LSC
479 posts
Jul 31, 2013
11:11 PM
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Can someone enlighten me please? I've kind of gotten hooked on old Gibson amps but like to be quite selective and I don't like using amps for furniture or display pieces. They've got to be a useful tool. I'm seeing some interesting older models but am unfamiliar with some of the characteristics.
I'm not at all familiar with field coil speakers. As I understand it, the output transformer is mounted on the speaker? Are there any tonal or practical differences compared to other vintage speaker types?
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LSC
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Greg Heumann
2289 posts
Aug 01, 2013
8:40 AM
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Modern speakers have a voice coil connected to the speaker wires, which creates an electromagnetic field proportional to the input signal. That field interacts with the field of a permanent magnet mounted around it to move the speaker cone.
Field coil speakers have a voice coil connected to the speaker wires, which creates an electromagnetic field proportional to the input signal. That field interacts with the field of an electromagnet mounted around it to move the speaker cone. The field coil (electromagnet) creates a constant steady field and is powered by the amp's power supply.
To a large extent, a magnetic field (whether permanent or electromagnetically generated) is a magnetic field. However this isn't entirely true when there are multiple fields - i.e., the one from the voice coil and the one from the field magnet. AlNiCo magnets' fields break down in the presence of strong electromagnetic fields, which is why AlNiCo magnet speakers offer a natural compression, while ceramic magnet speakers don't. I don't know what happens with field coil speakers in this regard - it is likely there is some difference from AlNiCo or ceramic that would subtly affect tone.
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Last Edited by Greg Heumann on Aug 01, 2013 8:42 AM
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5F6H
1656 posts
Aug 01, 2013
11:59 AM
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@LSC "I'm not at all familiar with field coil speakers. As I understand it, the output transformer is mounted on the speaker? Are there any tonal or practical differences compared to other vintage speaker types?"
I do not own, nor have ever owned or to my knowledge heard an amp with a field coil speaker, so can't comment on tonal properties.
An OT on the speaker frame does not necessarily mean you have a field coil speaker, some amps with permanent magnet (PM) speakers had the OT on the speaker frame including old Gibsons (GA-20 for example). Gibson dropped the field coils pretty much with the "BR" line.
The field coil usually has a winding that connects to and is part of the amp's B+ (high voltage dc) supply, because it needs the dc to generate the magnetic field. Field coils were pretty much obsolete on amps by the early 50's. It is more normal & much less hassle to convert from field coil (if the original speaker is shot) to PM, than the other way around. You need to factor in the power supply resistor when you do.
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Rick Davis
2182 posts
Aug 01, 2013
2:47 PM
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My 1947 Gibson BR-6 amp had a field coil speaker. I liked the tone a lot.
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chromaticblues
1430 posts
Aug 01, 2013
3:52 PM
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Every field coil amp I have tried that was in good working order sounded bad!
Part of that is the era that these were being made. The amp technology changed a lot from the forties/early fifties to the sixties.
I have had amps where the field coil speaker went bad and replaced them with alnico speakers only to sell the amp afterwards because it just wasn't the same after that!
I would say they are great for that thick old school sound.
It would be interesting to put one in a mid sixties amp just to compare to the original speaker of that era.
I honestly think the dimise of these were due to cost savings of useing alnico speakers and nothing more than that.
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shbamac
348 posts
Aug 03, 2013
12:42 AM
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In a guitar amp the voice coil doesn't induce a current in the field coil. The felid coil is powered by DC voltage drawn off B+ (usually as Mark stated above). The voice coil is powered by AC. AC can be induced into the voice coil by the felid coil. A hum-bucking coil is used wired in series with the voice coil in such a way that any AC induced into the voice coil from the field coil will be out phase with the AC induced into the hum-bucking coil by the FC (feild coil) reducing hum. It is more complicated than that but that is about the just of it.
The FC's magnetism is not constant. It fluctuates as the signal load draws the voltage up and down in strength. Like when you hit or play a guitar very hard you create a voltage draw which decreases the FC magnetism and gives you the compression just like a alnico. Alnicos and FC are very similar in the way they sound. You would have to create a huge voltage draw to really tell the difference. The FC'c inductance was sometime used as the filter choke in some amps and radios. But the main difference in the tone of the amps that used FC speakers was the preamp tubes and other values/combinations of other components. Not the FC speakers. These amps were also lo-fi.
In a way Leo Fender killed the lo-fi amp. He changed the tone/construction style in 1949 or 50 when music was going from radio to TV. By the mid 50's demand was for brighter/louder hi-fi amps when rock and roll was taking priority. Prior to that amps where lo-fi where the standard of "tone" was more important than loudness. But most people's opinion of good tone has shifted I believe. I much prefer the tone of the old lo-fi amps. Give me a Valco or an old Dano and I am in heaven.
Last Edited by shbamac on Aug 03, 2013 1:21 AM
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Banyan3
14 posts
Aug 04, 2013
12:01 AM
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I got a 1952 Gibson br6f with the original 10" Rola feil coil speaker in it. I'll post some sound clips . The highs are kind of ice picky.
Last Edited by Banyan3 on Aug 04, 2013 12:05 AM
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