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Power supply modification question

  1. Jul 21, 2016 #1
    Hi guys,
    Here is the amp schematic, http://www.classictubeamps.com/schematics/Marshall/1966%20JTM45.pdf [Broken]

    Here is the power supply schematic I would like to modify and use.

    I don't know how to get both of these schematics to display in this post...I guess I will never make it as a computer guy..lol

    Do you see any issues with this idea?


    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Jul 21, 2016 #2


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    like this


    and this ...


    when you are creating a thread, there are 3 buttons to the lower right of the text box
    "Post reply" "Preview" and "Upload"

    click on upload and it gives you the uploading option to browse you PC and attach the file


    hope that helps

    Last edited: Jul 28, 2016
  4. Jul 21, 2016 #3
    Thanks Dave,

    Now if I just had a nine year old kid around my house I could get them to show me how to do stuff on my smart phone...lol

    Now it will be easy to talk about the schematic. Anyway the idea is to use a twin rectifier and a switchable solid state power supply for the JTM45 amp. I don't understand the primary side of the transformer on the Mesa schematic.


  5. Jul 28, 2016 #4
    Nice Amp - wish I still had one!
  6. Jul 29, 2016 #5


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    The "Spongy" position reduces the supply voltages to the amp by changing the transformer turns ratio. This reduces the peak output power and may change the distortion characteristics a little due to the reduced filament voltages.

    If you use the identical power transformer, it should work. Original power transformer for two reasons:
    1) The dual 5U4 rectifiers draw 6Amps filament current and most xformers assume a single 5U4 at 3Amps.
    2) There is a minimum recommended transformer resistance for 'reasonable' 5U4 life. This limits the peak plate current of the tube.
    For 5U4 data sheets and design hints see:
    http://www.theremin.us/Rockmore Theremin/5U4G.pdf

    To equalize the reverse voltages across the 1N4007 diodes, I recommend a 1Meg 1/2W resistor in parallel with each diode.

    Realize that the various supply voltages, 422V, 406V, 402V, will depend on the current drawn from each voltage; but they are not likely to be critical.

    Have Fun!
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