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Old 31st Jan 2010, 2:47 pm   #1
Gonzini
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Default Cooper Smith Amplifier

Hi all
Good to see the forum is still very active
A friend has given me his Cooper Smith amplifier to repair.
We think its 1961 vintage, has 2 x ECL82 per side and an EZ81.
Looks like its based around the Mullard designs of the time.
Anyone have any data on these, there's not much if anything ive found on the web as of yet.
It needs re-caping for sure but im in need of some info about the output transformer wiring. Should be very straight forward but i think some external jumper's are missing which would explain a few things...
Cheers
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Old 31st Jan 2010, 4:00 pm   #2
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Default Re: Cooper Smith Amplifier

Well, you need to join the forum to get them. but...

http://www.wduk.worldomain.net/forum...ead.php?t=2915

Photos help by the way.

Cheers,

Steve P.
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Old 31st Jan 2010, 4:56 pm   #3
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Default Re: Cooper Smith Amplifier

Hi Steve
arr yes i found that website and have joined but i cant download or view the images as of yet.
Also i think this might be a differant model using EL34's
I think the one i have here will be around 10W max per channel.
l'll post up some pics when im back in the shed tomorrow.
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Old 31st Jan 2010, 4:57 pm   #4
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Default Re: Cooper Smith Amplifier

Hi, there were a number of Cooper -Smith designs, if you have no luck with the site you could try Radio Constructor mag. This used to publish the Cooper-Smith circuits.
If you are based at the Wickham in NE England I'm not far away and may be able to find the details.

Ed
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Old 31st Jan 2010, 5:18 pm   #5
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Default Re: Cooper Smith Amplifier

Hi Ed
Ive been looking to see if there are any scans available from Radio Constructor mag, found a few sites but not the info im trying to find.
Ive side tracked a number of times today theres so much interesting stuff you find along the way!
Im Wickham on the southcoast.
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Old 31st Jan 2010, 9:50 pm   #6
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Default Re: Cooper Smith Amplifier

At 2*ECL82 per side it sounds like about 7wpc. That's what the similarly bottled amp in my parent's Black Box stereogram managed.

The classic Mullard UL design is here: http://www.bonavolta.ch/hobby/en/audio/ecl82_2.htm

Flat out non UL about 9W maybe.
Chris
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Old 31st Jan 2010, 11:56 pm   #7
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Default Re: Cooper Smith Amplifier

Hi Chris
Thats looks very close without the amplifier in front of me.
I think you right on the power output.
Any idea what sort of DC resistance i should expect to see on each of the output windings to ground?
I think there might be a pair of jumpers missing on the rear that select the impedance, a quick check the other day didnt give any output but i could hear 1khz tone that i was using from the valves / output TX.
Someone has lashed up 2 bits of 2 core 1mm mains cable terminated into crocodile clips for the speaker outputs! It needs a fair bit of TLC...
I think HT was around 240V which seemed a little low to me, but im sure something would be happening with that.
Will post some pics up soon, its a pretty rare amp i believe.
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Old 1st Feb 2010, 12:53 pm   #8
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Default Re: Cooper Smith Amplifier

If it's wired like the Mullard circuit, I wouldn't expect to see more than a fraction of an ohm between any output tap and chassis. Anything higher would lose a lot of the power output if it was winding resistance!

Don't forget most multimeters have measurable lead resistance (short the leads together, note the reading, then subtract this reading from any subsequent measurement of low resistances).

If it's a different circuit, possibly without any feedback, the resistance might be anything- even open circuit, without it necessarily being a fault.

Well worth checking the continuity of ALL of the output transformer windings, though. The primaries on small p-p transformers like these are not particularly robust

Chris
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Old 1st Feb 2010, 3:21 pm   #9
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Default Re: Cooper Smith Amplifier

Hi Chris
Well.... The plot thickens.
Basically the output TX has 2 separate secondary windings.
I have a red / yellow winding which the red has been grounded, looks original.
Then theres another winding with blue / yellow / brown.
None of these are grounded. Also the wires are doubled up in pairs, same colour.
This would explain the lack of output as someone connected accross the grounded red on the single winding and on the blue on the other winding
I measure low resistance on the red / yellow @ 183 milliohms, then between the blue / yellow / brown 46 / 60 / 93 milliohms between them.
That all looks good, so whats what??!!
Negative F.B is taken from the brown wire.
I wonder if these 2 windings need series up together which might of been done on the rear plug when inserted?
The mullard design is differant, it has an extra ECC83.
Very similar it some respects with circuit design, some componets differant values and minus the ECC83.
Ive attached a few pics.
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Old 1st Feb 2010, 5:15 pm   #10
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Default Re: Cooper Smith Amplifier

Just as an aside those speaker sockets look just like the one on my EKCO Car Radio - see http://www.vintage-radio.net/forum/s...550#post300550.
Alan
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Old 1st Feb 2010, 5:46 pm   #11
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Default Re: Cooper Smith Amplifier

They are! I have no plugs either but we will end up with binding posts im sure if i can it going ok.
There are others on the chassis with HT, mains and heaters. All very close togother....

Last edited by Gonzini; 1st Feb 2010 at 5:54 pm.
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Old 1st Feb 2010, 6:02 pm   #12
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Default Re: Cooper Smith Amplifier

I suppose the good news is that they must have been commercially available plugs and sockets. The bad news is no-one has come out of the woodwork and said 'I've got lots of them'
Your original Pics, Post #9, are OK. Have you loaded some more? The process has been pretty straightforward since the last software upgrade.
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Old 1st Feb 2010, 7:52 pm   #13
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Default Re: Cooper Smith Amplifier

Hi Gonzini, it could be the Copre-Smith Prodigy from OCT/NOV 58 RC. This used 2, ECL82's in UL push-pull with an ECC83 as amp/phase splitter and an EZ81 as rect.
The circuit only shows a single sec winding on the trans, but there are connection details of a small 5 pin plug that can be linked for 3/8/16 ohms.
Main smoothers were rated at 350v, so HT would not be much above 250V.


ED
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Old 1st Feb 2010, 8:29 pm   #14
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Default Re: Cooper Smith Amplifier

Hi Alan, Ed

Our internet connection is very slow here at times, i didnt wait long enough for them load! Havan't posted any more but the bigger pics are on the server now.

ftp://bcs.zapto.org/ftp/upload/Cooper-Smith/

The owner wants to put 4mm sockets on there somewhere so yet work that bit out.

This chassis just has the ECL82's and the EZ81.
They must use one half as a phase splitter and the other as an input stage.
I think im going to have to sketch bits of it out, shouldn't take too long...
Defo on the 2 separate secondarys, ive attached a badly drawn diagram.
Not sure what to do about these windings then. I guess if i series them up for now and not present a too low a load i can at least test it somewhat further.
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Old 1st Feb 2010, 8:42 pm   #15
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Default Re: Cooper Smith Amplifier

Hi,
Not sure how big those 5 pin sockets are but they look remarkedly similar to the series of connectors (2,3,4 & 5 pin) used on the old ht/lt batteries. Actually have a 5 pin socket but only one 4 pin plug and it will plug into the 5 pin socket. Not sure which battery could have used the 5 pins.
Hope this may help.
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Old 1st Feb 2010, 9:00 pm   #16
Ed_Dinning
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Default Re: Cooper Smith Amplifier

Hi Gonzini, the Prodigy has the ECC83 as a pre-amp with tone controls, main amp could be considered as just the 2, ECL82's and the rect.
Feedback circuit in this one has 3n0F and a 680R, not too different from yours. Pri side of the transformer is the same.
The sec appears to have double yellow, single yellow, single red,double brown and double blue leads.
From your circuit the must be a return path for the fedback network, so I would linkyell and blue, this is the config for 15R, I would think. connect and 8R or 15R speaker to these points and see what you get. Running like this for a short time will not cause any problems, but may give increased levels of distortion.
This set may be the Cooper -Smith Bantam, but I havent located the circuit for that yet


Ed
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Old 2nd Feb 2010, 12:05 am   #17
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Default Re: Cooper Smith Amplifier

Hi Gonzini,

sounds like Ed has more info to go on than I do. I'd second the seriesing up of the output windings he suggests- otherwise the NFB has no complete circuit.

It'll be interesting to see whether the circuit is just the phase splitter and output stage a la Mullard, or whether it's a concertina phase splitter using one of the ECL82 triodes, with the other as a preamp.

In any case, given the size of those O/P TXs, it should sound quite good. They're about twice as big as the ones in the Pye Black Box amp- more like the size of a 5-10 output tranny!

Chris
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Old 2nd Feb 2010, 12:32 am   #18
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Default Re: Cooper Smith Amplifier

Hi guys
Thanks for thinking about this one. It will be good to get it all up and running.
l'll series up the 2 windings then, that does seem to be the way forward. Any ideas why they might of done it like that?
Are there any pics of the Prodigy or Bantam out there, i'd never heard of these until now though i dont do much old HiFi these days.
It sounds like the Prodigy is closer circuit wise than the Mullard, which would make sense i guess.
If i get a chance l'll sketch the circuit out and post it up. Looks like it would be a good addition to the collection.
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Old 2nd Feb 2010, 8:37 am   #19
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Default Re: Cooper Smith Amplifier

I think this is what you're looking for
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Old 2nd Feb 2010, 10:57 am   #20
Gonzini
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Default Re: Cooper Smith Amplifier

Hi Michael.
Getting much closer!
Output TX is still differant, but the ouput stage looks very close.
Have you got a higher res scan of the component values you could email on? Im struggling to read the values
Might even get a signal out of it later today...

Heres a new link to the server URL. I pasted the wrong one in above, but couldn't edit it by the time i noticed..

ftp://bcs.zapto.org/upload/Cooper-Smith/
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