Transformer identification, or mapping

Discussion in 'Shock Brother's DIY Amps' started by ArcticWhite, Aug 11, 2019.

  1. ArcticWhite

    ArcticWhite Tele-Holic

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    Found this little guy in my scrap pile.
    I pulled it out of a Hammond chord organ a while back but not sure which one.
    I Googled the numbers but came up empty.
    Is it a single ended OT? Will it work for a Vibro-Champ build?

    Wires are black red blue green.

    20190810_183318.jpg
     
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  2. peteb

    peteb Friend of Leo's

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    It appears to be that


    What is the resistance red to blue? Black to green?
     
  3. ArcticWhite

    ArcticWhite Tele-Holic

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    Red to blue 150 ohms.
    Black to green 1.2 ohms.
     
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  4. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Next, just apply a known AC voltage across the secondaries, remeasure the 'known' voltage across the secondaries because especially w/ single-ended OTs there seems to me to sometimes be a drop I don't understand but has been discussed a little in Shock Brothers. I like to use the heater voltage from a PT I'm confident in and is safely running on the bench, but other voltage sources are available.

    Then measure the VAC across the primaries and set up a ratio Pvac:Svac

    Reduce the ratio so that the Svac is 1 (Pvac/Svac : Svac/Svac)

    Square both sides

    Then make a little table by multiplying both sides by 2 ohms, 4 ohms, 8 ohms, 16 ohms, and the P will be the primary impedance for whatever the S (2 or 4 or 6 or 8 or 16) is loaded with.
     
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  5. ArcticWhite

    ArcticWhite Tele-Holic

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    I'm not sure I follow your math all the way.
    P Vac = 140, with SVac = 6.4

    P/S = 21.9

    21.9 : 1

    "Reduce the ratio so that the Svac is 1 (Pvac/Svac : Svac/Svac)
    Square both sides"

    Is that what you meant?

    480:1

    Square of 1 is 1.
     
    Last edited: Aug 11, 2019
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  6. peteb

    peteb Friend of Leo's

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    That looks good for an 8 ohm speaker.


    A 6v6 wants to see 4000 to 5000 ohms of impedance. If your ratio was 30 instead of 22, then 480 would be 900 or 1000. A 4 ohm speaker times 1000 is 4000. An 8 ohm speaker times 480 or 500 is 4000 ohms.


    Everything looks right on for an 8 ohm speaker, which may transfer more power than a four ohm speaker.
     
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  7. ArcticWhite

    ArcticWhite Tele-Holic

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    Thanks for the calculations above.
    Wondering what the resistance readings told us.?
     
  8. peteb

    peteb Friend of Leo's

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    The resistances look good. Red-blue is the primary, black-green is the secondary.



    If you subtracted the resistance of your meter from 1.2 ohms, do you get less than one ohm? That is what I would expect.


    The 150 ohms on the primary looks good. I have two bf champs and the OT primary on those is 260-270 ohms. I believe that tweed champs and Princeton’s are going to have a DC resistance on the primary of around 150 ohms.




    This is speculation: when we first saw the resistances, before you did the impedance ratio measurement, I was thinking 150 ohms is lower than what I am familiar with, and that maybe that meant that a higher impedance speaker would be right. Then you did the impedance measurement and that indicates an 8 ohm speaker is what is needed to match what the tube wants to see. It all seems to work out and make sense with an 8 ohm speaker.
     
  9. peteb

    peteb Friend of Leo's

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    Checking closer on the impedance ratio.


    AW measured an impedance ratio of 480. This multiplies the speaker impedance to the total impedance the power tube works into.


    4 ohms times 480 is 1,920 ohms

    8 ohms times 480 is 3,840 ohms.

    16 ohms times 480 is 7,680 ohms.



    The tungsol spec for the 6v6 tube calls out that the tube wants to work into a load of 5k rising to 8K as the plate voltage increases.



    It looks like an 8 ohm speaker will work, a 16 might be better, but since 8 ohm speakers are common and 16s are not, 8 is probably a good option.
     
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  10. peteb

    peteb Friend of Leo's

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  11. ArcticWhite

    ArcticWhite Tele-Holic

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    Wow, Pete. Thanks so much for this. I'm routinely amazed by how much generosity there is on this board. Thanks also to Tinfoil and others who helped me out.
     
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