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Radford STA25 Mk III -- a lot of questions.

With the Leak I felt it sounded a bit bigger and more natural on the 16 Ohm tap, but not much in it, both work. The JR149 has a really easy load IIRC dipping to 8 Ohms at its lowest point, hence being a great choice for valve amps despite the low efficiency.
 
The Leak is probably at its very best with a 16R load as the NFB is taken directly from the 16R tap. The Radford is rather more sophisticated than most as when you change the output impedance selector it reconnects all the sections of the secondary to be in parallel or series etc to suit the speaker load but uses all of the windings at all times. It also switches in a separate NFB network for each impedance setting, rather than a compromise one.

As you said, with a speaker that is actually say 12R there is not really a definite "correct" setting so it's suck it and see.
To be pedantic it could be said that generally in an "8R speaker" that measures higher over much of the frequency range it will often be not too far from 8R for low frequencies (but higher than resonance of port or driver) and that this range is where we need the most power and damping....
 
It's developed a fault! Hissing noise through the speakers, especially as tracks are changing, and occasional clicking sounds, and the amp itself is humming more loudly than before. It starts about 10 seconds after the amp is turned on, when the tubes start to glow. Same problem at 8ohm and 16 ohm.

I'll speak to Emporium tomorrow obviously, but I thought I'd mention it here just in case someone could think of what it might be.
 
Tony L said:
Sounds like it needs a full rebuild, as in fairness any amp of its age deserves.
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I’m sure you’re right, and it will get one, I just want to enjoy it for a while for a few weeks first!

However in this case it’s me who needs a complete rebuild too, since the hum/noise problem seems to have been caused by . . . . a faulty lead from DAC to amp!

Sorry for crying wolf.
 
mandryka said:
I’m sure you’re right, and it will get one, I just want to enjoy it for a while for a few weeks first!

However in this case it’s me who needs a complete rebuild too, since the hum/noise problem seems to have been caused by . . . . a faulty lead from DAC to amp!

Sorry for crying wolf.
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That’s good news. It can be easy to misattribute blame for these things when there are so many links in the chain, and particularly when one is on edge a little about something that’s been repaired.
 
mandryka said:
Plus, I want to be really fair to Emporium.

It is such a wonderful amp! I’m starting to enjoy music which has in the past completely eluded me!
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The Radford, Krell and a few others make me laugh at those who reckon "all amplifiers sound the same" or "amps are nearly perfect so concentrate on the rest of the system"!
 
By the way, this is the third time I’ve had leads fail, and it’s always the least thing I expect. Can someone recommend a brand of interconnects which is well made? I’m buying the cheapest I can find at the moment and I’m starting to think that that’s not the best policy!

One of the lead failures was the thing that goes from my router to the socket on the wall. BT replaced the router twice and were about to send an engineer before I realised what was making the problem!
 
mandryka said:
By the way, this is the third time I’ve had leads fail, and it’s always the least thing I expect. Can someone recommend a brand of interconnects which is well made? I’m buying the cheapest I can find at the moment and I’m starting to think that that’s not the best policy!

One of the lead failures was the thing that goes from my router to the socket on the wall. BT replaced the router twice and were about to send an engineer before I realised what was making the problem!
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I use the cheapest cable from Paul Coupe (RFC).
 
I mainly use free ones that came with CD players etc and haven't had any problems...
The odd dodgy cable now and then is inevitable and largely related to how roughly they are treated, the quality of the soldering and how well the cable grip has been designed/used...
 
Was the STA25 designed to go with the SC22 in sound terms? I mean, does the preamp colour the signal in such a way that, when combined with the power amp, the resulting sound is even more magical than the power amp alone? I mean, should I look out for an SC22?
 
mandryka said:
Was the STA25 designed to go with the SC22 in sound terms? I mean, does the preamp colour the signal in such a way that, when combined with the power amp, the resulting sound is even more magical than the power amp alone? I mean, should I look out for an SC22?
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I love the sound of my SC22/STA15. A lot of people told me the SC22 was poor, and being easily led I bought a Croft 25R to use with the STA15 instead. An excellent pre which does the job superbly, but I get the occasional indefinable spine-tingly moment with the 22 that somehow the Croft doesn’t give... distortion perhaps? I don’t really care!

The only problem is that the Radford combo is very sensitive, and high output sources will mean you have the volume set at nearer 0 than 1. The stereo tracking is rubbish down at the end of the scale, so you need to be able to attenuate the signal before it gets to the SC22. I use a passive pre between source and Radford pre to do this. (“You do what??? Why don’t you just ditch the the SC22 and...” I know, I know...)
 
Well it’s back after being inspected. The verdict was that it’s just fine! And the exploding cap was just very bad luck. The inside appeared clean, so whoever sorted it out at Emporium’s end seems to have done a careful job.
 
mandryka said:
One of these?

https://www.screwfix.com/p/lap-mas830b-digital-multimeter-600v/75337

Or if not, if you could find a link to one, that’d be great.
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snowman_al said:
That one is fine. You will need to check the 'bias' from time to time as the large valves will change from being used. Radford made it really easy as Jez says in #194.
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Well I’ve had a lesson on how to check the bias, and I must say it seems a fiddle. You have to hold one probe on the chassis and stick one probe in the little hole and then turn the screw with a screwdriver. That means you need three hands at least. I think the problem is that I just bought the cheapest multimeter and the probes don’t make a good firm connection.

So basically can anyone recommend a multimeter, whatever the cost, which will make it easy to adjust the bias on my Radford?
 
I’ve got a lovely Fluke up for grabs in the DIY classifieds room! A really superb multimeter.
 
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