Unknown Impedance OT

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mojoworker, Feb 17, 2012.

  1. mojoworker

    mojoworker Member

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    i am working on a harmony electra tube amp. unfortunately, the original speakers (2 10" in parallel) have no markings and are nonfuctional. they have no continuity and cant be measured.

    tube layout is 2x6V6, 5y3 rect, 3x12ax7 and 7199 PI
    the OT is labeled Z 3322.

    i uploaded som photos of the OT and a sketch of the signal path through an extension speaker jack here:
    http://s1055.photobucket.com/albums/s519/langlieger/

    the blue wire inside the chassis is discolored grey at the OT.

    if i see it correcty, using extension speaker jack will not use both the amp speaker and the extension cab, but will switch to only the other speaker and will use a different signal pole of the OT.

    can anyone tell me what impedances to use for these positions? is there a way to measure to be sure? i have searched for a schematic but have come up emptyhanded.

    any corrections or suggestions are greatly appreciated -

    chris
     
    Last edited: Feb 17, 2012
  2. pdf64

    pdf64 Member

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    Yes, that wiring from the OT to the jack socket looks pretty strange.
    You could try checking the impedance on both connections using the method described by Aiken http://www.aikenamps.com/OutputTransformers.html
    eg feed a small voltage into the OT secondary and measure the primary voltage; or feed a larger voltage into the primary and measure what you get at the secondary.
    The voltage ration is usually around 20-40, eg 1 Vac in, 30Vac at primary.
    Aiken covers the arithmetic.
    Pete.
     
  3. guitarcapo

    guitarcapo Senior Member

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  4. mojoworker

    mojoworker Member

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    sorry. 'harmony' was a mistake. it is an Alamo electra. i havent seen the schematic anywhere. i did however find another alamo circuit w/ 2 6v6s using 8 ohm, so i am sticking with that for now and leaving the extension cab jack alone.

    thanks for all replies -

    chris
     
  5. Ronsonic

    Ronsonic Member

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    Deja vu???

    Impedance ratio is the square of the voltage ratio.

    Inject a small, known signal at the speaker output, measure the voltage at the plates, divide, square. Guess that an old PP 6V6 amp will have about 8K (well something between 6-10K) at the primary.

    Repeat the procedure for the ext jack.

    Doncha love having a hobby that requires math.
     
  6. CaptainJake

    CaptainJake Member

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    I would pin 8 ohms to the donkey's tail without a second's consideration of a single math problem
     
  7. SatelliteAmps

    SatelliteAmps Member

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    Which might be correct, or it is high. A lot of smaller amps from that period used 4 ohm connections.

    If the OP has a variac, here's how to measure. Remove tubes from amlifier. Put the variac on the secondaries of the OT, and hook a DMM up to the outside primary (if it is push pull, then skip the center tap.) slowly raise the variac until you have exactly 1v on the primary side. Carefully measure the AC on the secondaries. Square this number (multiply it by itself). This gives the turns ratio. Should be a few hundred to the low thousands. This number times an output impedence will give you the impedence of the tranformer. So, if we got 30vac on the secondary, that would give us a turns ratio of 900:1. So, if we wanted to run a 8 ohm load, that would give us an overall value of 7200. A 16 ohm load would give a value of 14400, and a 4 ohm load would be 3600.

    The next part of this is to determine what powr tubes are being used and what they want to see. A push pull amp will want double the value of a single tube amp. Look up the data in a tube manual, and that should give you everything you need.
     
  8. Ronsonic

    Ronsonic Member

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    Adam, your voltage ratio will equal the turns ratio (something more like 30:1) you square that to get the impedance ratio (900:1 as in the example).
     
  9. SatelliteAmps

    SatelliteAmps Member

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    Ronsonic, you are correct. My math was correct, but the vernacular wasn't.
     

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