UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Audio (record players, hi-fi etc)

Notices

Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

Reply
 
Thread Tools
Old 20th Jun 2021, 10:36 pm   #1
RojDW48
Octode
 
RojDW48's Avatar
 
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 1,818
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?
Attached Thumbnails
Click image for larger version

Name:	P1010002.jpg
Views:	138
Size:	70.3 KB
ID:	236161   Click image for larger version

Name:	P1010001.jpg
Views:	132
Size:	54.9 KB
ID:	236162   Click image for larger version

Name:	P1000998.jpg
Views:	153
Size:	106.7 KB
ID:	236163  
__________________
'....don't go mistaking Paradise for that home across the road!' (Bob Dylan)
RojDW48 is offline   Reply With Quote
Old 20th Jun 2021, 10:55 pm   #2
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 3,591
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.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline   Reply With Quote
Old 20th Jun 2021, 11:00 pm   #3
RojDW48
Octode
 
RojDW48's Avatar
 
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 1,818
Default Re: Radford STA12

Good advice GJ - thanks.
__________________
'....don't go mistaking Paradise for that home across the road!' (Bob Dylan)
RojDW48 is offline   Reply With Quote
Old 21st Jun 2021, 5:22 am   #4
snowman_al
Heptode
 
snowman_al's Avatar
 
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 822
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!
Attached Thumbnails
Click image for larger version

Name:	IMG_7689.jpg
Views:	113
Size:	105.5 KB
ID:	236172   Click image for larger version

Name:	IMG_7805.jpg
Views:	99
Size:	54.5 KB
ID:	236173   Click image for larger version

Name:	IMG_0321.jpg
Views:	103
Size:	111.8 KB
ID:	236174   Click image for larger version

Name:	IMG_0313.jpg
Views:	98
Size:	87.8 KB
ID:	236175   Click image for larger version

Name:	IMG_0330.jpg
Views:	88
Size:	57.9 KB
ID:	236176  

__________________
Never Leave Well Enough Alone...

Last edited by snowman_al; 21st Jun 2021 at 5:36 am. Reason: PS
snowman_al is offline   Reply With Quote
Old 21st Jun 2021, 5:35 am   #5
snowman_al
Heptode
 
snowman_al's Avatar
 
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 822
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
Attached Thumbnails
Click image for larger version

Name:	Radford Transformers_and_Amp_circuits-001 - Copy-003.jpg
Views:	68
Size:	71.8 KB
ID:	236177  
__________________
Never Leave Well Enough Alone...
snowman_al is offline   Reply With Quote
Old 21st Jun 2021, 5:54 am   #6
PYE 405
Tetrode
 
Join Date: Dec 2009
Location: Chatteris, Cambridgeshire, UK.
Posts: 56
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
__________________
Andrew
PYE 405 is offline   Reply With Quote
Old 21st Jun 2021, 1:06 pm   #7
RojDW48
Octode
 
RojDW48's Avatar
 
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 1,818
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!
__________________
'....don't go mistaking Paradise for that home across the road!' (Bob Dylan)
RojDW48 is offline   Reply With Quote
Old 22nd Jun 2021, 4:55 am   #8
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 6,038
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.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 22nd Jun 2021, 1:14 pm   #9
RojDW48
Octode
 
RojDW48's Avatar
 
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 1,818
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.
__________________
'....don't go mistaking Paradise for that home across the road!' (Bob Dylan)
RojDW48 is offline   Reply With Quote
Old 22nd Jun 2021, 1:31 pm   #10
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 3,591
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,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline   Reply With Quote
Old 22nd Jun 2021, 1:40 pm   #11
snowman_al
Heptode
 
snowman_al's Avatar
 
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 822
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'!
__________________
Never Leave Well Enough Alone...
snowman_al is offline   Reply With Quote
Old 22nd Jun 2021, 4:08 pm   #12
RojDW48
Octode
 
RojDW48's Avatar
 
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 1,818
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.
__________________
'....don't go mistaking Paradise for that home across the road!' (Bob Dylan)
RojDW48 is offline   Reply With Quote
Old 23rd Jun 2021, 5:31 am   #13
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 6,038
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.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 23rd Jun 2021, 8:11 am   #14
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 3,591
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,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline   Reply With Quote
Old 23rd Jun 2021, 11:28 am   #15
RojDW48
Octode
 
RojDW48's Avatar
 
Join Date: Jun 2014
Location: Bristol, UK.
Posts: 1,818
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.
__________________
'....don't go mistaking Paradise for that home across the road!' (Bob Dylan)
RojDW48 is offline   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 11:58 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.