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Field coil speaker and PM speaker on the same amp (alternatively)

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  • #16
    If the decision is to replace the field coil spkr with a classic spkr, the field coil choke can be replaced with a choke used in:

    Vox AC15, 100mA DC, 10 Hy, D.C.R. = 270 Ω

    Vox AC30, 100mA DC, 20 Hy, D.C.R. = 419Ω

    Marshall JMP & JCM800, 120 mA DC, 5 Hy, D.C.R. = 115Ω

    Filtration with series resistor instead choke is not practical because requires high power resistors (> 10W)
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)


    • #17
      Chuck H, Helmholtz : I confess I'm tempted by your clear explanations. During Covid-19 lockdown here, on last Spring, I completed projects (both new builds and troubleshooting) that had been waiting for a dozen years! Including building a head cabinet for a SFSR and replacing its stock OT with a multi-tap one as a gift for my tired back, resurrecting an almost trashed old Magnatone 440 just to listen to that pitch shift vibrato for the first time in my life, etc.

      The only thing that still scares me is challenging that absurd Gibson GA-40 layout to check capacitors and resistors. I do think that a decent solution to service that amp not forcing techs to desolder wires going to tubes sockets and pots must exist somehow. Otherwise it's nothing but an evil joke!
      But that's another story anyway...

      Whenever a second lockdown will eventually (and unfortunately) be imposed I could try to convert the Oahu amp the way you suggested.

      I have a spare pair of connectors like this somewhere, and they could hopefully do the job:
      I'd have to figure out a possible not too invasive layout. I'd like to keep choke inside the amp's cabinet so that the PM speaker equipped extension cab could be absolutely standard.

      I'll let you know in case I decide to start such a moderately complex task.

      Thanks again for all the precious info you shared with me.


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