vanhalen5150 wrote:Sometimes I wonder if anyone actually knows what they are talking about.
Amen to that, don't believe any physicist that says they know 100% exactly what's going on. The exact nature of it all is still a mystery. Egghead types have a hard time admitting this. However, we do know enough to build useful things like motors, solenoids speakers etc. Having said all that, let me try to answer your questions pretending to know what I'm talking about
vanhalen5150 wrote:This is one of the problems I have with the whole induction thing. If you put a meter on a speaker and move the coil back and forth, it will create a charge through induction. Does this reflect back to the amp?
Short answer: If there's no speaker hooked up, then yes. If the speaker is plugged in, then it takes that induced voltage/current and makes noise with it.
Long answer: Before I get into that I have to point out an vital piece of physics info. A moving charge has a magnetic field that goes with it. That is to say, a wire with current going through it has a circular magnetic charge surrounding the wire. (google "Oersted compass wire experiment") Now if you wrap that wire into a bunch of loops, you focus that magnetic field to the middle of the coil (technically a dipole configuration) Throw in a soft iron core into that, or transformer laminations, and that magnetic field is focused even more.
The second piece of vital info: Conductors (like metal) hate it when a magnetic field changes near them either by growing or moving. (or when the metal itself moves around a magnetic field, it's all relative) Stationary magnetic fields are of no interest here, but for some reason a CHANGING a magnetic field causes special things to happen. When a magnetic field grows/moves, a metal conductor will magically try and move its internal electrons in such a way to generate an opposing magnetic field whose purpose it is to neutralize that initial intruding magnetic field change. This additional movement of electrons is induced current and opposes the initial current. Clever transformer/inductor makers harness this phenomenon to do useful work and make fun things happen. In a transformer, this current allowed to flow but in a different coil (the secondary) and then is passed along to a speaker to make noise. In an inductor (or a transformer with an open secondary which is the exact same thing as an inductor) that current has nowhere to go so it manifests itself only as an opposing voltage in the primary. Hence 'smoothing' out any changes in the voltage AKA letting more low frequencies through as opposed to high frequencies AKA low pass filter. However, that initial magnetic field does end up growing (unlike the transformer where the magnetic field gets to neutralize itself) This magnetic field is in fact energy stored as magnetism because the current IS flowing even though the voltage is appearing to hold more steady due to the opposing voltage being created.
As you know AC energy switches polarities frequently and when that initial source of current stops feeding in, there is a bizarre state of magnetism that has to be dealt with. This magnetic field that has struggled so hard to grow now does NOT want to shrink and once again opposing voltages are created to fight to keep this magnetic field from going away. Because conductors hate magnetic changes, an opposite voltage appears which tries to keep electrons moving to to reinforce this magnetic field. But ultimately it exhausts itself. This results in a choke or inductor using its magnetic field to send its stored magnetic power down the line (like a capacitor) smoothing out voltage in the similar but opposite polarity of the situation discussed earlier. Now in a transformer with no secondary this power has nowhere to go, so it goes backwards back in into the circuit as 'reflection' of what it was just fed in nanoseconds earlier. This charge reflected back into the amp can wreak havoc with the tubes that now have to deal with this strange backwards power surge. That's why a shorted secondary (Fender speaker output jack shorted when not plugged in) is better than an open secondary (Marshall non shorted) from a reliability point of view.
vanhalen5150 wrote:Since [the Weber] speaker motors themselves have no magnet, how are they the same?
The resistance in the Weber motors comes simply from the resistance of a very long thin piece of copper wire. That's why they get so hot. What makes the motors better than a simple resistor is they have an inductance quality too (due to the fact they are built like an electromagnet or solonoid with a soft iron core [which focuses magnetic fields]) and because of their construction being very similar to a speaker, the inductance will also somewhat mimic that of a speaker. You could get similar mathematical results with a store bought inductor and resistor in series to duplicate the values but as we know here studying the old magical ways of amp making, nothing quite sounds like exactly duplicating physical structures and such.
So actual speakers are in fact inductors too, the magnetic field created in the coil of the speaker opposes/attracts the permanent magnet in the speaker and that causes the cone to move and create a very complicated reactive circuit that resistors or load boxes can't quite imitate.
Hope that made even a little bit of sense.