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Transformer question

Discussion in 'Amps and Cabs [BG]' started by lexington125, Apr 2, 2015.

  1. lexington125


    Sep 11, 2013
    hollywood, baby......
    someday I will find 4 or 5 other guys who want to play the blues the way it was played before it became all about guitar heroics
    Looking at simple tube bass amp schematics and I came across the old Fender MusicMaster combo. In addition to the usual power transformer and output transformer, there is a 3rd transformer on the schematic, but it is NOT a filter choke. It is labelled as "audio transformer" and it is located between the preamp 12AX7 and the pair of 6v6 output tubes. What the heck is this and why have I never seen anything simiilar?
    here's the Musicmaster schematic -
  2. RoadRanger

    RoadRanger Supporting Member

    Feb 18, 2004
    NE CT
    Most amps use a direct coupled (AKA phase splitter) driver stage to the outputs instead of that transformer you see. No big advantage one way or the other...
  3. The output stage is what's called push pull. While one output tube is driving positive the other is tending negative. The tubes require two drive signals that are 180 degrees apart. This is normally provided by a tube stage named the Phase Splitter. I older amps this function is performed by a transformer as in this case.

    The advantage of a transformer is the drive signals are of a much lower impedance than that of a tube stage.
  4. lexington125


    Sep 11, 2013
    hollywood, baby......
    someday I will find 4 or 5 other guys who want to play the blues the way it was played before it became all about guitar heroics
    OK, thanks - I now understand its purpose, as an alternative to the usual phase splitter / inverter. (for comparison, the Musicmaster uses the transformer, while in my recently purchased Bassman 135, there is a 12AT7 instead of the transformer.) Good.

    But I've been looking at (and not necessarily comprehending) amp schematics for a long time, and I can't remember running across the use of a transformer in this location, at least not in any amps I am familiar with. Is this a design feature more common in hifi or other systems? Or was it used in lots of guitar amp designs, just not any I am familiar with?
  5. It was more common in transmitters and very powerful fixed bias amps. Using a bunch of KT88s it enabled the control grids to see almost zero ohms to ground which aided in stability.

    It was also used in many earlier SS amps until the technology matured.
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    It is called an interstage transformer. They were quite common in early audio tube amps. Juke boxes had them, they are found in hammond organs. Below is a demo.

    Last edited: Apr 2, 2015
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Transformer drive has several distinct advantages as well as a couple of disadvantages.

    First the advantages... lower drive impedance (due to the transform ratio), "perfect" phase inversion, doesn't drift with age (or defects) like tubes do, and 100% galvanic isolation across the transformer.

    Now the disadvantages... low frequency phase shift MUST be minimzed within the design for any amp with global feedback and sufficient LF gain or the amp can motorboat (LF oscillation), potential pick-up of noise or stray signal depending on location and orientation, and higher cost.

    I designed 2 (commercial product) tube guitar amps using a driver transformer, it's a very slick way to drive output tubes IMO.
  8. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The Stancor A53-C works in Fender and Gibson amps that use a transformer for the PI. Page 7 section g in the catalog below. They sell an equivalent at Antique Electronics: AES Interstage Transformers that works in these amps. The Hammond transformers that they list come with specs.

    You gotta smile when you see the clean inverted signal in that video.

    Attached Files:

    Last edited: Apr 3, 2015
  9. wcriley


    Apr 5, 2010
    Western PA
    Yep. I have an old (early 70s?) Carvin SS bass amp that uses an Xformer before the output devices.
  10. The first version of the SS Sunn Concert Bass also used transformer coupling (going from memory).

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