode resistors are 39R and not 33R, based on quite a few amps, so it's certainly not impossible.
 
You must have to derate the circuit to produce less power as you can't get 100 watts from a pair of modern KT88s reliably...
 
G T Audio said:
You must have to derate the circuit to produce less power as you can't get 100 watts from a pair of modern KT88s reliably...
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Well, based on a sample of size of about 4 or 5 amps, plus one more if you count a prototype new design I have on my desk (so 20 or so valves) JJs seem to tolerate the idle conditions in which 100W is possible. It is probably possible to exceed the anode dissipation under difficult loads for long periods of pure tones at high powers, but you can do that with any amp.

So not 100s of valves by any means but things seem OK so far!
 
It was the reason why manufacturers like Audio Research doubled up the numbers of output valves on all their designs...
 
G T Audio said:
It was the reason why manufacturers like Audio Research doubled up the numbers of output valves on all their designs...
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Quite likely. For whatever reason the STA/TT100s I've worked with have all been fine so far luckily. The main issue probably the high idle voltage the screens sit at.

My new design doesn't use UL, with much tamer screen voltages so should in fact be quite easy on the valves, despite being capable of 100W. It currently has a pair of JJs in it, I have abused the thing into all sorts of horrible loads including dead shorts with no ill effects so far!
 
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scotty38 said:
I have just put my name down on the list for an STA25 as it happens :D
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A Series 3 original or a "Radford Revival" reproduction?

A very good choice in any event.

I would trade one of my STA25/111 for a RR STA15 c/w Mullard GZ34 mains rectifier valve if RR produce them.
 
Darmok said:
A Series 3 original or a "Radford Revival" reproduction?

A very good choice in any event.

I would trade one of my STA25/111 for a RR STA15 c/w Mullard GZ34 mains rectifier valve if RR produce them.
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I wouldn't!! The other way round then yes... no issues with RR of course... just I'd take an STA25 over an STA15 all day long...
 
One enthusiast showed off his full active system in one of the Hi-Fi magazines way back in the 70's, comprising of a Radford STA100 for bass duties, a Radford STA25/3 for mid-range and a Radford STA15/3 for HF.

Loud Speakers were the IMF Professional Monitor IV.

The STA15/3 has the sweetest sound reproduction of all the Radford STA's.

I cannot comment on the ultra rare Radford STA7, as I have never heard or seen one in the flesh.
 
Umm, STA25 or STA15? Tough choice.
I own an STA15 III, I too think it is a little sweeter and a touch less clinical, but could live with either.

Sorry, getting off topic here...
 
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