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    Dynaco A-470 cloth lead output transformers - plate lead ? screen lead ?

    Bob Latino

    Posts : 2639
    Join date : 2008-11-27
    Location : Massachusetts

    Dynaco A-470 cloth lead output transformers - plate lead ? screen lead ?

    Post by Bob Latino on Sun Jan 03, 2010 4:41 pm

    I had someone Email me because he was building up an ST-70 from parts and couldn't tell the plate lead from the screen lead on the A-470 output transformers he was using. What happens on the older cloth lead A-470's is heat fades the dye on the cloth to the point where on some transformers the BLUE and GREEN leads and the BLUE/WHITE and GREEN/WHITE leads look virtually INDENTICAL in color.

    Now sometimes these wires have taken a set and from the LENGTH of the wire you can usually tell which is which. On both transformers the GREEN and GREEN/WHITE unltralinear screen tap leads (which go to pin #4) are just a tiny bit (maybe 1/4 inch) longer than the BLUE and BLUE/WHITE plate leads (which go to pin #3).

    But what are you going to do if "Bubba" removed the transformers and CUT THE LEADS and the leads are now about the same length and look about the same color? That is what happened in the case of the Email that I got. He had bought the A-470's on Ebay. OK - Whaddya do now ? Get out your multitester and check the resistance from the RED lead to the GREEN and the BLUE lead. The resistance of the RED to the BLUE (plate lead) will always be HIGHER than the resistance to the GREEN (screen lead) and the resistance from the RED lead to the BLUE/WHITE (the other plate lead) will also be higher than the resistance to the GREEN/WHITE (the other screen lead).

    Why is this true? On the primary side of the A-470 the RED B+ lead is placed at the CENTER OF THE WINDING and as you go further towards either side of the winding you will come first to the GREEN and GREEN/WHITE screen leads. At the very end of each side of the primary winding are the BLUE and BLUE/WHITE plate leads. Simply put, there is MORE WIRE in the winding from the center RED wire to the BLUE and BLUE/WHITE wires and MORE WIRE will give MORE RESISTANCE.


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