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cyl 23rd April 2007 07:21 PM

Push Pull Output Transformer wiring problem
 
I recently bought a pair of vintage push pull output transformer but I have a problem to identify the wires. I can identify the primary section and the secondly section. But I don¡¦t know how to find out the plate wire and the power input wire.

Suppose the correct connection is P1-B1 connects to B2-P2. What will be happen if I guess it wrong? Let say, 1) B1-P1 connects to P2-B2 or 2) P1-B2 connects to P2-B2.

I know in the second case may damage the coil. What about in the first case? Is there any side effect? How to identify the wires?

gofar99 23rd April 2007 09:05 PM

Greetings, One way to determine which windings are plate taps and which is the center tap (for power) is to use a meter that can read fairly low resistances. Even premium transformers have a certain amount of DC resistance in the windings. From your description it seems that the transformers are for push pull use. If so, and they aren't for ultra linear use there will be two plate taps and one center tap on each transformer. If you measure the resistance between them, the reading between the two plate taps will be higher than to the center tap. For example if the reading plate to plate is 20 ohms, the reading to the center tap from either plate tap will be half that reading or 10 ohms. The secondary windings (speaker) will have rather low resistance readings and unless your meter can read very low ohms (0.1 and lower) it will be difficult to separate the various taps. If there is a single secondary (only 8 ohms or 4 or whatever) then you are home free. If the transformer is for ultra linear use, it may be possible to measure the resistances with an accurate meter. The values for the screen taps will be less than that for the plate taps but not zero. For example if the plate to plate value is 20 ohms then the screen to screen might be 12. from one plate to its corresponding screen would be around 4 ohms. The configuration would be: plate 1 to screen 1 to center tap to screen 2 to plate 2. I hope this helps.

Good listening
gofar99

billr 24th April 2007 03:13 AM

As gofar99 says a multimeter will do the first stage.

work out which of the wires are common to each other, then note the colours of the wire etc.

then as gofar99 says, write the colurs of wire down, then measure the resistances between them all, and write them down too. From this it will soon become apparent which is which.

One trap though, the primary resistance between the two halves is very unlikely to be the same, it will be within a few tens of ohms of each other, for example, i have some old trannies that are 270 and 300 ohms each half. this is to do with the geometry of the windings, and as one in nearer the centre, it has less wire per turn, and so over hundreds of turns, it makes a big difference.

One wy to work out the Z ratio, is to connect a 100V ac source to the primary, and measure the output on the secondary, this gives the direct turns ratio, squaring this gives the Z ratio.

hope that this helps

bill

gofar99 24th April 2007 03:39 AM

Greetings, Quite correct. I would caution that you be careful if you apply voltages to the transformer. If you have the wrong set of windings it an act as step up transformer and pose a serious electric shock hazard. I would suggest you apply a low voltage (for instance from a 6 volt heater transformer) to what you believe are the primary (plate to plate) and measure the outputs. The lowest impedence (like 4 ohms) will have the least voltage. The actual voltage will depend on the ratio of the windings in the transformer and may be quite low. This may require a meter that can read in the 0.1 volt or less range if everything is connected and identified correctly. Start off with the meter in a high range though (2-300 volts) to protect the meter, just in case you have identified the windings wrong. If you accidently apply the low voltage to the secondary windings, the transformer will act as a step up and mayl pose a shock hazard, but not nearly as serious as if you use a higher voltage. In either case, I would place a series resistor between the applied voltage and the transformer. This will limit the current available and if you have the windings identified incorrectly will protect the transformer and your voltage source. Pretty much any value will do. I would suggest something like 100 ohms for low voltage input and 1000 if you go fo the higher voltage. Since it won't draw much current a 1 watt resistor should be OK. Keep us posted on how it goes.

Good listening
gofar99

cyl 24th April 2007 05:08 AM

I can determine the primary section and the secondary section. The primary section has higher resistance. My problem is in the primary section. The primary section separates into two coils (Totally 4 wires, all in the same color). I can check each pair of coil is about 100ohm. So, the problem is which two wires connect together? Which wire is for plate and which wire is for power in?

tjl 24th April 2007 09:43 AM

Please mark your output transformer primary coil1 terminals as 1A &1B, coil 2 as 2A & 2B,

then apply any low AC input (6.3 or 12 V) to 1A & 1B terminals, First connect 1B to 2A then use AC meter measuring 1A and 2B ,

IF meter reading display voltage double (12 or 24 V), It means 1A compare to 2B is out of phase, 180 degree reverse each other, then 1B & 2A will be B+ input, 1A will be P1, and 2B will be P2.

IF meter reading become to ZERO, it means 1A compare to 2B is in phase, then 1B & 2B are B+, 1A will be P1, and 2A WILL be P2.

cheers

cyl 24th April 2007 03:32 PM

Thanks a lot. Now I know how to connect two coils together out of phase. But still have two combinations, (1A 1B)-(2A 2B) and (1B 1A)-(2B 2A). Does it affect the preformance? or just inverse the output in the secondary.




Quote:

Originally posted by tjl
Please mark your output transformer primary coil1 terminals as 1A &1B, coil 2 as 2A & 2B,

then apply any low AC input (6.3 or 12 V) to 1A & 1B terminals, First connect 1B to 2A then use AC meter measuring 1A and 2B ,

IF meter reading display voltage double (12 or 24 V), It means 1A compare to 2B is out of phase, 180 degree reverse each other, then 1B & 2A will be B+ input, 1A will be P1, and 2B will be P2.

IF meter reading become to ZERO, it means 1A compare to 2B is in phase, then 1B & 2B are B+, 1A will be P1, and 2A WILL be P2.

cheers


Dave Cigna 24th April 2007 11:30 PM

Quote:

Originally posted by cyl
Thanks a lot. Now I know how to connect two coils together out of phase. But still have two combinations, (1A 1B)-(2A 2B) and (1B 1A)-(2B 2A). Does it affect the preformance? or just inverse the output in the secondary.

To put it bluntly, nobody here could possibly know which way is 'better' (unless they happen to have inside info, but I doubt that...)

I can think of no reason why there would be any risk in either connection. The transformer is not going to burst into flames either way. Or, if it is, then it will do so both ways. :)

Your only option, it seems, is to try it both ways and decide for yourself. If you have a signal generator and an oscilloscope then you can measure frequency response; you might find that one connection has greater high frequency range due to different capacitance at the two ends of each primary winding. But, maybe not. If you don't have access to the equipment then just use your ears.

BTW: I'm assuming that there is absolutely no markings on any of the wires, that they are all exactly the same color, etc.

-- Dave

tjl 25th April 2007 02:23 AM

Thanks a lot. Now I know how to connect two coils together out of phase. But still have two combinations, (1A 1B)-(2A 2B) and (1B 1A)-(2B 2A). Does it affect the preformance? or just inverse the output in the secondary.
.................................................. ..................................................
Two kinds of connecting methodes are same effects for power output stage,

But if get feedback form secondary 8 ohm or 16 ohm or other terminals to voltage amp or driver stage, it may be negative feedback, that is correct: or posiyive feedback that make loud noisy from output stage and speaker.

Positive feedback need 2 to 3 seconds gradually creating oscillation after tube warm up, then you must turn off power and reverse polarity of the secondary (feedback)2 wiring connection

Or change plate1 & plate 2 wiring connection,

chose one methode as your converience.

If you have signal generator and scope as Mr.Dave Cigna mention about, may be more helpful for your testing.

cheers


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