Mag Amp Power Scaling

[silverfox]

Had this posted in Other Topics. Thought I'd try it here. Let me know if it doesn't belong. Did some Forum Research prior to posting and didn't see any like this.


Has anyone considered using Mag Amp theory to incorporate Power Scaling? Is that what London Power uses?

A transformers gain may be controlled by superimposing a magnetic field upon the core, thus influencing the amount of power transferred between the windings. Would this work on a audio amp by adding a circuit that saturates the core of the transformer? Perhaps using an ultra linear grid winding as the injection point? London Power? I don't think that would be patentable since various forms to this have were in service since the inception of Mag Amps.

Basic Mag Amp with filament transformers:
http://sparkbangbuzz.com/mag-amp/mag-amp.htm

This site has several really cool concept proof pages that made me wonder if high speed levitation devices are not extremely  simple to build using naturally occurring minerals. Yikes maybe I shouldn't have said that. This whole site could disappear from the web now.

Related pages
This page, same site, shows home construction of a Triode:

http://www.sparkbangbuzz.com/els/vt-vac-el.htm

http://www.sparkbangbuzz.com/els/hm-vac-diode-el.htm

Could be useful in the future???

Silverfox.

[spacelabstudio]

Had this posted in Other Topics. Thought I'd try it here. Let me know if it doesn't belong. Did some Forum Research prior to posting and didn't see any like this.


Has anyone considered using Mag Amp theory to incorporate Power Scaling? Is that what London Power uses?

A transformers gain may be controlled by superimposing a magnetic field upon the core, thus influencing the amount of power transferred between the windings. Would this work on a audio amp by adding a circuit that saturates the core of the transformer? Perhaps using an ultra linear grid winding as the injection point? London Power? I don't think that would be patentable since various forms to this have were in service since the inception of Mag Amps.

Basic Mag Amp with filament transformers:
http://sparkbangbuzz.com/mag-amp/mag-amp.htm

This site has several really cool concept proof pages that made me wonder if high speed levitation devices are not extremely  simple to build using naturally occurring minerals. Yikes maybe I shouldn't have said that. This whole site could disappear from the web now.

I don't know, but I suspect they're doing something far less exotic.

The application that comes to mind to me is a compressor.

Chris

[Willabe]

Is that what London Power uses?

No. It's a power mosfet Kevin uses to drop the voltage. Same thing with VVR.


                   Brad       

[HotBluePlates]

Has anyone considered using Mag Amp theory to incorporate Power Scaling? Is that what London Power uses?

London Power's Power Scaling is used to reduce the power of an output stage (or entire amplifier) by reducing the supply voltage. It does this with what is basically a MOSFET regulator, whose output voltage can be turned down with a circuit that allows a simple pot to be used as a control mechanism.

Part of the genius of the original powerscaling circuit is the fixed-bias version, which allows the bias voltage to automatically track the changed B+ voltage, in the proper direction to keep the output tubes correctly biased.

Has anyone considered using Mag Amp theory to incorporate Power Scaling?

Unfortunately, the website doesn't give as full an explanation for the audio amplifier test. But you should note he's trying to increase power to the speaker, where powerscaling is used to reduce power to the speaker (specifically, reduce the power level at which the output tubes run into distortion).

When I say "not as full an explanation" I mean the schematic shows a Low-Z input for the audio, but what impedance that really is isn't specified. I assume based on the diagram that this will be roughly speaker-impedance level. That implies an entire amp... preamp, power amp and output transformer, ahead of the "Low-Z Input" shown in the schematic.

Also, you might have overlooked that the 35kHz oscillator is a power source, or what you could view as an additional complete power amp at the "AC 35kHz" input on the right side of the schematic.

So ultimately what his circuit is doing as an audio magnetic amplifier is to use the pot and d.c. voltage to control how much of the 35kHz oscillator power is used to boost the output of the audio at the speaker.

[HotBluePlates]

Before you think you completely messed up, London Power (at least used to) sells Super Scalers, which take an existing amplifier's output and boosts it to a higher power level.

For example, say you have a tweed Champ and you want to play it with a band onstage. Its 3-4w output won't cut it onstage. So you unplug the speaker from the secondary of the output transformer, plug a Super Scaler into that jack, then connect the output of the Super Scaler to a set of speakers. A simple version of the Super Scaler can boost a Champ from 4w to 60w output.

The circuit for a Super Scaler that I've seen doesn't use a magnetic amp, or even have adjustable power boost. It uses 2 typical guitar amp output transformers (one is configured as the Super Scaler's input transformer), a pair of 6L6GC's, a power transformer and a fairly simple power supply. The 6L6GC's are run in class B, and the input transformer applies a high voltage a.c. to the screens of the 6L6's.

So the Super Scaler doesn't have the adjustability, but performs the function of the 35kHz oscillator from the mag amp since it is the power boosting element.

[HotBluePlates]

Another point: you may have thought the mag amp gives "something for nothing". It doesn't.

In a guitar amp, the output tubes take a small signal from the preamp and has a large power gain to drive the speaker. It does this by taking the power available from the amp's power supply and controlling the flow of that power through the amp's output transformer. The preamp signal is used as the control mechanism for the output tubes.

This is like the mag amp in that the preamp is like the Low-Z audio input (and in a way like the d.c. control voltage). The 35kHz oscillator and its power is equivalent to the guitar amp's power supply. The transformer between the two is effectively equivalent to the output tubes.

A guitar amp's output tubes don't make power, they only control the power that is otherwise available from the power supply. If you put 6550's in a 6V6 amp (with no other changes), you don't get any real power output increase because you haven't made the power supply bigger (so it can deliver more power) or optimize the output transformer to allow it to work with the bigger output tubes to transfer more power to the speaker.

And so it is with the mag amp. It's not just whether you add a transformer and have the control mechanism available; you have to have the extra implied parts his schematic leaves out to supply the additional power you want to use to boost your signal.

We probably don't see these more because they're better suited to non-audio applications, and aren't economical compared to just using tubes (or transistors) and an appropriate power supply.

[silverfox]

The first link in the post points to the construction of a Mag Amp using two transformers. I wasn't referring to the other page on that site regarding the use of torrid's to build an amplifier.

I'm not proposing obtaining something for nothing. What I'm asking is: Would a Mag Amp type transformer be another method to control the output power in the power amp.

By controlling the level of saturation in the core of an inductor, the amount of power transferred to the output may be controlled.

A Mag Amp has 3 or more (possibly) sets of windings around the core. A primary, a secondary and a control winding. By passing a DC current through the control winding the core is saturated to some point between the min and max core saturation levels. I believe the remaining saturation level is left for the primary to induce a signal, "Flux" into the core. If there is a large amount of flux level left to use in this process, the voltage or current transfer is large. A smaller degree of available flux level will result in a small power transfer.

Flux level is not the correct term. Don't remember the term that relates to degree of permissible flux for a given substance.

This would of course require changing the OT of an amp with a Mag Amp transformer. As strange as this may seem it may be possible to simply wind a control winding around the outside of the transformer. Don't know. Haven't tried anything yet. The link that leads to old electronic theory books has some good information on Mag Amps. Don't know what link that is at the moment so here's a Wiki http://en.wikipedia.org/wiki/Magnetic_amplifier

Silverfox.

[HotBluePlates]

The first link in the post points to the construction of a Mag Amp using two transformers. I wasn't referring to the other page on that site regarding the use of torrid's to build an amplifier.

The first page is great if you need a light dimmer. Which is not trivial, cause as he mentions, that's normally done with transistor SCR's or triacs, and spews electrical noise. So the mag amp method is quite elegant in that setting.

I think you totally missed the points I raised. Let me say them more directly...

What I'm asking is: Would a Mag Amp type transformer be another method to control the output power in the power amp.

...

This would of course require changing the OT of an amp with a Mag Amp transformer.

-  A magnetic amplifier is not a reasonable add-on for control of power inside an amplifier.
-  A magnetic amplifier cannot replace the output transformer in a tube amp and still have tubes as the amp's output device.
-  A magnetic amplifier would take the place of the output tubes if you wanted to use it as a way to raise/lower power delivered to a load. That's because the magnetic amplifier performs the function of the output tubes.
-  A magnetic amplifier could be used between the speaker output of a typical guitar amp and the actual speakers. However, at that point it boosts the power level.
- A magnetic amplifier may be heavy and expensive compared to simply using tubes (to boost power) or solid-state (to reduce power, as in powerscaling/VVR).


You would not replace the typical output transformer in a guitar amp with a "magnetic amplifier transformer". The guitar amp's OT performs the function of impedance conversion, and is needed unchanged for the output tubes to have a load they can actually work into. Meaning, the speaker is a low impedance and looks like a short-circuit to a tube's plate, so the OT is needed to make the speaker look like several-thousand ohms.

By controlling the level of saturation in the core of an inductor, the amount of power transferred to the output may be controlled.

... By passing a DC current through the control winding the core is saturated to some point between the min and max core saturation levels. ... If there is a large amount of flux level left to use in this process, the voltage or current transfer is large. A smaller degree of available flux level will result in a small power transfer.

All of that sounds reasonable.

But as you move to saturation, the primary inductance drops. The result is that power output of your lows/low-mids falls off before the output of highs. I'm thinking at a minimum, you will have an undesired tone control.

If you want to build something like this to satisfy curiosity, go for it. I know a guy (who briefly posted on this forum) who built an "inverted-mode" amplifier. The signal input was on the plates of the output tubes, and the speaker output came from the control grids. A horribly inefficient way to avoid having to use an output transformer, but he went through the design process and built it just to see for himself that it could be done. You won't see any commercial amplifier using that approach because it is so wildly uneconomical for the modest performance it offers.

In a similar way, You could use a mag amp in place of a guitar amp's output stage (output tubes, OT). I propose that you don't see this done anywhere because it is wildly uneconomical for small audio output powers (meaning, not on the order of several to hundreds of kilowatts).

For normal audio amplifiers, if you want to reduce the power output, it is much easier and cheaper to reduce the screen voltage of the output tubes (as in some early table radios), or plate-and-screen voltage (as in powerscaling/VVR), or use any of the very many master volume schemes out there. Also cheaper and easier for power reduction are any of the many ways to waste off a portion of output power (in a resistor, lightbulb, fan, etc) that attenuators use.

You should note that even the wiki article you linked points to this fact. The output tubes in a guitar amp control a relatively large output power by using a relatively small input power from the preamp as a control signal. The applications shown in the wiki list a number of ways of controlling very large powers with a relatively small control power, when other and simpler means aren't practical.

[spacelabstudio]

You couldn't just change the "gain" of the output transformer applying a magnetic field, if that's what you're asking.  At least, not in any useful way.  If you did it, you'd just lessen the impedance seen by the power tubes by decoupling them from the speaker load.  Will give you less audible output, but also fry your output tubes probably.

What he's doing is essentially putting the secondaries of the transformers in series with some signal which attenuates the signal.  Applying DC to the primaries then takes away the attenuation by saturating the core.  So he calls it an amplifier but you can also think of it as a voltage controlled attenuator.  Probably not much of a practical use in a guitar amp.  You couldn't reuse any of the existing transformers.  You'd need to choose a spot in your amp where you want to attenuate signal add a couple of transformers there.  Gets expensive with dubious benefits.  But you could always do a weekend science experiment with a couple of radio shack power transformers and see what happens.

[Willabe]

Isn't this how the saturable reactors that are used for the variable impedance in the Hammond organ (Sluckeys Warbler project) pitch shift vibrato work?

               Brad      

[silverfox]

HPB- Okay. What you say makes sense now. Particularly the decoupling of the impedance from the output tubes.

Harmonics of a transformer may lead to uncontrolled tone. Also agree with that. The harmonics of my Mosfet Trace Elliot Power Amp arising from the output transformers are actually quite good and add to the tone; But it was designed for that. They may be tube output transformers.

Willabe- Today I went to a large Flea Market. I saw an old radio that had two coils in it. One coil was fixed, larger and mounted parallel with the front panel. The smaller coil, centered inside the larger coil, was parallel with the chassis and rotated through two axis of orientation as I turned the control knob. Ultimately the inner coil ended up inside of and parallel with the outer coil. To say it another way, the smaller coil rotated through 90 degrees on two axis'.

It appeared to be some sort of variable phase reactor. Phase shifter? It was very simple in design. Perhaps it could be used as a variable power tap.

There are speakers that achieve power limiting through rotation of something on the back. I may have seen them posted here.

Whatever I try, can't affect the output impedance of the amp.

How would I use inductors to control power to the speakers without changing, by much, the output impedance the amp sees. Also it must be affordable and easy to build.

How about something that changes the distance between two inductors and like an arc welder transformer, simultaneously moves a bar into or out of the winding to maintain impedance matching. This would be attached to the output of the amp? Lots of copper and magnetic Iron. sounds expensive. Specialized market. Hmmm. Death Knell. But would it work? Is this a cheap mechanical power control?

Thanks for the considerations,

Silverfox.

[Willabe]

PRR knows/understands radio circuitry, HBP, DL and I think Sluckey too. Guys please forgive me for leaving any one out.

That sounds/seems more like a variable capacitance for tuning in a radio station to me?


              Brad      

[PRR]

> sounds more like a variable capacitance for tuning in a radio station

It is a variable inductor.

I think wrong trees are being barked.

[Willabe]

I think wrong trees are being barked.

Well, every dog has his day and every tree needs it's bark.       It was just a shot in the dark.  

It is a variable inductor.

Why a variable inductor, what's it's purpose?


         Brad      

[silverfox]

I freely admit I could be mistaken.

The variable inductor may be a band tuning component. There was a variable capacitor for station tuning and 1 tube. Perhaps the tube was a diode. There was no speaker or any other components for an amplifier.

Now why would a variable inductor be a wrong tree to bark up if one were trying to control the output power of a tube amp? Assuming of course there were also a means of maintaining the load impedance? Couldn't variable inductance lead to phase shifting with other components? Isn't phase shifting a means of controlling output power?

These are for me, legitimate questions in a though process. As you may have noticed, I dropped the Mag Amp as it sounds like that concept wouldn't work.

Fox.

[silverfox]

Eminence FDM Technology. I found the video. At about 1:28 in the video. The guy turns a mechanical part on the speaker. I believe it is akin to the two coil setup in the radio I saw today.

Eminence FDM Reignmaker Speaker


See, I knew I wasn't crazy. See SEE!!!!

Seriously, I probably saw this some time ago and combined with conversations on the forum started thinking about it resulting in the post.

So how does that speaker control work and could an output coil setup like that be constructed and attached to an amp??


Silverfox.

[HotBluePlates]

I saw an old radio that had two coils in it. One coil was fixed, larger and mounted parallel with the front panel. The smaller coil, centered inside the larger coil, was parallel with the chassis and rotated through two axis of orientation as I turned the control knob. Ultimately the inner coil ended up inside of and parallel with the outer coil. To say it another way, the smaller coil rotated through 90 degrees on two axis'.

It appeared to be some sort of variable phase reactor. Phase shifter? It was very simple in design.

Reminds me of the antenna loop in this radio.

Until I realized you were talking about a radio, the first thing that came to mind was a syncro, as part of a synchro/servo system (another place mag amps actually apply).

A synchro has stationary windings and a movable rotor winding, and is a control that you would manipulate to cause similar rotational changes in big equipment that is moved by a servo. "Big equipment" meaning things like battleship turrets or large rotating radars. In other words, things that aren't effectively controlled with any other technology.

Now why would a variable inductor be a wrong tree to bark up if one were trying to control the output power of a tube amp? ... Couldn't variable inductance lead to phase shifting with other components? ...

You tell Me how that controls power. What does phase have to do with anything?

Meaning, I've tried to provide you the theoretical and practical basis for why it doesn't make sense; but since you're convinced it does, then you provide me the theoretical/practical basis for why it will work.

Imagine you have a tweed Champ: no tone controls. Every coupling cap shifts the phase of the guitar signal some amount, when referred to the signal at the input jack. Each common-cathode gain stage inverts the signal (roughly same-as 180-degree shift). The output transformer may shift phase relative to some middle frequency as you move to extreme low and high frequencies. The speaker shifts phase slightly due to the combination of resistance and inductance.

But unless phase shift manifests itself by way of a squealing feedback loop that is creating positive instead of negative feedback, shift of absolute phase of a mono signal is not something your ear can detect.

[HotBluePlates]

Eminence FDM Technology. ... The guy turns a mechanical part on the speaker. I believe it is akin to the two coil setup in the radio I saw today.

Nothing like the two-coil setup you saw. And totally unrelated to a magnetic amplifier.

So how does that speaker control work and could an output coil setup like that be constructed and attached to an amp??

It cannot be attached to an amp, except by buying one of Eminence's FDM speakers.

How does it work? Well don't listen to the dude playing; in fact don't listen to the video at all except between 3:45 to 4:48 when you actually hear it without volume-normalizing.

I will upfront admit I can only guess based on what I see in the video. However, a speaker only has one coil in it: the voice coil. That voice coil is attached to the speaker cone and spider, so the knob on the back very likely does not attach to the voice coil in any way (if it did, it would inhibit the speaker's ability to move at all).

Normally, the voice coil sits in a very thin gap which either surrounds a plug magnet (alnico speakers) or is between a magnet assembly and a polepiece (ceramic speakers). The strength of the magnet, the close spacing to the voice coil, the stiffness of the speaker motor and its ability to move freely all contribute to a speaker's sensitivity: how many dB SPL ("sound pressure level") you get at 1 meter for 1 watt of applied power.

Some speakers have high efficiency and high sensitivity. Their acoustic volume with a given amount of power is higher than a different less sensitive, less efficient speaker.

I'd bet a month's pay that this is what is happening in the FDM speaker:
-  The knob is probably attached to either the primary magnet of the speaker or to the polepiece (or both). When twisted for maximum loudness, the entire magnet is moved to focus the maximum amount of its flux into the gap where the voice coil sits, yielding maximum efficiency for that speaker and maximum acoustic volume.
-  When you twist the knob for less volume, what you are probably really doing is moving the magnet assembly back away from the speaker basket and that voice coil gap. Less flux is focused into the gap, so the speaker's sensitivity is reduced. The result is that with the same applied power, you get less acoustic volume.

You see, this is not something to apply to an amp, but is a speaker-only thing. The volume is reduced not by manipulating how the amp is working, but by making the speaker less efficient. You should also notice that the volume can't be reduced to zero with this; it looks like there is some lower limit to how much the volume can be reduced.

[HotBluePlates]

Eminence says their FDM speaker can reduce output volume by nearly 9dB over the range of the knob's rotation.

I find it interesting that their standard guitar amp speaker have sensitivities ranging from 96.1 dB up to 103.5 dB... a range of 7.4dB in speakers optimized for their individual application.

It seems they've hit on how to get the speaker sensitivity to be sub-optimal as a volume control mechanism. Actually, quite a useful thing. I think I'll look into them as more detail becomes available.

[kagliostro]

Other Brand "same" result but different technology ?

http://www.fluxtone-speakers.com/Vin_article.html

http://www.fluxtone-speakers.com/FluxTone_Speakers_reduce_sp.html


----


May be this is of some interest

http://www.google.com/patents/US8249292




K

[HotBluePlates]

May be this is of some interest

http://www.google.com/patents/US8249292

Nice find! Yep, that's Eminence's patent and appears to describe exactly what I said, but with much legal jargon mostly to make the patent as general as possible.

Other Brand "same" result but different technology ?

http://www.fluxtone-speakers.com/Vin_article.html

http://www.fluxtone-speakers.com/FluxTone_Speakers_reduce_sp.html

Yes, but the big coil on the back of the speaker makes me think his idea is basically to create a field coil that counteracts the existing magnet flux to achieve the same end-result. Although if he can truly get 25dB of reduction, it goes further than the physical adjustment method of Eminence.

I'm guessing that with an a.c. inlet on the control box for the Fluxtone that there is a power supply to create some d.c. from the line voltage, and the pot adjusts how much current gets out to the field coil on the speaker.

Problem is the Fluxtone seems to take existing off-the-shelf speakers and mod them with the assembly, so $300 Celestion Blue or Gold alnico speakers are now around $1,000.

Even if the Fluxtone is patented now, and gives greater power adjustment, I think the high price will prevent its wide use. Eminence FDM speakers appear to list at $140.

[kagliostro]

appears to describe exactly what I said

Yes, that is, your description fits very well

----

but the bog coil on the back of the speaker makes me think his idea is basically to create a field coil that counteracts the existing magnet flux to achieve the same end-result.

If I understand well, in the text they say they remove the permanent magnet and use an electromagnet like in the old school movie-theater sound systems plus a variable controller that allows to reduce the strenght of the electromagnet

---

Yes, about cost $1000.00 instead of $140.00 is all a different affair

K

[silverfox]

kagliostro- Thanks for that link. As I recall old radios had an energized speaker magnet or coil. How could you maintain the impedance through variations of power?

HPB- First of all I stopped talking about Mag Amps a couple of posts ago and began considering other possibilities. The Radio antenna in your illustration is not what I saw at the Flea Market. I saw a variable reactor based on changing both the proximity of two coils and the orientation one had to the other, effecting the angle of incidence of the flux lines which would lead to variable power transfer resulting from the change of inductance- I think. Here is a link to a version of one:

 http://www.123rf.com/photo_12764389_variometer--variable-inductance-for-the-power-amplifier-isolated-on-white-background.html

Here is another but only one degree of orientation

http://www.wireless.org.uk/vario.htm

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.
Variometer

in radio engineering, a variable-inductance coil designed for tuning oscillatory circuits. The type of variometer used in the 1920’s and 1930’s was divided into two sections connected in series: a fixed outer coil (the stator) and a movable inner coil (the rotor). The inductance of the variometer is equal to the sum of the inductances of the sections and the mutual inductance between them, which changes significantly when the rotor is turned 180°. This type of variometer was widely used to retune the frequency of the oscillatory circuit of a radio receiver. The variometer subsequently lost its original form and use and is now used in radio receivers as an inductance coil containing within its winding a core made of a magnetic dielectric that changes the induction within insignificant limits in order to adjust the frequency of the oscillatory circuit.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.



On Phase Shifting and Power Control- There are two components to an AC waveform. Voltage and Current. When the voltage and current are in phase, that is to say, both increasing at the same rate over time and the peaks in alignment, the power factor will be about 1, or unity.

As one of the two either leads or lags the other, the available power decreases until the point at which they are 180 degrees out of phase and the Real Power is 0. Consider how the receding ocean waves begin to stand still as the water that washed ashore flows backward into the wave now approaching. Does that wave have any ability to perform work on an object in it's path, no. However it is performing work on it's internal (local system) state. Unlike the seashore an electrical system that is not designed to operate out of phase will burn things up.

Phase control is used to regulate the amount of power delivered to a machine or transmission system.

I'm not sure you realize, the purpose of my questions are not to debate or cause distension, rather, they advance my learning process. For example:

I began this post in hopes of finding/understanding a means of Power Scaling. As a result of the contribution, some of which were yours, I have learned a great deal on the subject. I believe one of the simplest methods would be to use the antiquated approach of the energized speaker. However there must be a load matching method incorporated.

Another method would be controlling the phase angle of the current to the speaker or perhaps the phase angle of the voltage and that is done with capacitors and inductors. That's already been done.

Perhaps another method would be to change the mechanical gain of the speaker.

Well, I certainly have a ton of information to consider now and I thank you all for your contributions, I think at the end now the variometer combined with a capacitor may hold promise,

Silverfox.

[HotBluePlates]

I'm not sure you realize, the purpose of my questions are not to debate or cause distension, rather, they advance my learning process.

I understand. I apologize if my responses to the hypotheticals you've raised seems argumentative. I'm not trying to be dismissive, but I think as you explore more about each topic you've raised (and there have been many!), you'll find they are largely not feasible. Originally, I wanted to help you avoid wasting time, but I think I've realized that learning more about each of these things is not time-wasted, even if it doesn't lead you to the answer you thought you'd get.

I began this post in hopes of finding/understanding a means of Power Scaling. ... I believe one of the simplest methods would be to use the antiquated approach of the energized speaker. However there must be a load matching method incorporated.

No load-matching. I was thinking more about the Fluxtone solution, and think I can refine the description which will hopefully show you how it works. I think it is less-complicated than I first imagined.

Fluxtone appears to take standard available guitar speakers and remove the existing permanent magnet structure. Once they do that, they replace it with a field-coil. This is an electro-magnet: d.c. applied through the field coil creates a magnetic field which takes the place of the magnetic field lost when the permanent magnet was removed.

Notice the Fluxtone has 4 terminals at the speaker; two of them are for the voice coil (the + and - from the original speaker) and the other two are for the field coil. The power supply (the box with the power cord, which connects to the amp's speaker output and to the Fluxtone) no doubt has a small d.c. supply in it. The knob (however the internal circuit is arranged) almost certainly controls the amount of direct current sent from the box through the field coil.

Rather than phase, the quantity of current is what's important here. The reason is that current through a wire (or coil) sets up a magnetic field around the wire, and the larger the current the stronger the magnetic field. If you reduce the current through the field coil, you weaken the magnetic field of the Fluxtone speaker which reduces the "foundation" for the speaker's voice coil to push against. The weaker magnetic field is kind of like trying to jump from a surface, and going from bedrock to quicksand as the surface you're jumping from.

The end result is less speaker cone movement, and less acoustic volume. A speaker's voice coil uses the alternating current applied to it to push off against the static magnetic field of the speaker's magnet (or field coil). Strong magnetic field + strong applied a.c. = big speaker cone movement. Cut the strength of the static magnetic field (by reduced field current in the Fluxtone, by moving the magnet in the Eminence), and the speaker doesn't move as much.

Eminence's approach is a similar idea but evades Fluxtone's patent (if one has been assigned as they imply by "patent applied for") because it works on the basis of changing magnetic field strength by physically moving a permanent magnet, rather than changing the current in an electromagnet to change magnetic field strength. A side-effect is Eminence's approach seems cheaper to manifest, at least for a manufacturer.

... I think at the end now the variometer combined with a capacitor may hold promise ...

Look into all of it.

I think you'll find that phase control only applies to 3-phase (or multi-phase) motors, and that LC circuits to mess with reactive power vs real power apply to tank circuits and RF where a single frequency is applied to the LC circuit and that the LC circuit is either tuned to resonate at that frequency or is tuned away from resonance.

So why do I even bother trying to dispel some of these things? It is not about trying to "prove you wrong" but has to do with a realization I had last night. For the first time in nearly 20 years of tinkering with this stuff, it finally dawned on me how the correct load impedance, output tube and supply voltage is selected for a guitar amp output stage.

In reality, it is dead simple and I should have grasped it after a week of learning about amps. But the mass of misinformation from prominent authors either omitting or misapplying technical information when talking about an amp's output stage confused the issue to the point where it seemed like it must be something "magical". I hoped to help you avoid the wasted time I experienced hanging on to things that cloud an issue rather than make it as clear as it really is.

[kagliostro]

As I recall old radios had an energized speaker magnet or coil. How could you maintain the impedance through variations of power?

I think HotBluePlates describing the Fluxtone unit give you the answer

K

[silverfox]

kagliostro- Yes, I went back and read everything again this morning and looked at the diagram.

Fox