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Old 21st March 2009, 07:45 AM   #1
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Location: Simon's Town, Cape Town.
Default OPT Trafo

Hello all.
I have stumbled upon a old looking OPT and have no idea what it's specs are or where it came from. I guess its from the 25 (ish) watt range. All I can tell it has a faded lable with anode, screen and CT taps on the primary and all I can make out on the secondary is a 15 ohm tap for shure. There are two other numbers: 3020 and MA 307
The core (EI) is about 95 X 80 X 34 mm. I have googled it with no joy.
Hope anyone can help.
Cheers, Carl
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Old 21st March 2009, 11:42 AM   #2
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Have you got a good digital multimeter?


Can you identify the leads? It's important to identify the A1 and A2 (outer) leads and the HT lead (primary centre tap). Typically you'll have 100 - 300 ohms series resistance between A1 and A2 and approximately half that between either A1 and HT and A2 and HT.

You didn't mention it but are there any ultralinear taps to identify?

The secondary taps - in the heyday of valve amplifiers, speakers were either 16 ohms (series secondaries) or 4 ohms (parallel secondaries). The secondaries should have about 0.7 ohms each on your DVM.

To identify the impedance of the primary connect A1 and A2 to household current. Fashion yourself a plug with a short lead (best to use an inline fuse too) and read the resulting voltage across the primary accurately.

Calculate the turns ratio (N) by dividing the mains voltage by the voltage across one of the secondary windings.
For example with a 250V mains voltage and a hypothetical 5.6 V secondary reading:

N = 250 / 5.6 = 45

Impedance is changed by N^2 so N^2 = 45^2 = 2025 and because the secondary voltage was measured across a 4 ohm section the primary impedance must be:
2025 * 4 = 8100 ohms.

(with credit to Morgan Jones)

Hope this helps.
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Old 21st March 2009, 01:41 PM   #3
Funker is offline Funker  Germany
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Hi Baldrick,
the way you desribe to check out unknown iron is the same I would do so.

But it is not a good idea to fiddle about with the mains voltage especially for newbies. To work on hot mains circuits is for experts only.

The other thing is some countries have mains supplies without a neutral wire. The have two hot wires against earth. That makes essential to use an isolating transformer at the bench.

Here in Germany and the most of the EC countries having a 4 wire 230/400V 3 phase+N AC Mains. The domestic outlets are connected to neutral and one of the 3 phase .

I have a differential current circuit breaker in my fuse box( also known as GFIs E.L.C.Bs or RCDs or whatever) for my personal safetety .


But the best way is to use a lower and mains isolated Voltage so as 12, 24 or 42 Volts . Any Trafo is good which you have in your part drawer. The results are the same but safe.

And even 115V can be dangerous !

Never haggle with the safety rules as you have only one life!

My workshop safety equipment:

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

Isolationstransformer/Variac

Mains stabilizer

Bench panel

regards
Wolfgang
DF6ZC
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Old 21st March 2009, 03:05 PM   #4
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Hi<
Thanks for the replies.
Ok, I have 6 taps on the primary. The 1st one I can't make out, the next, "A" which I assume is anode, then SC which I assume is an ultralinear (43%?) tap, then "CT" which seems to be the centre tap. From there another "SC" and again a "A".
If my assumptions are correct I recon I've got the primary sorted, but for the first tap. The lable is old and faded.
On the secondary there is a clarly marked 15 ohm tap but the other 3 taps have been faded or moth eaten.

I'll definitely use your help, I'm in the process of budgeting for a good variac so I'll have one soon enough.
I was hoping if anyone perhaps knew which amp it came from.
Many thanks again.
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Old 21st March 2009, 04:06 PM   #5
pointy is offline pointy  United Kingdom
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i would think you have an out-put transformer

marked:-

anoid(end)-screen grid(tap)-centre tap-screen grid(tap)-anoid(end)

one side

and the other 32 ohms-16 ohms-8 ohms-4 ohms

a 1950s onwards out-put transformer (british)........nice!
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Old 21st March 2009, 08:53 PM   #6
Funker is offline Funker  Germany
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Hi

You wrote:

On the secondary there is a clarly marked 15 ohm tap but the other 3 taps have been faded or moth eaten.

To find this out, apply a very low ac voltage between the "15 ohm" and common. It may somewhat about 6V or less from a heater winding of a mains trafo or an ordinary Bell trafo.

Be carefully , using alligator clips and secure all parts against slipping from the bench, cause you get high voltage on the primary while do the checkout. Better you insulate the primaries with a piece of eletrican Tape. Your fingers will thank you for that.

If everything is well prepared check the voltage between all the secondary taps. Wrote down all values on a piece of paper with a schematic of the secondaries.

For example you find a tap with 3 ,5 Volt and the feeding voltage is 7V (normally a 6,3V heater winding with no load) you actually found the 4 Ohm tap.

the impedance relation is square by voltage relation. Half the volt is quarter the impedance.

But maybe you have a special version of an output transformer , the one with an extra nfb ( negative feedback winding.

Now its neccessary to find out which one is the nfb winding . They do not have to deliver much current to the feedback loop, so the wire is much more smaller than the one for the speaker windings. But the voltage are somewhat in the same range like on the other taps.
So the dc resistance of the nfb winding is greater , the speaker windings have somewhat of about 0,5 ohms but the nfb winding raise up to about 3-5 Ohms.

Finally ,
check dc resistance of the secondary, if the are in a low range by 0..... ohms there probably no extra nfb winding present.

If you find a 2-5 Ohm one there will be a nfb winding or a 100V line output.

7 Volts on the 16 ohm tap give you
5 Volts on a 8 ohm tap
3,5 Volts on a 4 Ohm tap
2,5 Volts on a 2 ohm tap ( rare but common on jukje box trafos

the 100V tap has different voltage , the impedance of this output depends on the power for what the trafo is designed.
A 50 watter has 200 Ohms on the 100V line, in your example you will find 17 Volts
and 34V for a 25 watter.
These trafos comes normally from PA Amps which have do drive long speaker lines( supermarkets, stations, hotels an so on.

So I hope I bring a little darkness in your brightend world :-))

And excuse me for possible mistakes in grammar , Im not natve english.

regards
Wolfgang
DF6ZC
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Old 22nd March 2009, 04:38 PM   #7
Jaap is online now Jaap  Netherlands
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Default I have a problem

I found an old EHRL OPT (type A-472) that someone probably took from an old amp or radio.
the specs are: prim 10K; sec 3,5 ohm; 3VA

I put 10,32 volt on one side (2 thin wires) and get 0,394 volt on the opposite (4 thicker wires, 2 windings of each 0,3 ohm dc resistance which i connected in series).

So the winding ratio measured by me is 1 : 26; 26 x 26 = 676; so with 3,5 ohm impedance secundary the primary impedance should be 2366.

What or who is wrong here ?
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Old 22nd March 2009, 05:26 PM   #8
Jaap is online now Jaap  Netherlands
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I think I solved it. If I take one of the secundary windings the transformers windings are 1:50. With 4 ohm speakers you get a primary impedance of 10K.


What can you do with a 10K se transformer that is 3VA (leistung in German) ?
Why are there 2 secundary windings ?
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Old 22nd March 2009, 06:15 PM   #9
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Hi Dynsdale!

There are several folks in your vicinity who can help you. Either e-mail me privately, or put your problem on www.avforums.co.za under 'Vinyl and Tube Amps' and you will get a reply.
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Old 22nd March 2009, 06:18 PM   #10
Funker is offline Funker  Germany
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Hi Jaap,
you have probably a small opt from a radio or tape recorder which worked with an EL95 in the output stage. Have a look at the data sheet here. http://tubedata.tigahost.com/tubedat...030/e/EL95.pdf

The sign VA means apperance Power which contains the reactive component power which is always present in AC circuits with inductive or capacitive components.

look here about AC Power:

http://en.wikipedia.org/wiki/AC_power


The 2 secondairies allow different speaker configurations e.g. both winding in series for 16 Ohms and both in parallel for 4 Ohm . Older trafos are normally designed for 5/ 15 Ohms load.

But this means that both windings are not equal an should not be paralleled. The windings relations in this case are than 1:1,73 . They shoud be connect in series an so you alter for a 15 Ohm speaker from end to end or a 5 ohm speaker from low end to tap.

That trafo will give a nice little amp for office use on your PC.
good luck!

regards
Wolfgang
DF6ZC
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