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O/P Tranny Primary/Tube question...

Discussion in 'Building the Classics' started by Stephen H, Aug 9, 2021.

  1. Stephen H

    Stephen H Member

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    In Joe's list "Common Output Tranny Primary Z ratings" there is "Marshall, 50W 2xEL34 3,400 Ohms", and "Park 50W 2xEL34 5,000 Ohms", so let's say for the sake of this question that I had a 5,000 Ohm primary O/P tranny and swapped it for the 3,400 Ohm in my 2204 what difference would I notice?
    Cheers, Steve
     
  2. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    The output section power would increase about 15-18%. The headroom would also increase while the output section was clean and not clipping and the voltage would sag more once clipping occurred causing more compression.
     
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  3. Pete Farrington

    Pete Farrington Well-Known Member

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    What is meant by 'headroom' in this context?
    I normally assume people use it to mean an amp's max unclipped output power, but it seems to refer to something different here.
    Gain, maybe?
    A lower load impedance will reduce the open loop gain, but the NFB will tend to counteract that to some degree.
    Also the clipping will 'sharpen' with less rounded corners / an increase in higher order harmonics, but NFB causes that anyway.
     
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  4. Spanngitter

    Spanngitter Member

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    Aiken has the best information on this (see citation below)
    There is several parameters influencing power amp performance but in a nutshell said the higher Zprim does give a better low frequency response and less clipping:

    Primary inductance

    The transformer does, however, have a primary inductance, which has a direct effect on the low frequency response of the transformer. The -3dB low frequency cutoff point can be determined by the following formula:
    f = Z/(2*Pi*L)
    where Z is the primary source impedance (generally speaking, this is the reflected impedance in parallel with the source impedance presented by the tube's plate) and L is the primary inductance.
    This means that if you want better low frequency response from your transformer, you have to increase the primary inductance, which means a larger core and/or more turns on the primary.
     
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  5. Stephen H

    Stephen H Member

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    Thanks for all the replies, simple explanations I can understand, you've been a great help.

    I've "read" that Aiken article but back then it kinda floated off the screen in a blur, I should go back and re-read the whole site, I hope my level of understanding (much of it gleaned from these hallowed halls) will extract and retain more useful info than the first time...

    Cheers
     
  6. Pete Farrington

    Pete Farrington Well-Known Member

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    I can’t see that, would you mind explaining how you’ve drawn that conclusion from the linked page?

    You may be mixing up primary inductance with (the reflected) primary impedance.
    It may help to think of primary inductance as providing the foundation upon which the reflected impedance is developed. ie if the inductance is inadequate, then the bass response will be compromised, because the inductive reactance will shunt across the reflected impedance.

    eg broadly speaking, if we wish to support a 5k primary impedance down to a -3dB corner freq of say the typical 50Hz, a primary inductance of 16H is required.
     
    Last edited: Aug 9, 2021
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  7. 351

    351 Member

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    You'd hear more of a change due to the winding pattern and lamination composition than the change in primary z rating.
    If you had two identical core o/t's and at least similar winding pattern transformers and wound one to 5k and the other 3.4k, the 5k one would be louder, but not by much, and it would break up a little later.
    Find 5 of the same primary z o/t's but different brand o/t's , jumper them in one after the other and get ready for an education about the importance of o/t in shaping tone.
    My rolling stock is about 50 o/t's from the 50's 60's and 70's.
    I build the amp, then select the correct o/t by jumpering.
     
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