|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
Hi all,I recently picked up a pair of Jensen F-12-NS speakers and I would like to ask a couple of questions about them. First, there is a 1/4" fine crack in the cone where it turns into the surround. What is the best adhesive to seal and strengthen this with? Second, they have DC resistances of 5000 and 250 ohms and it would be more convenient to be able to run them from one voltage source, but I haven't been able to find good information on the web for calculating wire sizing and other parameters for winding iron core coils. My understanding is that the current carrying capacity in a coil is different than in free air. Can anyone suggest a good source of information on this? When I tried to measure the inductance of the coils I came up with approx. 4.5 H for the 250 ohm and something around 65 H for the 5k, does this seem ballpark? I estimated (guessed) that these speakers should be capable of 15 W of output given that the old Hammond amps I've seen on ebay run 4 to 8 6V6 output tubes. The limited information I have states that the power dissipation in the field coil should be 100-140% of the undistorted output power so that gives around a 65V drop and 230 mA on the 250R and 275V and 55 mA on the 5k. Seem right?
Lastly, I've seen it suggested on this forum that these might be good in a front loaded horn or on an open baffle, but I didn't find any success stories or specific designs. What designs has anyone tried and did they work well or not?
Thanks for any information you can pass on to me.
Follow Ups:
Yo Steve and others:I recently acquired the above speaks but I can't find anything about them at the Jensen vintage sites I've visited. Anyone have any tech info or care to help me with an enclosure, maybe something easy for starters?
Hi Michael,Congratulations on the Jensens; they are really fine units.
I have repaired many cones with damage like you have described with a small amount of Titebond wood glue, thinned slightly with water and applied with a fine artist's brush. The object is to coax the cone fibers back together in their original relationship, and saturate the area of the crack or tear with just enough glue to restore the strength of the cone when dry.
These Jensens are often found with these mismatched field coil values, as the 250 and 5,000 ohm resistances were used in the Hammond amplifier circuit. It may be easier and more practical to obtain more drivers of the same resistances and pair them up, rather than disassemble and rewind the one(s) you have. Nevertheless, I have a few spare Jensen 12" field coils in several values and may be able to help you out. I can also supply you with winding info and advice; please write to me privately for this.
These drivers should be able to handle 15 watts of power safely, but you might not want to listen to it...their sensitivity is very high! In most hi fi uses you will be feeding them milliwatts. Although they were mostly used in direct radiator (often open backed) organ tone cabinets, they would be excellent in front loaded midbass horns. These are marvelous drivers, and you will have a hard time finding their equal in a permanent magnet driver.
Hello,Have a pair of 12" Jensens and a pr of Wurlitzer 6420 field coil tube amps (6L6's, 6SN7, 5U4G) and am planning on using them in my 180 cycle tractrix horns. Haven't gotten to it yet. How far up do you think these 12's will go in these horns? Would they be better suited for my bass horns (good to around 400 cycles)?
Thanks.
Magnetar
StartURL
Hi Mike,Your 12" Jensens would probably measure well to at least 3kHz., but they may be into cone breakup and soundin' snarly by 2,500 or so. I think they would best be used in a horn system as midbass drivers. I am using a pair of 15" field coils in my 70Hz. exponential midbass horns, with crossover points of 80 and about 350Hz. The relatively low upper cutoff is due mainly to my love of big a** mid horns, but the horn/driver combo does begin a natural rolloff at around 400Hz.
These field coil drivers are a bit of a hassle to use, but (to me) offer the ultimate in clarity and punch.
PS-- Do you know if these Wurlitzer amps any good? Got em for almost free.Thanks again.
Magnetar
StartURL
I've never tried any of the organ amps, although I have several of the parallel push pull 6V6 Hammond units. By reputation these amps are simple, conservative designs that sound very good, but perhaps lack extension to the frequency extremes due to the less-than-spectacular output transformers.
Done!! The woofer is now mounted in 16"x24" (WxH)baffle with 19" top and bottom wings and 15" side wings. Randomly wired it up to a University 4420 L/C crossover at 2k and crossed it to a Uni 4409 old model (1" driver)that I sat on top box and moved forward and back until I lost a little dip in response ~700-800 Hz. The gods must be with me, everything in phase, big stereo center. Attenuated the tweeter to match the output of my other speaker. And then it happened. Poof, the speaker became invisible. Music just appears in my room. Whoa. What's that about?
Sounds like you are on the right track! Welcome to the vivid sound of field coils. Just wait until you hear some f.c. compression drivers. This is a technology that, in my opinion, should have never gone away.One of the best speakers I've ever heard is a circa 1936 Lansing Monitor 500B. This two way horn system uses a 284 driver (daddy to all the later Altec large format drivers) on a tar filled 500Hz., eight cell horn. Below is a 15" woofer in a small W bin. The sound from this simple, early system is so natural, relaxed and complete, it takes my breath away.
Hi Steve,Thanks for the information. Really looking forward to hearing these speakers! To repair the crack, should I open it to dab glue on the facing surfaces or should I just let the glue wick into it? It is completely closed and almost undetectable as it sits now. Should I work from the front side or the back? Should I clamp it in any way while it dries? As a completely off the wall idea, would there be any advantage to wiring it up and sending it a low level fixed signal, say 200 Hz, for a minute or two after glueing, to sort of microscopically distribute the glue?
What would you suggest as the optimum frequency range to design for with these drivers? I like the simplicity of construction with an open baffle, but I'm not sure what all of the tradeoffs for the various options are.
Ahhh, I am supremely impatient to hear these. I think I have the materials at hand to knock up one baffle, cross it over at 2k to a University 4409 and run it off the center channel from my Scott 222C. Any objections? I will start as soon as I can decently turn on a power tool on a Saturday morning.
Thanks,
Hi Michael,I would just apply glue to the edge of the crack; it will seep in to where it is needed. After it begins to tack up (couple of minutes, you can gently push from both sides simultaneously with your fingers to make sure the fibers are mended back together. When dry there should only be a slightly dark line to remind you of the crack.
You'll have to experiment to see what method of baffling these drivers works best for you. I recommend front horn loading, but then I'm a horn guy. They probably have a resonant frequency of 60 or 70Hz., which gives them a nice bass snap in an open baffle. They will extend up very high, as they were often used full range in the organ cabinets, but will sound pretty rough for hi fi use above 2kHz. I'd cross them over to mid drivers at 1kHz. or below.
Hi,I am running the 250 ohm speaker with a 30V wall wart at the moment (to be upgraded) and planning the power supply for the 5k ohm coil. Do I need to take any special precautions to deal with kickback when I power the coils down?
I like many other have a pair of 500's & 5000's too. How about rewinding the field coils to run on say 12v? I remember someone mentioning to me that you should aim for 12W max dissapation.
Hi,Rewinding was my first thought too. But I was kind of put off because I have never disassembled any sort of speaker before nor have I wound an inductor. That means new skill set, tools possibly and time. Plus batteries and a quality charger so they don't explode when you forget to turn it off. On the other hand I have junk parts and a soldering iron. So this is my plan.
Please (double please with sugar on top) anyone feel free to correct my judgement or calculation errors.
For my 250 ohm or your 500R, any number of discarded or ultra-cheap transformers or wall warts should work. Right now I'm running a 24V 1.2A wall wart I picked up at Goodwill for two bucks. It puts out 31V at load for 3.8W and sounds fine. I'm shooting to have 15 watts available and mount a couple different dropping power resistors on a switch and maybe a 25-50R 3W pot to give adjustibility. I want 61V available for 15W, you need 77.5V for 12W. A 48V secondary should work for me, or a 60V for 500R, with a voltage doubler circuit.
For the 5k, you could run off the mains (something they wouldn't have blinked at when most of these speakers were made), but not recommended or get a cheap isolation transformer and run a voltage doubler circuit. You should also be able to take almost any two 6 or 12 or whatever volt matched transformers and run them secondary to secondary to get isolated 120V without exceeding any specs. 15W gives me 275V and 55 mA. So two diodes, two 200uF 160V caps, one 120uF 300V cap and a 100R 5W resistor gives me 294V at the load with 30 mA of ripple, per Duncan's power supply designer software. That's good for 17W, attenuate as needed.
These ideas and calculations were done in the middle of the night when I couldn't sleep, so therefore they may contain any number of errors. YMMV, etc.
Going this route, I should have two speakers running tonite and be relaxing in the world of auditory bliss.
I was considering getting the coils wound by a local power transformer winder (no way I'm doing that myself). Shouldn't be more than a few bucks.
I was much more worried about damage to and realignment of the voice coil than the winding part. If your field coils are like mine, they basically have the former built in as the pole piece and the support for the humbucking coil. Should be akin to spooling line onto a big fishing reel.I finished the HV power supply and the second cabinet, so I now have a somewhat matched pair. They are nice, period. Some info I can pass on is that I am running 248V into the 5k coil for about 12W. The coil gets nominally warm after several hours of power, say maybe 100-105 deg F. The output appears to be about 4-5 dB higher than in the speaker with 250 ohm, 31V, 4W setup. More importantly, the detail seems more there, defined, clear, low level whatever. I think I will bump it to 15W and if all goes well try 18W which is the limit of my PS. Just to see what it does to the detail. There is no hum whatsoever. I haven't measured it, but the PS design software models 300 mV ripple in my HV supply and, absent signal, the speakers are absolutely silent. I am listening right now at a comfortable early morning level of ~70dB with an RMS input of 50 mV (0.325 mW).
This post is made possible by the generous support of people like you and our sponsors: