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Thread: How do I replace field-coil speaker with magnet speaker?

  1. #36
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    I have the same amp, but is a Gretsch Electromatic 1955. There is 1000-1300 ohms through the field coil. It is may favorite all time amp. It is Holy Grail! I am considering doing a scratch build reproduction with a magnet speaker. I would prefer using a choke instead of resisters. I cannot find one that looks right. You said you were going to use a fender deluxe style. The ones I see are 150 ohms. Which choke (exactly) did you use. Thanks

  2. #37
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    When Hammond moved to PM speakers, they replaced the field coil with a 250 Ohm, 20W resistor. Of course this does not mean that 250 would be the correct value for your amp.

  3. #38
    Supporting Member Chuck H's Avatar
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    Right. I wouldn't sweat it beyond actual tube safety and performance. The high DCR of the field coil speaker is no doubt because of it's dual purpose as an electromagnet. Since that is no longer necessary the thing to do to preserve the tone is focus on the impedance and voltage drop in the circuit. I'd bet that most guitar amp chokes will be ok for impedance and any resistance for DC voltage drop can be achieved with an additional resistor. Probably getting very close to stock performance from the amp but with the freedom to use a PM speaker.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

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  4. #39
    Senior Member Enzo's Avatar
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    Replacing the coil with a choke will likely bring higher voltages, the circuit design was based around the resistance of the coil. I might suggest using a 1K resistor and observe the results. Then decide if you really need to added expense of a choke. WOuld the choke really add anything? On the other hand, it is just a guitar amp. It isn't going to make a ton of difference.

    The speaker is the single thing in any amp that has the most effect on the sound. SO while a magnet speaker might sound great, it probably won't sound like the original field coil speaker.

    I have a large 5k resistor in my tool drawer, a large ceramic one with screw terminals. I have a couple clip wires soldered to it. I have used it for decades in old jukebox amp repair as a generic replacement for speaker field coils. (I would receive amp modules, while the speaker assembly remained in the jukebox.)
    Education is what you're left with after you have forgotten what you have learned.

  5. #40
    Supporting Member Chuck H's Avatar
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    I really think Enzo is right on this. I only mentioned the impedance you would get from a choke because some of my recent bench tests for design have shown it to be a significant parameter for clipping performance. But it's nitpicking on my part.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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