Is This An 8 Ohm or 4 Ohm '66 Blackface Showman - Photo

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by hardys, Jan 26, 2015.

  1. hardys

    hardys Member

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    I recently acquired a '66 Showman a few months ago. I've been using it with an 8 Ohm cab, assuming that all Showmans have 8 Ohm OTs and Dual Showmans have 4 Ohm OTs. While trying to educate myself a bit more on the Showman's history, I stumbled on an web article on that states Fender made Showmans with both 4 & 8 Ohm OTs, depending on what speaker cab was paired with it.

    The article also states that if the OT has the number 125A29A (which mine does), it is 4 Ohms. Is the article correct? Is mine in fact 4 Ohms? Will it damage the amp running it with an 8 Ohm cab?

    Thanks for any help!

    [​IMG]
     
    Last edited: Jan 27, 2015
  2. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    All my info states the 125A29A has a 4 ohm secondary. The 125A30A is the 8 ohm Showman OT.
     
  3. smolder

    smolder Gold Supporting Member

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    Would love a link to that article... my understanding has been the same as yours (previously).

    Fender put two output jacks on there. I'm assuming that for combos they did not anticipate using two cabs aside from the combo, and given how Fender did things they would not rethink this for head units. From everything I'm told those OT's will handle the mismatch.
     
  4. hardys

    hardys Member

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    Here's a link to the article I found about the transformer codes.

    https://books.google.com/books?id=k...29a fender transformer 125a29a 4 ohm&f=false
     
  5. hardys

    hardys Member

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    So, my next question is...how can you measure the impedance at the output jack, assuming that I must keep a speaker connected as to not damage the transformer?
     
  6. mark norwine

    mark norwine Member

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    125A29A is a 2K:4-ohm transformer....this isn't really in question, so I'm not sure what you're asking.

    If you want to know how to determine a transformer's turns ratio ( and, by squaring, the Z ratio), I'll type it out.....if that's really what you're asking.

    Is it?
     
  7. hardys

    hardys Member

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    I didn't realize that the 125A29A was definitively 4 Ohms, so I was trying find out how to measure the impedance of the transformer with a meter and verify that it really is 4 Ohms. I guess I don't need to do that now, with the input I've received from you guys verifying this. Thanks!
     
  8. mark norwine

    mark norwine Member

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    Here's the inconsistency:

    Showman (on the faceplate) = 8 ohms = 125A30A
    Dual Show (on the faceplate) = 4 ohms = 125A29A

    ...in a perfect world.

    But the world isn't perfect, and Fender had a habit of sticking 4-ohm OTs in "Showman" amps on a seemingly-random basis. I've seen it plenty of times on my bench.

    But there's no inconsistency in the OTs themselves: A29A=4 ohm....A30A=8 ohm. You have a 4ohm OT.

    ~~~~

    If you want to verify the Z ratio of a OT (any OT), you will need 2 DMMs and a signal generator.

    - disconnect the OT from the circuit
    - connect DMMs to the sec and pri leads. Leave the pri ct unconnected. Set both DMMs to AC volts
    - apply a small (but stable, constant) ac sine wave to the secondary. Let's say 3vac
    - record the ac voltage on the pri leads. It will be higher....let's say it reads 67vac
    - divide 67 by 3. This give us 67/3=22.3
    - 22.3 is the turns ratio.
    - The Z ratio is the square of the turns ratio, so 22.3*22.3 = 500
    - The Z ratio of this OT is:
    --- 500:1, or
    --- 1000:2, or
    --- 2000:4, or
    --- 4000:8, etc.

    remember that OT's don't have "impedance"...they have "impedance ratios".

    Makes sense?
     
    Last edited: Jan 27, 2015
  9. donnyjaguar

    donnyjaguar Member

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    I do what Mark says with the exception I run 120Vac from the wall outlet into the primary and measure the volts out. If you think about it, the primary can deal with an awful lot more than 120Vac. :)
     
  10. hardys

    hardys Member

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    Yes, thanks...it does make sense. Thank you so much your detailed response! :aok
     
  11. mark norwine

    mark norwine Member

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    donny's way works, too....
     

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