My Grandad has given me a single ended transformer that years ago he used for an EL84 Output Stage, however he can't remember what impedance the primary and secondary is. Its an RS Transformer that says 12833 on the side. Can't find anything on google. All four primary lugs are connected so I'm guessing there are two impedances available. Lug 1 on the secondary side is connected to the lug holding the board to the transformer and lug 3 isn't connected to anything.
Hopefully someone will recognize it!
Transformer Identification
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Littlewyan
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Transformer Identification
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Stevem
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Re: Transformer Identification
Do the old pump in low ac volts in thing to find out what impedance it is!
Pump in 1/2 a volt in on the secondary and then times what you read on the primary by 2, then square that and then times that by what you think the secondary tap is!
IE) 14 x 2 = 28, 28 x 28 = 784.
784 x 4=3136
Pump in 1/2 a volt in on the secondary and then times what you read on the primary by 2, then square that and then times that by what you think the secondary tap is!
IE) 14 x 2 = 28, 28 x 28 = 784.
784 x 4=3136
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Littlewyan
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Re: Transformer Identification
Ah yes I was going to do that test as well I'm just not sure about the secondary. There is one thick insulated wire connected to lug 2 and an uninsulated wire connected to lug 4. I'm guessing lug 2 is the ground? But then lug 1 has an uninsulated wire connected to it and is connected to the pin that holds on the board on top and I'm not sure if grandad did that.
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Stevem
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Re: Transformer Identification
Oh, got you!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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gui_tarzan
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Re: Transformer Identification
Lucky guy. My grandfather was an upholsterer. 
--Jim
"He's like a new set of strings, he just needs to be stretched a bit."
"He's like a new set of strings, he just needs to be stretched a bit."
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Littlewyan
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Re: Transformer Identification
My grandad is an amateur radio enthusiast so has a valve parts lying around. I've tried to convince him to build a valve amp or a valve radio but hes just given the parts to me instead. I'm not giving up on him though, there is always birthdays and Christmas .
With regards to testing it I THINK there might be an AC Power Supply at work but I believe its 24VAC. I guess using a 6.3VAC Heater winding won't hurt?
.With regards to testing it I THINK there might be an AC Power Supply at work but I believe its 24VAC. I guess using a 6.3VAC Heater winding won't hurt?
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martin manning
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Re: Transformer Identification
A 6.3VAC filament transformer will work fine for a low-voltage AC source. Your transformer may well be set up just like the modern offering below. I'd confirm that 1-4 and A-D are all on the same windings, check the DCR to see if that is in the ball park, and measure a few voltage ratios.
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Re: Transformer Identification
You can use the 6.3V or 5V winding on any PT that you might have. Also, if you have a AC wall wart (brick) that puts out anything up to about 13V, that should also be fine. Most wall warts are DC, so be sure to read the imprinting on the case.
If you need to, you can probably purchase an old filament transformer for very little money on eBay or possibly a new one in a store.
If you need to, you can probably purchase an old filament transformer for very little money on eBay or possibly a new one in a store.
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Littlewyan
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Re: Transformer Identification
Ok I'm going to use a 6.3V winding off of my little JTM1 amp as I've got the virtual center tap on the board so its easy enough to clip onto. So which lugs on the secondary should I hook onto? As one of the lugs is grounded with the pin holding the board onto the top but he has clearly lifted the other two lugs and soldered onto them. Also whats interesting is how the primaries are labelled F A B C.
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martin manning
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Re: Transformer Identification
Maybe start by measuring using the ones that were soldered. You could figure out the order of all the taps on both windings, but that'd take some work.
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Littlewyan
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Re: Transformer Identification
Ok here are my readings. I don't think I can use Stevem's equation as I'm using a different voltage. But I have done a few calculations using a website.
7VAC on the secondaries, here are the readings for the primaries with the calculation impedance ratio. Not sure about the lower voltage readings I took, also not sure about the secondaries as it seems 1 and 2 give the same readings as 2 and 4.......
1 and 4 - F and C = 227V - 1024:1
1 and 4 - A and B = 20V - 9:1
2 and 4 - F and C = 298V - 1849:1
2 and 4 - A and B = 26V - 16:1
1 and 2 - F and C = 299V - 1849:1
1 and 2 - A and B = 26V - 16:1
1 and 4 - F and B = 211V - 900:1
1 and 4 - A and C = 39V - 36:1
2 and 4 - A and C = 51V - 49:1
2 and 4 - F and B = 275V - 1521:1
Edit: I unsoldered Lug 1 from the pin on the transformer and it didn't make any difference to the readings. Connecting Lug 1 and 2 together gave the same reading as 2 and 4, not sure what thats about. Anyway looks like a rather useful transformer, judging by it's size its good for at least 5W I'd say. Not sure about the Primary Lugs A and B as I always got a low reading from them.
7VAC on the secondaries, here are the readings for the primaries with the calculation impedance ratio. Not sure about the lower voltage readings I took, also not sure about the secondaries as it seems 1 and 2 give the same readings as 2 and 4.......
1 and 4 - F and C = 227V - 1024:1
1 and 4 - A and B = 20V - 9:1
2 and 4 - F and C = 298V - 1849:1
2 and 4 - A and B = 26V - 16:1
1 and 2 - F and C = 299V - 1849:1
1 and 2 - A and B = 26V - 16:1
1 and 4 - F and B = 211V - 900:1
1 and 4 - A and C = 39V - 36:1
2 and 4 - A and C = 51V - 49:1
2 and 4 - F and B = 275V - 1521:1
Edit: I unsoldered Lug 1 from the pin on the transformer and it didn't make any difference to the readings. Connecting Lug 1 and 2 together gave the same reading as 2 and 4, not sure what thats about. Anyway looks like a rather useful transformer, judging by it's size its good for at least 5W I'd say. Not sure about the Primary Lugs A and B as I always got a low reading from them.
Re: Transformer Identification
Steven gave you the proper model, but applied it to a specific case, in other words, an example.
The general rule to use is that the impedance ratio is the square of the turns ratio. So, 7V in yields 227 out. 227/7 = turns ratio of 32:1. You make an assumption at some point to get primary:secondary impedance.
For example, applying the general rule or its corollary (the square root of the impedance ratio is the turns ratio) you have to figure out what you have. If you 8K:8 impedance ratio, that's the same as 1000:1. Sqrt(1000)-31.6. In this example, the turn ratio is 31.6:1. So, you might guess the 32:1 is 8K:8. The rest more or less flows from a single assumption and then you see if it is making sense in the context of what you have.
You've observed so far turns...likely impedance
32...8K:8
43...8K:4
30...7.2K:8
39...6.1K:4
You really should continue the tests until you have a full chart of all possible combinations. To do this you've got to develop assumptions to help you do the testing.
I'd start by assuming the transformer has one primary coil and one secondary coil (as opposed to having split coils that you might join in a variety of ways.) I'd assume the terminals are in sequential order of where the taps are placed on the coil. This assumption suggests that F-C is the full length of the primary and 1-4 is the full length of the secondary.
Now, there is nothing that says you can't use 2-3 or 3-4 as the secondary, so there are lots of possibilities to test. Cycle through the whole thing.
The best way to record this is in a grid. I think the attached covers it. After you get the voltage readings, convert the turns ratio to impedance ratio. Simply square the result.
The general rule to use is that the impedance ratio is the square of the turns ratio. So, 7V in yields 227 out. 227/7 = turns ratio of 32:1. You make an assumption at some point to get primary:secondary impedance.
For example, applying the general rule or its corollary (the square root of the impedance ratio is the turns ratio) you have to figure out what you have. If you 8K:8 impedance ratio, that's the same as 1000:1. Sqrt(1000)-31.6. In this example, the turn ratio is 31.6:1. So, you might guess the 32:1 is 8K:8. The rest more or less flows from a single assumption and then you see if it is making sense in the context of what you have.
You've observed so far turns...likely impedance
32...8K:8
43...8K:4
30...7.2K:8
39...6.1K:4
You really should continue the tests until you have a full chart of all possible combinations. To do this you've got to develop assumptions to help you do the testing.
I'd start by assuming the transformer has one primary coil and one secondary coil (as opposed to having split coils that you might join in a variety of ways.) I'd assume the terminals are in sequential order of where the taps are placed on the coil. This assumption suggests that F-C is the full length of the primary and 1-4 is the full length of the secondary.
Now, there is nothing that says you can't use 2-3 or 3-4 as the secondary, so there are lots of possibilities to test. Cycle through the whole thing.
The best way to record this is in a grid. I think the attached covers it. After you get the voltage readings, convert the turns ratio to impedance ratio. Simply square the result.
You do not have the required permissions to view the files attached to this post.
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Littlewyan
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Re: Transformer Identification
There is no lug 3 so I've pretty much tested all possible combinations. Although I didn't try Lugs F and A on the primary side, not sure why. Bit weird about A and B though can't work out why they'd be like that as you can't do much with a 9:1 Impedance ratio unless I'm missing something.
Re: Transformer Identification
This is what I believe to be a "universal" type. In such a configuration, I don't think it would be unusual to encounter an unusable combination. It is an artifact of the design and unlikely anyone thought it would be of use.
If you fill out the chart, omitting the 3 lug, we might be able to form a more complete picture and make a better guess.
What do the lams and bobbin measure? That's a hint for the watts rating.
If you fill out the chart, omitting the 3 lug, we might be able to form a more complete picture and make a better guess.
What do the lams and bobbin measure? That's a hint for the watts rating.
Re: Transformer Identification
Littlewyan, use your Ohm-meter to find windings first.
You will have, most likely, one high Ohm reading, witch is primary, other one maybe 1/10 of it on the same side, witch is B+ choke and one or two connected together, very low reading - your secondary.(your 1,2 and4)
Reading ac voltage without finding real windings will be confusing, your digital multimeter will show some fake ac voltages even not connected to the both sides of one winding.
After measuring the voltages you will calculate easy 4 , 8 or 16 Ohms secondary by assuming primary impedance around 4 to 5K. ( your calculated primary impedance will fall in that range)
So hook up 7v ac to 1 and 4, measure 1 to 2.
Measure F to A, B to C.
(Judging by you pictures F-A and B-C are separate windings, and 1-2-4 are connected secondaries for maybe 4 and 8 Ohms or 8 and 16Ohms, I'm not sure)
You will have, most likely, one high Ohm reading, witch is primary, other one maybe 1/10 of it on the same side, witch is B+ choke and one or two connected together, very low reading - your secondary.(your 1,2 and4)
Reading ac voltage without finding real windings will be confusing, your digital multimeter will show some fake ac voltages even not connected to the both sides of one winding.
After measuring the voltages you will calculate easy 4 , 8 or 16 Ohms secondary by assuming primary impedance around 4 to 5K. ( your calculated primary impedance will fall in that range)
So hook up 7v ac to 1 and 4, measure 1 to 2.
Measure F to A, B to C.
(Judging by you pictures F-A and B-C are separate windings, and 1-2-4 are connected secondaries for maybe 4 and 8 Ohms or 8 and 16Ohms, I'm not sure)