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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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20th Jun 2021, 11:36 pm | #1 |
Nonode
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 2,074
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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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20th Jun 2021, 11:55 pm | #2 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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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. Cheers, GJ
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21st Jun 2021, 12:00 am | #3 |
Nonode
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 2,074
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Re: Radford STA12
Good advice GJ - thanks.
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21st Jun 2021, 6:22 am | #4 |
Octode
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 1,092
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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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Never Leave Well Enough Alone... Last edited by snowman_al; 21st Jun 2021 at 6:36 am. Reason: PS |
21st Jun 2021, 6:35 am | #5 |
Octode
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 1,092
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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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21st Jun 2021, 6:54 am | #6 |
Pentode
Join Date: Dec 2009
Location: Chatteris, Cambridgeshire, UK.
Posts: 176
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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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Andrew |
21st Jun 2021, 2:06 pm | #7 |
Nonode
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 2,074
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Re: Radford STA12
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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22nd Jun 2021, 5:55 am | #8 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,637
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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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22nd Jun 2021, 2:14 pm | #9 |
Nonode
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 2,074
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Re: Radford STA12
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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22nd Jun 2021, 2:31 pm | #10 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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Re: Radford STA12
Quote:
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
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22nd Jun 2021, 2:40 pm | #11 |
Octode
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 1,092
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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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22nd Jun 2021, 5:08 pm | #12 | |
Nonode
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 2,074
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Re: Radford STA12
Quote:
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23rd Jun 2021, 6:31 am | #13 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,637
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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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23rd Jun 2021, 9:11 am | #14 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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Re: Radford STA12
Quote:
Cheers, GJ
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23rd Jun 2021, 12:28 pm | #15 | ||
Nonode
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 2,074
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Re: Radford STA12
Quote:
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9th Aug 2021, 3:58 pm | #16 |
Nonode
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 2,074
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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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9th Aug 2021, 4:14 pm | #17 |
Octode
Join Date: Feb 2009
Location: Wallasey, UK.
Posts: 1,308
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Re: Radford STA12
Such an interesting thread! Please post pics of the final refurb.
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