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Thread: Combo Preamp Transformer Current Rating

  1. #1
    p Piano Drawbar Dave's Avatar
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    Combo Preamp Transformer Current Rating

    I am waiting for a 117 vac 020875 combo preamp to arrive and I will be converting it to run on 230 vac. Both the US and Non US versions of the power supply show 34 vac on the transformer secondary. As I select a replacement power transformer, what current rating would be best? I donít want to go bigger than I have to!

    I already have a couple of 230 vac neons to replace the originals.

    I know that the preamplifier design is primitive by todayís standards but Iím mainly going to be running my Farfisa Compact Duo through it and the preamp in the Farfisa is probably just as basic!

    Many thanks
    Dave
    1937 Hammond BC with Trek II Preamp, Percussion and Reverb
    1970 Leslie 147
    1968 MK2 Farfisa Compact Duo/1964 Binson Echorec 2/Leslie Combo Preamp

    Restored and rehomed; RT3, C3, M102, L122, T202, 122, 147, 760 x3, 330, 710, 825
    MK1 Solina, Rhodes Stage 73, MK1 Farfisa Compact Duo, Hammond XK3, Analog Korg BX3 and CX3s

    Formerly tonewheel1966

  2. #2
    mp Mezzo-Piano Jyvoipabo's Avatar
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    1 members found this post helpful.
    Hello,

    According to my calculations the power is very low 5mA @ 48VDC ~ 1/4 VA
    But you will not find such small transformers.

    Example :
    https://www.conrad.com/

    product Nį 609884

    Toroidal (ō) 60 mm
    Input voltage (number x voltage) 1 x 230 V
    Output voltage (number x voltage) 2 x 15 V AC
    Height 39 mm

    You must have to wire the outputs in serial to have a 230V / 2x15 = 30V 15VA transformer.
    And the no load output voltage will be around 30V + 15% = 35V.
    This value is not critical and many other transformers can fit.

    JP
    Last edited by Jyvoipabo; 08-12-2018 at 06:30 PM.

  3. #3
    p Piano Drawbar Dave's Avatar
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    Thanks JP!
    1937 Hammond BC with Trek II Preamp, Percussion and Reverb
    1970 Leslie 147
    1968 MK2 Farfisa Compact Duo/1964 Binson Echorec 2/Leslie Combo Preamp

    Restored and rehomed; RT3, C3, M102, L122, T202, 122, 147, 760 x3, 330, 710, 825
    MK1 Solina, Rhodes Stage 73, MK1 Farfisa Compact Duo, Hammond XK3, Analog Korg BX3 and CX3s

    Formerly tonewheel1966

  4. #4
    mp Mezzo-Piano Jyvoipabo's Avatar
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    check also that it fits in the box.....

  5. #5
    p Piano Drawbar Dave's Avatar
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    Well the CPA arrived today and I switched out the transformer and added the 100k resistors in series with the neon pilot lights. My Compact Duo sounds awesome through the 147 and I can finally do that wibbly Farfisa sound on Pink Floyd’s Time!
    1937 Hammond BC with Trek II Preamp, Percussion and Reverb
    1970 Leslie 147
    1968 MK2 Farfisa Compact Duo/1964 Binson Echorec 2/Leslie Combo Preamp

    Restored and rehomed; RT3, C3, M102, L122, T202, 122, 147, 760 x3, 330, 710, 825
    MK1 Solina, Rhodes Stage 73, MK1 Farfisa Compact Duo, Hammond XK3, Analog Korg BX3 and CX3s

    Formerly tonewheel1966

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