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Old 20th Jun 2021, 11:36 pm   #1
RojDW48
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Default Radford STA12

My uncritical devotion to old Radfords has made me buy this rather rare auction find. Looks to be complete with the odd competent looking repair. I am waiting for the PS caps to arrive before warming it up (a little bulge in the 20 + 20 can). I wondered if any members had done up one of these? Had a any wise words to share?
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Old 20th Jun 2021, 11:55 pm   #2
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Default Re: Radford STA12

I own an STA15 which I repaired quite a few years ago now. I'm afraid the electrolytic capacitors don't age well. They might be reformable, but one of mine (an HT smoother) was essentially open-circuit (I guess completely dried out). The main effect of this was to cause a very low frequency oscillation to start in that channel - something like 2Hz. It's a testament to the very broad bandwidth of the Radford output transformer that this got through it, and I spotted the fault by seeing the woofer cone in my speaker moving slowly in and out !

The other issue is that the insulated stranded wire used to connect the tagboard to the valve pins can be very prone to stiffening, particularly where it's been running warm, close to the valve sockets. It might look OK at a glance, but closer inspection can reveal that almost all the strands have fatigue-cracked and the connection is literally hanging on by a thread or two.

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Old 21st Jun 2021, 12:00 am   #3
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Default Re: Radford STA12

Good advice GJ - thanks.
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Old 21st Jun 2021, 6:22 am   #4
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Default Re: Radford STA12

Yes I have one. It is a gem.
It was a complete wreck when I got it. Yours looks really nice and very nearly un-touched in comparison.
I had to strip it down completely and reform the chassis, its much thinner and smaller than the STA15s, and was badly bent. Also one of the output transformers was dead, but Will at Radford Revival made 2 new ones for it. There is a AOS thread about it here: https://theartofsound.net/forum/show...-Rebuild/page3

Alan

PS get rid of the two Toffee Wimas straight away!
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Last edited by snowman_al; 21st Jun 2021 at 6:36 am. Reason: PS
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Old 21st Jun 2021, 6:35 am   #5
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Default Re: Radford STA12

The Original MA15 (ECC83 phase splitter) circuit is a very close match for board layout and component values etc.
I re-hashed a copy for the STA12 with some voltage values.
Alan
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Old 21st Jun 2021, 6:54 am   #6
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Default Re: Radford STA12

Watch out for the two dark browm Wima capacitors at either side... could be very leaky.
Looks a lovely example of this amplifier
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Old 21st Jun 2021, 2:06 pm   #7
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Default Re: Radford STA12

Quote:
Originally Posted by snowman_al View Post
The Original MA15 (ECC83 phase splitter) circuit is a very close match for board layout and component values etc.
I re-hashed a copy for the STA12 with some voltage values.
Alan
Thanks Alan - that is really helpful (and a superb job of work if I may say so!). Will at RR pointed me at that circuit diagram, but I am happy to see the added voltage values - high hopes!
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Old 22nd Jun 2021, 5:55 am   #8
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Default Re: Radford STA12

Nice amp Rog... in pic 1, post 1 there two hemispheres sticking up (look like frigate radar), what are they and what is their purpose?

Looks essentially a Mullard 5-20 so why only 15w?

Andy.
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Old 22nd Jun 2021, 2:14 pm   #9
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Default Re: Radford STA12

Quote:
Originally Posted by Diabolical Artificer View Post
Nice amp Rog... in pic 1, post 1 there two hemispheres sticking up (look like frigate radar), what are they and what is their purpose?

Looks essentially a Mullard 5-20 so why only 15w?

Andy.
Hi Andy - They are directly beneath the RCA inputs on top of the chassis, so I am guessing shielding? Though it seems a bit over the top! The amp is rated at 12W pc. Radford always under-rated as I understand it, based upon distortion levels - maybe that explains it.
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Old 22nd Jun 2021, 2:31 pm   #10
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Default Re: Radford STA12

Quote:
Originally Posted by Diabolical Artificer View Post
... Looks essentially a Mullard 5-20 so why only 15w?
It is a bit odd, but there is some insight here http://www.audiomisc.co.uk/HFN/Radford/revisited.html.

At the other extreme Beam-Echo's DL7-35 is the most faithful commercial reproduction of the Mullard 5-20 that I've come across and that does deliver 27W at 0.1% THD, and can be got down to ~0.05% if enough care is put into selecting the valves.

Cheers,

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Old 22nd Jun 2021, 2:40 pm   #11
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Default Re: Radford STA12

My guess is the same as yours, it's for shielding. It is not there on later amps.

Output power is dependant on the +HT volts?
Mullard 5-20 it is around 430 volts at the anodes.
MA / STA15 has about 355 volts at he anodes, and
STA12 has about 310 volts on the anodes.

The Radfords are under run with respect to the Mullard design and the output power is suspect is quoted at a specific distortion level. Plus they do give their output valves an easier life.
They both do more in actual life if you 'turn them up'!
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Old 22nd Jun 2021, 5:08 pm   #12
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Default Re: Radford STA12

Quote:
Originally Posted by snowman_al View Post
My guess is the same as yours, it's for shielding. It is not there on later amps.

Output power is dependant on the +HT volts?
Mullard 5-20 it is around 430 volts at the anodes.
MA / STA15 has about 355 volts at he anodes, and
STA12 has about 310 volts on the anodes.

The Radfords are under run with respect to the Mullard design and the output power is suspect is quoted at a specific distortion level. Plus they do give their output valves an easier life.
They both do more in actual life if you 'turn them up'!
My 'other' amp is Woodside's Radford Renaissance STA35. After I got it I called Mike Davis (Woodside) to ask about the difference. He said that, apart from more modern components, there wasn't any - the STA25 was always good for 35W but rated 25 in relation to THD - 0.1% I think.
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Old 23rd Jun 2021, 6:31 am   #13
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Default Re: Radford STA12

Thanks all. Those shields are weird, you wouldn't think they'd do owt, but they must have had an issue or else why fit em.

With an anode HT of 310v, why use EL34's and not EL84's? Again there must be a reason; better THD specs? That said my limited messing with valve amps shows 0.1% THD is easily achieved without much fuss.

Andy.
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Old 23rd Jun 2021, 9:11 am   #14
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Default Re: Radford STA12

Quote:
Originally Posted by Diabolical Artificer View Post
... That said my limited messing with valve amps shows 0.1% THD is easily achieved without much fuss.
I take it that's irony Andy . As for the EL34/EL84 thing, it is possible to make a nominally 12W EL84 amplifier with 'point one' percent distortion. Harold Leak's TL/12 Plus is the most famous example (and it would be easier these days, when we don't need the thing to have 125mV-input-for-full-output sensitivity). But boy, was there some pain involved ...

Cheers,

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Old 23rd Jun 2021, 12:28 pm   #15
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Default Re: Radford STA12

Quote:
Originally Posted by GrimJosef View Post
Quote:
Originally Posted by Diabolical Artificer View Post
... Looks essentially a Mullard 5-20 so why only 15w?
It is a bit odd, but there is some insight here http://www.audiomisc.co.uk/HFN/Radford/revisited.html.

At the other extreme Beam-Echo's DL7-35 is the most faithful commercial reproduction of the Mullard 5-20 that I've come across and that does deliver 27W at 0.1% THD, and can be got down to ~0.05% if enough care is put into selecting the valves.

Cheers,

GJ
Thanks GJ - that explains it in terms even I can understand. In the 60s I worked at the university of Bristol School of Chemistry we must have had 60 or 70 Labpacks there alone - they were legion! Heaven knows how many they sold altogether.
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Old 9th Aug 2021, 3:58 pm   #16
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Default Re: Radford STA12

Many thanks are due to JohnBS for taking on the re-furb when it got beyond my paygrade (didn't take long)! A lot more work than I had expected and an excellent result
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Old 9th Aug 2021, 4:14 pm   #17
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Default Re: Radford STA12

Such an interesting thread! Please post pics of the final refurb.
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