Fender Twin Reverb UL 135watts

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Carlosraj

Member
2010-01-17 11:12 am
  • #1
    Hi


    I have an old Fender Twin Reverb Ultralinear 135watts(1981) amp with a blown output transformer. I have ordered a replacement from Hammond, model No: 1750WR. It is stated that it is a replacement for Fender's 013691 output transformer. This 1750WR transformer has a primary impedance of 2000 CT Ohms. Will this impedance match 4 of 6L6GC tubes. I do not understand how the output impedance of a output transformer is used to match the load impedance of the tubes. I have attached the datasheet for this transformer and 6L6GC tube. Please assist.


    Thanks


    Carlos
     

    Attachments

    • 1750WR.pdf
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    • datasheet(2).pdf
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    JMFahey

    JMFahey

    Member
    2009-03-18 7:32 pm
    Buenos Aires - Argentina
  • #2
    Carlosraj said:
    I have an old Fender Twin Reverb Ultralinear 135watts(1981) amp with a blown output transformer. I have ordered a replacement from Hammond, model No: 1750WR. It is stated that it is a replacement for Fender's 013691 output transformer. This 1750WR transformer has a primary impedance of 2000 CT Ohms. Will this impedance match 4 of 6L6GC tubes. I do not understand how the output impedance of a output transformer is used to match the load impedance of the tubes.
    Click to expand...
    Twin Reverb Ultralinear 135W usas 4 6L6 tubes in push pull parallel (2 tubes per side) and expects tp "see" a 2000 ohm plate to plate lload.

    The Output transformer does not have impedance by itself but converts speaker impedance to that needed for the tubes.

    So IF you connect an 8 ohm speaker to 8 ohm secondary tap (from black to green) or a 4 ohm speaker (from black to grn/yel), you will have 2000 ohms plate to plate (brown toblue).

    No need to dig deeper, that´s enough data to repair that amplifier.

    Hammond is excellent quality iron so it will work fine.
     
    PRR

    PRR

    Member
    Paid Member
    2003-06-12 7:04 pm
    Maine USA
  • #6
    Kay Pirinha said:
    ...two pairs of those 6L6-GC's I'd expect half the datasheet's plate-to-plate load value...
    Click to expand...

    Many combinations of V and Z will make the same power.

    Fender could estimate or experiment his own answers.

    The 4k/pair loading seems to come from 6L6 AB2 condition (3,800) where Vg2 is held down to suit original 6L6 spec and Z is dropped to suck the current. The 5881 had higher Vg2 rating and could pull 4k/pair in AB1 for about 40 Watts (Bassman 5F6a). The 6L6GC has even higher rating for P and G2, and you can do 65 Watts per pair in about 4k, and even in UltraLinear.

    You can build a marketable radio or amplifier on the datashet suggestions. Building a market-beating amplifier may need a little thought and much smoke-testing.
     
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