transformer purpose

Discussion in 'Shock Brother's DIY Amps' started by aaronftnm, Feb 18, 2009.

  1. aaronftnm

    aaronftnm TDPRI Member

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    I feel really silly asking this question since alot of you guys know so much about amp building, but what does an output transformer do?
    I think that I understand the concept of a power transformer, but correct me if I am wrong...does the power transformer take the wall current and "convert" it to usable voltages that are small enough to power the tubes?
    Its kinda of difficult just reading through a bunch of information without any discussion involved, I always think that I grasp the concepts until I talk to people about it.
     
  2. Papa Joe

    Papa Joe Friend of Leo's

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    Never feel silly about asking questions!!! Silly is not asking..
    Some one here will give you the answers..PJ...
     
  3. celeste

    celeste Friend of Leo's

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    You have the PT mostly right. In the case of tubes, they usually want a voltage that is higher then what comes out of the wall, so the PT raises the voltage to what the tubes want. The ratio of input to output is the same as the ratio of the number of turns on the primary and secondary windings. It also has a safety element. If you look at the schematic of a power supply, you see that a PT has two sets of windings that are not connected, the secondary winding is isolated from the primary. This isolation is very important because to get shocked by the secondary, you have to complete a circuit with something else connected to the secondary winding. Because the primary is connected to the mains supply and everything in the building is connected to the mains, and the mains are connected to a rod driven into the earth, there are a lot more things that could shock you to death without the isolation.

    The OT is an impedance matching device and a DC blocker. Most tubes are high impedance devices, most speakers are low impedance devices. There are exceptions to both, but they are rare and present a new set of problems to try to use. The OT matches the high impedance the tubes want ( about 2.5k ohms for say a KT88 in push pull to maybe 22K for a 12AU7 PP used in a very low power amp like a Firefly) to the low impedance a speaker wants, usually 4,8 or 16 ohms. The impedance ratio is the square of the turns ratio.

    The DC blocking parts happens because the primary and secondary windings are not connected to each other and only transfer voltage through changes in magnetic flux. The changes in flux are caused by changes in the current in the primary windings. Because music signal is an AC voltage riding on top of a stable DC voltage the flux changes with the AC and ignores the DC.

    There is a lot that is simplified here, maybe to much, maybe not enough. I hope this helps you to understand.
     
  4. aaronftnm

    aaronftnm TDPRI Member

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    It seems to make sense to me pretty well. I'm sure that its much more complicated then that, but it gives me an idea of what I need to understand.
    I went to the thrift today to scrape up some parts from old radios and such, and I found a few good ones. I mostly wanted them for the knobs, pots, steal a few capacitors for pedals, but a few of them had some transformers in there. How would I go about "testing" the transformer to see the voltages and such. I don't want to seem like a total newbie, I promise I will be extra careful. Would it help if I posted a picture of the transformer?
     
  5. muchxs

    muchxs Doctor of Teleocity

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    Use a voltometer, test them with all the tubes installed so you have an idea of how they'll perform under load. The tube lineup give you an idea of how much current you can expect from the transformer. Current, while often ignored in more important than voltage. It doesn't matter what the voltages are if the transformer melts from too much load!

    Look up an identify various rectifier schematics, that will tell you where to poke your probes. Most "5" series rectifiers (5Y3, 5V4, 5AR4, 5U4G) output pulsing DC on either Pin 2 or Pin 8. Trace your circuit, it should be a straight shot to the first filter cap.

    Bag and tag everything carefully. Identify AC mains, filaments, high voltage and centertaps. Tag the leads before you bag the transformer. Toss a note in the bag about the original tube lineup, you can refer to it later if necessary.

    The leads on old transformers are often faded so it can be difficult to tell what the leads were by color code out of circuit. Old transformers without documentation may be so much scrap metal.
     
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