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Hi PPL I have just recieved my field coil today.It has F12-N C5 142 on basket rim along with AO 21055-1 and possibly a serial 220925.Checking the driver out everything seems in order.One thing that worrys me is that the feild coil wires have insulation tape around a join in them.This cant be right.Perhaps it has had a replacment feild coil at some stage.The spider is phenolic.All so the cone seems to be thinker then it should be.I am worried it may have been reconed.I have never seen one before .But I have read here that there very fragile.Well this one doesnt seem to be.Certainly not compared to my my 8and1/2 inch german radio drivers.Thiner then a jbl cone yes but quiet think all the same.The cone has 6 ribs and 2 up and 2 down rolls in the suround.Dome paper dust cap.
I need to know if it original because I want to find a mate for this one if I like it. If it is not original I wont bother.I will find 2 more.
Field coil DCR is 980ohms and voice coil is 6.8ohms .What does this mean.
I was going to build a clc supply with what ever voltage you guys recomend and put a resister in it and work up.Can anyone tell me what voltage this should have on it.Cause I dont even know what impeadence it is yet.
It's going to go in a JE lab baffle with some filtering to test.Again HELP please I have to know what it sounds like Soon!
Follow Ups:
The simplest supply for this power is a 5-10 A silicon bridge followed by 2,000 uF 200 V electrolytic capacitor. This is plugged directly into 120 V wall outlet. You may include 10-100 Ohm 20 W resistor between the bridge and the capacitor to fine tune the output voltage.
At 25 W, field coil and yoke will be barely warm to touch.
F12N is much better though at 40 W on the field coil.
I was speaking from my personal experience. As I said, 25 W on F12N makes field coil barely warm to touch. The limiting factor is temperature rating of magnet wire used in the field coil. Antique wire was rated at least 100 degrees C. Transformers of that time were rated to run continuously at 80 C body temperature. There is no reason why field coil should be rated differently.
At 30 W, F12N coil heats to 50-55 C.
To run it at 40 W, the winding should be impregnated with silicon oil, and ~1000 cm2 heat sink added to the yoke. The temperature reaches 65-70 C, which is normal.
Needless to say the driver should not be placed in any kind of box. It should be on an open baffle with free air circulation.
Factory FC ratings were veeeery conservative, and for driver inside wooden cabinet with a lot of heat from tubes, with ambient temperature reaching 70 C...
There is a reason why a field coil would be designed for lower temperature rise than some other devices. The speaker's voice coil has its own heat dissipation to accomplish, all the while living within the motor structure. If there is excessive temperature rise in the motor it will make for a very hostile environment for the voice coil. Although I haven't measured it, I would also expect the dynamic compression of a driver with a hot motor to be greater than one running at normal temperature.
> Although I haven't measured it, I would also expect the dynamic compression of a driver with a hot motor to be greater than one running at normal temperature.
Hi Steve,
What makes you think that? The resting voice coil temperature will stabilize with the field coil and that will cause different parameters than with the field coil cold but energized, but beyond that, given power dissipations in the coil should cause the same temperature rise in the voice coil which would cause the same increase in DCR. Actually I'd think that a hot field coil might have less compression, as the same increase in DCR would be a smaller percent change relative to rest (with the hot field coil), so would cause less compression. I don't think this would be a very big difference though. However, I must admit it's been a while since I reviewed heat transfer so I could be wrong, and I don't have time to look it up right now.
Other than that, you're definitely right - you're adding the temp rise of the field coil to the voice coil, so depending on what the insulation's like, you could be asking for trouble.
John
Hi John, great to hear from you.
So maybe that's why field coil drivers sound good... they run warm! You're gonna make me put on my thinkin' cap about this, but what you say makes sense. Hmmm.
There are practical considerations that weigh against running these old drivers at high motor temperatures. At what temperature would a paper former spontaneously combust? The other poster was talking about motor temperatures up to 212 degrees Farenheit. I've only smoked one coil former all these years, but when I was done with it there was nothing left but charcoal.
Running a motor really hot will probably expand the center pole a thousandth or two; not sure what the effect would be on the top plate. The voice coils I've seen that warped out of round probably did so due to high temperatures.
For what its worth it ran for 6 hours and drifted up to 44 watts and lived.You could put a water cooling block on the back if you where keen enough.Or perhaps just a heat sink.
Feeding 40 watts to the field coil of any Jensen 12" speaker is dangerous and ill advised. It might sound great, but not for long. Remember the words to that Foreigner song? "...live all of my years in a single minute."
I looked through some old Jensen catalogs. In a circa 1940 catalog, the early pedestal based version of the A-12 has a rated field wattage of "Maximum 18; normal 14; minimum 10." In a 1950 catalog the F-12N has a rated field wattage of 17.5, and the F-12Q is rated 12.
These speakers were designed conservatively for a long service life. The operating point of the field coil is usually well into the "knee" of the saturation curve of the pole pieces, and there is little to be gained by forcing more dissipation from the field coil than it was designed for. With the multi kilo-ohm coils, the wire is like human hair and any over voltage or overheating situation is likely to cause failure. I have seen many open field coils on old Jensens.
When operated correctly a field coil motor only become mildly warm to the touch when fully warmed up, which may take several hours. If you cannot leave your hand on it continuously without discomfort, it is running too hot.
All the Jensen literature I have at home suggest 20w maximum. Even the larger F15LL will not take 40w.
Regards,
John
Yep I left it on and it got real hot.I will set it down to 20watts tomorow and give it a try.If it sounds as good.I will will report back tomorow.I am worried I might have magnestised the pole piece as the voltage climbed up to 220v so 44 I think.
Well I got it up and running today in the JE Labs baffle.I had it running at just over 40watts I think it drifted up to 41 watts at times.I used a converted pa amp with the valves out.~270volts 20uf/7Hchoke/20uf/400ohm/20uf.I ran it for 20 minutes. At first it shouted so I threw in a 1mh choke.Very effecient and good vocal projection.But nothing really spectacular.Seems to have that thick paper cone sound but who knows.I had to turn it off as the coil was starting to get hot.Hot enough to start getting to hot to leave my fingers on the coil for a extended length of time.So I am wondering if it has a problem or it just cant handle 41 watts.I could definetly smell it to.Hot laquer perhaps.All so one of the voice coil wires seems to go in a coil around the field coil.Iam thinking it probably is a choke.
Any way I have no decent compression driver horns for on top.I do have my pm4a and oris horn.I will get around to mounting it up on top.It almost has enough bottom end to not need a sub in the je labs baffle perhaps not quiet.Should make a good workshop system if it doesnt over heat.I will try lower voltage tomorow.
As I posted above, 40 watts is WAY too much dissipation for a 12" jensen field coil. It is overheating and smelling bad because you are in the process of burning it out. The only 40 watt rating I have seen is the maximum dissipation rating on a Lansing 15" theatre woofer, which has a much larger motor than your Jensen.
I would recommend 14 watts as a good figure to work up to. Start with 10 watts for several hours, checking the temperature frequently, then proceed to 14 watts if the coil remains fairly cool. It should never become too hot to leave your hand on it comfortably.
Steve my poor speaker.It got really fried.Well maybe not really. I measured the dcr when it was hot last night and it was up from 980 to 1200ohms.I will measure it again today when I adjust the supply.At least the lower supply requirements will make it easyer to build .Steve have you experienced any difference in the sound when using minimal amount of capacitiace verses huge capacitance in fc supplys. And finally thank you your input.
I have seen a few references in the old theatre service literature to "cold" and "hot" field resistance readings. It is true that the resistance increases with temperature; sounds like it sure did in the case of your speaker! No matter, if the field coil still has continuity then it is okay.
So far I have not noticed much in the way of sonic changes due to the field supply used. Many folks insist that it makes a huge difference, but that has not been my experience so far. However, a friend and I made up some new tube field supplies for the Lansing Iconic two way speaker. When my friend installed the new supplies on his speakers he noticed a considerable improvement in sound quality over the solid state supplies he had used previously. To some extent it might depend on the supply voltage. The Iconic runs on 330VDC, while most of my work involves 12VDC field coils.
Hey Steve I just halved the voltage down to 20 watts.It hardly is getting warm.Musicly I little less power but perhaps more bottom end.And yes the coil did come back to 980 ohms once it had cooled.All so there is a coil that goes around the feild coil that the voice coil goes through before it enters the speaker.Is this a choke.I can wire around and see I supose.Which bring to wonder if choking it this way has advantages. Oh and I added on 100uf to my supply due to the now large resistance comprimising it.Perhaps it helped maybe they do need a good supply.I have 8 batterys there I will get around to trying them eventually.Thanks again.PS are they your altex feild coil compression drivers on ebay?
That thin coil under the top plate is a humbucking coil, and it is placed in series with the voice coil. Most of these speakers were used in situations where the field coil functioned as part of the smoothing circuit for the amplifier's power supply, so they were fed some gnarly DC. You can either leave it alone of disconnect it; I remove them when taking the drivers apart.
No, those weren't my field coil converted 288s on ebay.
If it were, then it was to be supplied with a hum signal out of phase with field coil hum.
The extra coil around the pole piece is a demodulator coil. Its purpose is to counter the modulation of magnetic field in the gap by the voice coil. It is connected in series and out of phase with the VC. It makes magnetic field in the gap immune to modulation by the field of the voice coil. Another advantage of the demodulator coil is reduction of driver's Le, which allows better high frequency extension.
With an underpowered field coil that you advocate, demodulation coil has most benefits and should not be disconnected. With high FC power, when the yoke is close to magnetic saturation, demodulation coil becomes magnetically uncoupled from the voice coil and all its benefits disappear.
Sser2, your comments are most interesting and thought provoking. I have never thought of that little extra coil in this way, but I like your thinking. Do you have any citings from the literature that indicate that this coil functions as a demodulator? It would be fascinating and would open up new avenues of thought if true, and I am all ears when it comes to improving field coil technology.
I have deduced that this coil is a humbucker mostly because the field coils in most of these Jensen units serve as filtering components in the amplifier's power supply. If the audio signal contained some hum components then their presence in this coil, if in opposite polarity to the field coil, would tend to cancel the hum in the speaker's output.
I am not sure why you state that I advocate an "underpowered" field coil. I have stated the recommendations of the manufacturer in response to what appeared to me to be your reckless recommendation of almost three times the nominal field dissipation that these units were designed for. As Paul Klipsch used to say, "Any mechanical or electrical device can be overdriven to the point where failure must occur."
Ok thank you.I will try with out it.It running nice and losening up at 20 watts. good bass impact.
Oh this supply is getting harder to build everytime I look here.I got 240 here anyway.But I dont think I am would pluging a bridge into the wall anyway.Thats seems scarry somehow.I just woke up here but I think that means I need 200ma and 200 volts to feed it to 40watts.This choke I have to use today is getting harder and harder to find.At 200 volts that makes 200volt electos out of the question too.Have I got this right?I just wanted to use like 100uf of paper caps anyway.I didnt think ripple was a major concern here.Though I think current demand is.I thought maybe a big choke can deliver that though.I will see what I can find.But first I got to take my little girl to her first swimming lesson cause shes sweeter then all the field coils in the world :)
Hi.
If you post a picture, someone here can most likely tell you if the cone is original or a replacement. The original Jensen A12/F12N cones do tend to be of lighter construction than the more modern replacement cones.
If I remember correctly, the F12N field coils can take about 17.5w, or about 130v and 134ma for your specimen. I run my A12's at about that wattage and would not take them any higher.
Regards,
John
Thanks for the reply. Iam reasonably sure its the original cone now.I found a good picture to compare.134 ma *2 for stereo would be a big supply. Have to be solid state.I think I might have been better of with the higher impeadance ones.But I will put somthing together I am sure.I am wondering now just how much ripple I can leave on it.I have a some 20uf paper caps I want to use.I have a feeling it might make a difference.Unless someones states different .I will use these figures as a guidline starting lower of course.Thank you again.
I think it is the original cone.I found some photos of one.I am going to keep searching these archives.Perhaps I have missed some answers to my questions.
I have searched and searched.I thought I would need to know the impeadance but most are referring to resistance.Standard are 250ohm and 5000 .Mine is 980.I might start at 10 watts and work up.
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