Question about transformers for the gurus out there

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

  1. ahiddentableau

    ahiddentableau Tele-Meister

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    This isn't strictly a guitar amp question, so apologies. But I know there are a lot of knowledgeable folks out there in TDPRI-land and I'm hoping for some help.

    I'm in the process of building a tube mic preamp. I'm going to start with the Scott Hampton design from the TapeOp article that came out about 15 years ago.

    My question is to do with the output transformer. I thought I would use the VTB2290 by Carnhill. It's sold by AML in the UK. Series connection should be 10k:600R, which would seem to fit the design nicely, and the price is attractive. (OTs from places like Jensen appear to be over a hundred bucks, and that's too rich for my blood.)

    My issue is this: I don't understand the information as given in the AML/Carnhill information packet. The iron in question is on page 6. It confirms that a series connection would give about 10K:600R. So far so good. But look at the stated turns ratio: 1+1:1+1. I just can't square this with the impedance table. If it is possible to wire this thing up 1:1 or 2:2, then how can the impedances be like that? Wouldn't there be two possible connections that would be unity gain? I'm far from knowledgeable in this area, but I can't
    understand this.

    Could somebody help me out? Is this in fact possible, and I'm just in a state of ignorance? Or is it likely a mistake/typo?

    I would sincerely appreciate any help/info you guys can dole out!

    Edit:
    I should mention that I emailed AML about this, but I never got a reply. Maybe I'm too small potatoes to merit it? Anyway, that bridge led nowhere.

    AML transformer information packet:
    http://www.audiomaintenance.com/downloads/carnhill_design_guide.pdf

    Link to AML OT page:
    http://www.audiomaintenance.com/acatalog/Output_Transformers.html

    The basic schematic for the tube mic pre is available here:
    https://web.archive.org/web/20030408091534/http://www.hamptone.com/figure1.htm

    And more info on it is available here:
    https://web.archive.org/web/20030407232915/http://www.hamptone.com/mic-pre_kit.htm
     
    Last edited: Aug 11, 2019
  2. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Just bumping this to the top for you in hopes that someone may be able to help.
     
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  3. Teleterr

    Teleterr Friend of Leo's

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    I ve wondered about Z and turns also. I assumed different wire gauges are used, but that's just a guess. I got a top quality tranny w those specs off eBay super cheap; nobody wants them any more. I m going to use mine in reverse w an Op amp so the Tube amp responds just like a pickup.
     
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  4. ahiddentableau

    ahiddentableau Tele-Meister

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    That thought had crossed my mind as well, and I'm sure it's possible that's what's going on. But I'm still kind of reluctant to believe that's what's going on here for these reasons:

    Everywhere else I look at xformer specs (Lundahl, Jensen, Edcor, etc.) I've never seen this before. If a turns ratio is listed, it's always as though there is a single consistent wire used. You can follow the ratios to the impedance tables and it all makes perfect sense. And it has to be this way, doesn't it? Otherwise what good is the ratio on the datasheet? You'd have to know the wire gauges and the relative impedance/resistance difference in order for it to mean anything. No point in having a shorthand if it isn't short--or at least communicative. When I look through the rest of that Carnhill sheet, I don't see any other instance of this. Every other set of specs I glanced at looks like it works out.

    I'm stumped. I just wish AML would have answered my stupid question.
     
  5. peteb

    peteb Friend of Leo's

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    Not claiming to know anything more than what has been said.


    If we follow the rule




    EFBE1931-2AB3-4F33-8FA2-29908B05F14B.png





    The turns ratio is the square root of the impedance ratio



    In the example given, 9600/600 = 16, the turns ratio is 4:1



    As they list it it should be 4 + 4 : 1 + 1




    My guesss is that the turns ratio is in error. I see some others that they list with similar errors.


    Maybe people pay more attention to the impedances than the turns.
     
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