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Thread: 1946 Masco MA-35 PA Conversion To Guitar Amp

  1. #1
    Senior Member
    Join Date
    May 2011
    Out West, Canada

    1946 Masco MA-35 PA Conversion To Guitar Amp

    Hi folks, I recently purchased a used 1946 Masco PA head. 35W. Price was good, it needed work. I found one bad 7N7 phase inverter tube, replaced it and fired it up. It works.
    I had bad crackling on the pots so I pulled the original coupling caps in the tone stack and replaced. The crackling cleared up great. There are a few more coupling caps I will replace and have ordered them.
    All the B+ caps have already been replaced by previous owner to new F&T caps. These look new and will remain. 3 prong cord has been installed. It appears to have a 270K negative feedback resistor.

    I have questions about what I should consider doing to an old PA such as this to convert it to a guitar amp? Should I put 68K resistors off the input jacks? Or maybe 33K as the jacks are not wired to be a shared input?
    Any tips on what I should do are appreciated.

    I traced the signal circuit through the schematic here. Green is tone stack. Orange arrow is cathode resistor. Blue is through phase inverter.

    Here are a few pics of the amp. Any tips appreciated. Thanks, Keith

    Justin Thomas likes this.

  2. #2
    Senior Member trobbins's Avatar
    Join Date
    Jun 2010
    Lower output stage coupling cap value and add grid stoppers.
    Add cathode current sense for each 6L6.
    Add grid stopper to V5a.
    Add protection 1N4007 diodes to 5U4 anodes.
    Change one input stage to cathode bias, and remove it's input coupling cap and add grid stoppers and 1Meg leak. Seto other stage grid leak for ok anode idle voltage.
    Play and fault-find all creaks and groans.
    Check hum level is not objectionable.
    Check how much feedback is applied - may need to reduce feedback to ensure no instability for all load types.
    Check which stages are overloading first, and maybe attenuate or increase stopper to move overload more to output stage.
    Justin Thomas likes this.

  3. #3
    Supporting Member mozz's Avatar
    Join Date
    Oct 2015
    First, i would change any and all wax caps, that looks to be about all of them. Chances are also that most every high value resistor is going to be way off.
    Justin Thomas likes this.

  4. #4
    Old Timer
    Join Date
    May 2006
    Chill-Ville, VA
    Call it "Pentode-A-Saurus." Love all those pentodes!

    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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