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Jensen speaker bumps

Discussion in 'Amp Tech Center' started by mrgroo, Jan 3, 2020.

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  1. mrgroo

    mrgroo TDPRI Member

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    Hiya. Does anyone know why some old 50s Jensen’s have raised bumps around the speaker basket (and extra holes) near where it meets the bell? Were these for mounting transformers to? Does it mean they aren’t speakers from a guitar amp and came from an organ or something like that? This is a p15 by the way.

    upload_2020-1-3_12-33-40.jpeg
     

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  2. Wally

    Wally Telefied Ad Free Member

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    I looked at a variety of vintage Alnico Jensens. The only ones I saw that had those holes and bumps were the ‘N’ speakers. I will take a WAG that Jensen was concerned about two things with that higher capability speaker....weight of the bigger magnet and heat dissipation. The holes would possibly allow for better heat dissipation, and the bumps would strengthen the metal to avoid deformation the basket and the resulting misalignment of the voice coil. ?????
     
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  3. mrgroo

    mrgroo TDPRI Member

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    Cheers Wally! Any idea why other N’s of similar period don’t have them? A one year thing maybe or a different purpose for the speakers?
     
  4. Wally

    Wally Telefied Ad Free Member

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    Who knows....and like I said that is just a guess. What year was the speaker above made. I’ll take a look at my P15N in my Alamo Paragon to compare. The P15N in my Premier Super 88 is less accessible.
     
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  5. mrgroo

    mrgroo TDPRI Member

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    Thanks. This is a 57 but the 59 doesn’t seem to have it and I can’t find many pics elsewhere. Cheers
     
  6. mrgroo

    mrgroo TDPRI Member

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    Ps I think that’s a good idea re the heat sink etc
     
  7. schmee

    schmee Doctor of Teleocity

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    It's a good question for sure. Maybe it's related to accommodating a different version of the speaker, magnets etc?.
    Also, in sheet metal work, sometimes making bumps and ridges are for stabilizing a flat surface from distortion OR for drawing metal away from an area that tends to warp or wrinkle in stamping. Not sure if any of that applies here or not though.

    Notice the impressions on each leg of that basket? Projecting out? Those are not decoration, they are to stabilize those legs from warping. Most Jensen's just have a narrow long impression there though. Another weird thing about your speaker.
    All those holes have got to be to mount something on a different version. For cooling or whatever they would be more symmetrical I would think. SWAG's here...
     
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  8. mrgroo

    mrgroo TDPRI Member

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    Very interesting - thanks for the thoughts too.

    it makes me realise how little documentation there is out there on speaker design changes / colours / bells etc over the years. Very hard to find a history of those iconic Jensen speakers and how they evolved.

    ps I’m surprised that it seems impossible to find reproduction stickers of the old Jensen 50s logos for adding to bell covers that have lost them. I’ve looked and can only find a few of the blue ones, not the gold or green labels.
     
  9. Wally

    Wally Telefied Ad Free Member

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  10. Wally

    Wally Telefied Ad Free Member

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    Those older labels look to me to be a bit more expensive to make that the later flat paper labels.
     
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  11. mrgroo

    mrgroo TDPRI Member

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    My hypothesis is that the four holes were meant to seat a transformer on the back of the speaker. Then due to the loss of metal they designed it to be strengthened with the bumps. What do you think?

    Anyone know of any amps or other amp like objects that were mounting something to the back of the 15” P15N’s?
     
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  12. lammie200

    lammie200 Friend of Leo's

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    My guess is that those bumps are meant to stiffen the metal the same way a ridge in a car body panel stiffens the body panel. I would think that that surface should be very stiff since it is the interface between the speaker magnet and the speaker diaphragm. Just a guess, however.
     
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  13. Wally

    Wally Telefied Ad Free Member

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    mrgroo, you don’t say what the date code is on that speaker. The 1957 P15N in the array that I showed is too late for a field coil speaker, which would entail having a large electromagnet. I owned a 5B5 Pro from 1952 that ran an F15N field coil. I have not seen field coils later than that in a Fender amp. I don’t recall if the basket for that F15N looked like the P15N you show us. But....seek and he shall find....

    here....https://reverb.com/item/30016502-vintage-1957-jensen-f15n-field-coil-concert-series-speaker-2

    we can see that indeed that basket is like the basket used for the F15N. The four large holes were used to,mount the plate holding the field coils magnet. And,,,the bumps are there.
     
  14. mrgroo

    mrgroo TDPRI Member

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    Awesome - that’s great finding thanks!

    It’s still a little strange to me as it seems like this was a bracket then for a field coil but it was a factory Alnico P15N. Codes:

    ST 654 P15N c4964 3
    It’s a date code of 220712

    I don’t own it yet but am trying to buy it at the moment and just struggling to reach the seller, so doing some research in the meantime.

    I’ve much appreciated your opinions and thoughts guys!
     
  15. Wally

    Wally Telefied Ad Free Member

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    Mrgroo, how about tis. What if that speaker is not a 1957 but rather is a 1947 and an early example of a permanent magnet P15N. The permanent ‘P’ Alnico were introduced in the late 1940’s. The 1952 F15N was a late comer for a field coil in a Fender. fwiw, the speaker you are look8ng at...which may be the one in the array I linked????....also has the bracket for mounting the output transformer to the basket, which the later Jensen’s did not have, ime. I don’t think you are looking at a 1957 speaker....I think it is ten years older than that.
     
  16. mrgroo

    mrgroo TDPRI Member

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    Aha! That might be right by the sounds of it yes! It’s not the same speaker as the one you linked but your theory sounds very plausible indeed. What a shame as I was looking for a 57 to pop into my new purchase of a 1957 Fender Pro. Currently it has the incorrect speaker in it a 1960 P15P). I might just keep looking and you have probably just solved an annoying mystery for me!
     
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  17. powerwagonjohn

    powerwagonjohn Tele-Meister

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    Interesting the F15N has the semi-round plate under the magnet as was discussed in an earlier thread about early P15N speakers. It seems like the holes/bumps and the plates were left over designs from the f series baskets.
    Thanks John
     
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  18. schmee

    schmee Doctor of Teleocity

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    I have an old Jensen non paper label or two around... as the used car salesman in Joysey says: "ow mush ya wanna pay"? :lol:
     
  19. schmee

    schmee Doctor of Teleocity

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    I had a ~1960 Dano 1 x 15 amp that had that bracket and a ballast transformer mounted on it. (go figure) It was a C15R speaker though. None of us could figure out why it had that ballast tranny, but eventually we decided it was to avoid blowing the cheap P15R speaker. BTW, I sold that speaker to Alexander Dumble on Ebay. Gawd knows why he wanted it!
     
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  20. schmee

    schmee Doctor of Teleocity

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    There are so many different P15N frame configurations out there it's bewildering!
    One wonders if those bumps are to keep the bell cover centered? Seems a bit much for that!

    Look at the weird magnet/plate structure on this one! and... those holes... still there... WTH?
    [​IMG]
     
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