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Thread: Build = simple 50w tube Power amp.

  1. #1
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    Build = simple 50w tube Power amp.

    This might be a stupid idea.......


    Im thinking of building a simple 50 watt (2 el34) power amp to rackmount.

    Looking at a Marshall 2204 amp schematic...Id love to just make the power section of this.

    I just want to plug in any popular 1 ru rack preamp into it , like a Jmp-1 , or ADA , Fractal , Rocktron , or similar.

    SO....If i was to copy the 2204 output section , where exactly would my "input" be ?

    The Master volume pot ? Would that be an "Input Level " ? phase inverter tube would be first inline (PPIMV) ? or some extras in the front of the circuit ? I want to keep this simple as in no Op-amp / transistors .......

    i realize the power section would have less current draw due to no preamp section/tubes...

    Is there a schematic out there that is similar to what im talking about ? Theres a bunch of tube guitar style power amps out there..(Peavey Classic 60 / Marshall 9200) i just want to build a way more simple versionClick image for larger version. 

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    Lifetime Member Enzo's Avatar
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    There are not usually line level points in the signal path of tube amps, unless you copy some amp from the return of an FX jack pair. Usual FX loop has a signal level drop to the send, then a boost back up to tube levels at the return.

    Amps don't get much simpler than a PV Classic 60. Power tubes, phase inverter tube, input buffer tube.

    Same with a Marshall 9005: two power tubes, phase inverter tube, buffer tube. They chose to do the phase splitting at low signal level THEN boost, but either way...

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    I know you want to avoid SS but the simplest way is to use an lnd150 input stage feeding the PI.

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    Maybe take a look at the AX84 Project power amp:

    http://www.ax84.com/static/corepower..._Schematic.pdf

    Seems simple enough.

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    That may not have sufficient voltage gain to make full power with the signal level available from some sources, eg delay pedal.

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    I had a mil spec rack mount power amp years ago that was line level input. No volume control. Came out of a ship or something. 2x 6L6 push pull. Had the metal tubes. Went in the dumpster with a bunch of stuff in the early 80s. May be some literature on something similar if you look around.

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    Quote Originally Posted by Fazer View Post
    Maybe take a look at the AX84 Project power amp:

    http://www.ax84.com/static/corepower..._Schematic.pdf

    Seems simple enough.
    Exactly what i was talking about--so will the output level of any popular preamp be perfect for this ( the input to the MV ) ?

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    Quote Originally Posted by Valvehead View Post
    Exactly what i was talking about--so will the output level of any popular preamp be perfect for this ( the input to the MV ) ?
    I think so. I use the one I build with a Madamp bass preamp kit, this one:

    http://www.madamp.de/index/lang/en/artikel/19

    I can't find my build notes with the voltages, but the Madamp schematic give a 1.2V RMS max output, so a line level output (or something less) should do.

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    If 'any' preamp has a +4dBV output (which if my math is right, means just under 3.5vpp, same as Fazer's 1.2vrms) then that may not be enough to drive the 2xEL34 power stage pictured to clipping, given the NFB in circuit. Is that what you want, a clean-ish output stage?

    [I hate to suggest] Otherwise, add a recovery stage to the input of the power amp, and give yourself a true volume control instead of an attenuator.

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