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  • Gulbransen APF-4

    I'm trying to decide if a Gulbransen APF-4 amplifier would be a good donor for an 18 watt Marshall clone. It has an EZ81 rectifier, a 12AX7 preamp and PI and 2x EL84 PP output. That's the right tube complement. I think the rather beefy PT ran other organ related stuff as well so there should be enough heater current for another couple of 12AX7s. Lots of room on the chassis as well. My concern is that the B+ might be kind of low. I don't have the amp and I can find no literature on one. Any one out there have info or experience on this amp? Thanks on advance.

  • #2
    So power it up and find out what the B+ might be. What voltage are the main filter caps. Chances are they wouldn;t have used 500v caps on a 300v circuit. Your cap voltage is a good clue to B+ voltage. Of you can disconnect the transformer from the power supply and measure the high voltage winding AC, and from that determine what the B+ DC was.

    Oh, wait, you don;t have the amp. Then never mind...

    How about this, make some amp out of it anyway. Build your Marshall preamp into it, and if the output becomes a 12 watt instead of 18 watt, so what?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      You can always use solid state rectification to raise the B+.
      WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
      REMEMBER: Everybody knows that smokin' ain't allowed in school !

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      • #4
        Originally posted by loudthud View Post
        You can always use solid state rectification to raise the B+.
        That is true. I'm trying to get 18-20 watts out of it. IMHE, that type of amp clone sounds the best when you are cooking the outputs. If I can get it cheap enough I will just do it.
        Last edited by olddawg; 10-12-2012, 04:52 AM.

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        • #5
          The filter cap I can see in the picture are 40uf at 475 volts.

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          • #6
            I used one to make a Spitfire circuit.
            Had some hum problems and I figured it was due to the EZ81.
            Switched to a 5Y3 and it sounded fine.
            Don't see why you couldn't do a TMB.

            Mine is in storage at the moment, waiting for a combo cab.

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            • #7
              Originally posted by Cygnus X1 View Post
              I used one to make a Spitfire circuit.
              Had some hum problems and I figured it was due to the EZ81.
              Switched to a 5Y3 and it sounded fine.
              Don't see why you couldn't do a TMB.

              Mine is in storage at the moment, waiting for a combo cab.
              Hi Cygnus....Why did the EZ81 hum? I have another build with an EZ81 that is whisper quiet. Does the amp have a filament center tap or did you have to make a virtual? Do you remember what the B+ was or what the PT secondary voltage was?

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              • #8
                Sorry, been a while.
                I did not use anything other than whatever the Gully provided from the start.
                I'm guilty of changing several things "while I'm in there" so I can't say 100% if the hum was due to the EZ81.

                IIRC B+1 was 340V

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                • #9
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