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Upgrading the RS Musical Instrument Cab to a Bass Cab

Discussion in 'Amps and Cabs [BG]' started by Thor, Apr 11, 2005.

  1. Thor

    Thor Gold Supporting Member In Memoriam

    Edit 12/5/2014: Went over this DIY thread, fixed all the pics and links I think. Let me know if you spot anything off. Figured I may as well bump it too. Keep in mind I was a total n00b to this in 2005. Enjoy, it came out great and I still have uses for it despite some technical shortcomings.

    Edit 2/11/2009: NOTE: This is an old build thread that had the pictures lost during multiple forum software upgrades. I have been able to reconstruct the thread which chronicles the driver selection process and the rebuild process. It has excellent technical input from some well known speaker designers and aficionados and I recommend it for beginners to WinISD, cabinet building and design as an example of the issues involved in a successful project outcome.

    All driver selections are made and the cab is finished, so input on that level is
    no longer needed.


    I am working on a rebuild or upgrade of the Radio Shack ‘Building Speaker Enclosures’ project from a book published by David Weems in 1980 for Radio Shack. I built this cab from scratch in 1982. Subsequent advances in design technology lead me to believe this can be upgraded with significant boosts in performance at reasonable cost.

    I will post some pics below of the project, but essentially, it is now a 2x12” sealed box with 3 piezo horns, presented at 4 ohms. All speakers are wired in parallel, and the piezo line has a 10-ohm resistor in series. I am going to leave that part alone and only focus on the woofers.

    The author provided specs on some drivers cited in his long out of print book, though none are available on this driver, Radio Shack Cat No. 40-1314. The driver has a silver dust-cap, I am told from a source at Bose Corp that this was typical of Altec-Lansing. Confirmation, denial or discussion of that would help me evaluate if they are suitable for this project. I have searched the web extensively looking for specs on these drivers to no avail, if anyone knows where I could find them please let me know.

    With the specs in hand, I was considering porting the bass cabinet. I would prefer to do this without a shelf if at all possible because of the cab construction, it has a removable panel at the rear. It would be quite easy to port the front once the parameters are available.

    I did acquire a recent copy of Vance Dickason'’s Loudspeaker Design Cookbook to broaden my understanding of ported box design.

    The internal dimension's - ID - of the cab are roughly 11.5" x 16.5" x 34.5" = 6546 cu. in and deducting the volume removed by the piezo tweeter shelf I come up with an estimate of 6486 cu, in, or 3.75 cu. ft. for the internal box volume.

    Plan A would be to port and dampen the cab. ( see below)
    EDIT: This was not an option

    Plan B would be to replace the drivers with other drivers suitable for my needs.

    Just to define my needs more clearly, I would like to use this as a bass cabinet, and reproduce down to Low E 41 hertz without farting, I do not play with a low B 30hz, so rolloff in that area is not an issue for me. I don'’t need a lot of real high end performance, based on my playing style and tonal taste.

    I use an Eden WT-300 hybrid SS amplifier, with an internal 12AX7 tube pre-amp. It will deliver 300 watts @ 4 ohms and 220 watts at 8 ohms. So it should have plenty of power to punch those little bad boys. I have a bigger rig as well, an Acoustic 140 head with a 106 bottom, with 2 x 15 drivers. My back is aching looking at it, and it no longer fits in my car, so this upgrade is beginning to look real interesting to me.

    part II.

    I fired up the cab after rewiring and punched it pretty hard with the Eden. Though I don’t have the Thiele/Small parameters, it is obvious to me that the RS drivers do not have enough low end to handle the bass signal delivered by my passive/active P/J 81 Ibanez Roadster and the Eden wt300.

    TB'er and friend secretdonkey has an Accugroove Whappo jr., and mentioned to me that they have one 12” that functions as a woofer, and another 12” configured as a mid-range driver. This suggests a low cost solution of finding a suitable 12” woofer replacement and designing a crossover circuit to limit the low end of the RS driver.

    Does anyone have a suggestion for a suitable woofer? Anything for under a $100?
    ( EDIT: The driver selection has been made...)

    Bottom line is that I can get an Eden or Aggie 2x12 for not a lot more than 400 bucks, so throwing more than 200 into this cab probably doesn'’t make sense.

    This thread is intended to demonstrate how an old cab can be revived and modernized with a limited expenditure.



    Footnote: In order to link all the pictures some had to be attached to different
    posts. All build pictures imaged (not attached) are in the correct sequence. Some
    attachments are not because of the 2 attachment per post limit.

    Attached Files:

    Last edited: Dec 5, 2014
    Stumbo likes this.
  2. Thor

    Thor Gold Supporting Member In Memoriam

    This is the interior of the original cab.


    Attached Files:

    Last edited: Dec 5, 2014
  3. Thor

    Thor Gold Supporting Member In Memoriam



    Attached Files:

  4. wow...you mean this book..... :D


    Sorry I can't be any help to you.
  5. Craigle

    Craigle "Careful with that joke, it's an antique!"

    Mar 10, 2004
    New Orleans, Louisiana
    I'd probably PM Bill Fitzmaurice. He seems to know a good bit about identifying drivers and cabs and such.

    Unfortunately, I couldn't even begin to answer your question.

  6. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics

    I think it's highly unlikely that those are Altec drivers, which were mostly pretty expensive cast frame drivers by the 80s. If you want to know the T-S parameters, why not test the speakers yourself? Here's one of many sites I found with a method for doing just that: http://sound.westhost.com/tsp.htm

    I it were me, I'd look at BGavin's spreadsheet, find a couple of drivers that would be happy in that size enclosure that are realistically priced for what you're willing to spend, and have at it.
  7. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Those most definetely aren't JBLs. Quite likely they are Utah OEM for Rat Shack, the aluminum dust covers were an attempt at resembling JBLs. The Piezo tweeters are OK, but they shouldn't have been horizontally arrayed. Dave wouldn't do it that way today. I'd plug off where they are and remount them vertically to one side of the top of the box or go with just the one in the middle. The box should have a shelf brace between the drivers to beef the box up, holes cut in it to allow passage of air. The box needs to be fully lined with poylester batting or acoustical foam. Download WinISD to figure the correct tuning for the box and the duct size(s) required. You would only get specs on the drivers by measuring them yourself, but they aren't really worth keeping anyway, even by 1980 standards they were not high quality. A couple of Delta 12LFs would work pretty well in that size box with a nice response hump around 80 Hz.
  8. Thor

    Thor Gold Supporting Member In Memoriam

    (Edit: The response above was from Bill Fitzmaurice, a well known and respected audio engineer and is absolutely spot on target.)

    Thanks for your comments. :)

    Adding additional bracing to the box is not difficult. A shelf with air holes is not a problem, though the box is very strong as is with the cleats in it.

    There is currently 24 oz of polyester pillow stuffing in the box, 2 bags from 'The Fabric Place', which pretty much fills the whole thing, though I did not show that in the pics.

    Bill, I assume those are "Eminence" brand Delta 12LF's correct?

    Any favorite sources you can recommend?

    I will disconnect the 2 diagonal piezos rather than do the major surgery. If I was going to go that far, I would just start from scratch with a new box, I guess.

    Thanks for your help btw! :)


    JP, yeah, that was the book. Feeling as old as me yet?
    That cab is older than most members here, I suspect! ;)
  9. Funkengrooven

    Funkengrooven Turn it down? You gotta be nuts!!

    Those Speakers were almost certainly Utah or someone like him.
    They will last about 10 seconds with the Eden pumping it out.
    WinISD is an outstanding Theile-Small program, contact me off list and I'll email it to you if you can't find it.
    As far as hi-power 12's I use WeberVST Michigan Ceramics and pound them with an SWR 550x all the time. If you prefer the Alnico sound Weber builds them like that too. and I'm in agreement with the vertical tweeter array. Vertical will reinforce and add definition and directivity, horizontal will smear and throw it everywhere and definition will be lost. If you want Weber's net address contact me off list, I don't wanna run afoul of the rules. And I also have that RS book but it gives only the barest directions, all the cabinets I have built I used JBL or Altec guidelines or just built off the top of my head. Oftentimes the design criteria are dictated by the size you are willing to move and vehicle space you have available. Once those considerations are out of the way the drivers and interior volume of the cabinet will then dictate the type of cabinet and porting, if any.
  10. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The box may seem strong enough but you'll find out otherwise with the first open E you hit. Stuffing the box is OK just so long as there are no exposed internal surfaces that sound can reflect off and neither the rear of the drivers nor the entrance to the port (s) is blocked. One of the better places for Eminence drivers is Pi speakers. Go to their site and send an email message to Wayne for a quote.
  11. Thor

    Thor Gold Supporting Member In Memoriam

    :D I am getting this WHO like feeling all of a sudden :D

    email coming

    I am not sure I understand the difference, perhaps you would define that for me, (and other readers)
    Edit: I checked the website, on the link here and looked up the Michigan Ceramic 12's. they look pretty good as well.

    Ok, I will alleviate that then as well.

    No rule book infraction there, as neither of us have any commercial interest in the company. People post useful links and resources for others to see all the time.

    Well, what it essentially comes down to is using the existing box and beefing it up. Calcualting the port size and choosing the drivers. Final step will be to remove 2 of the tweeters, and fill the holes. Well, they looked good at the time!

    Thanks again for your help and ideas!
  12. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Last edited by a moderator: Dec 5, 2014
  13. Thor

    Thor Gold Supporting Member In Memoriam

    Ok, a brace will be step one then. I will get a pic up when that is complete.
    Roger that.

    Thanks for the tip on that, I'll get in touch with him as well.

    I found these specs on the Delta 12 LF's on the Pi Speakers site. This is on heck of a driver! I like that freq curve a lot! That one looks like a winner for sure!
  14. Petebass


    Dec 22, 2002
    QLD Australia
    I noticed you based your calc of 3.75cu'ft based on the external dimensions. The pic is gone now but that timber looked about an inch thick. Taking that into account and that cab has aprox 2.7cu.ft. of internal volume.

    The Delta 12lf's is what Avatar use. I'm not sure how big the Avatar box is, but if you base the cab size on the Thiele small parameters, that speaker prefers a cab of 2.4 cu'ft PER DRIVER. So your box might be too small.

    Thor you can get WinISD for free from Limearteam Website . I recomend the "Pro" version. Don't worry that it's still in Alpha testing, it works well and does a lot more than the older version.

    Before settling on the 12lf, I'd also be looking at The Omega 12 Pro which doesn't go as low as the LF, but having 2 of them would compensate for that, and they would work better in a cab that size. Or you can go all out and get a couple of Beyma 112nd/w speakers. Why build your own Avatar-like cab when you can build your Accugrove-like cab? Depends on how much money you want to sink into this I guess.
  15. Thor

    Thor Gold Supporting Member In Memoriam

    Actually the OD of the cab is 1.5 inches thicker.

    I deducted 1.5 " from all dimensions as the material is 3/4 of an inch thick. I think I did state the
    " ID of the cab is " i.e.the internal dimensions. I then approximated the volume taken up by the outside of the shelf and deducted that.

    I will take a minute to recalculate this tonight, but I think it is reasonably accurate. I don't have the book at hand now, but I will come back to that, as that is an important issue, I gather.

    I did not take into account the volume of the speaker basket in the calculation.

    Guess off of this hint ....;) My son's college just sent a letter with a 10% tuition hike for next year. The total tab would buy a small foreign pickup truck, a bass boat and a 3 new bass rigs to put in the truck bed. And I would have money left over for a pizza and beer party for all my lowlife friends. So, as little as I can get away with.

    Yes, I would to hear about the speakers, let me know. GAS
    Thanks !
  16. Petebass


    Dec 22, 2002
    QLD Australia
    Oops my bad. Sorry, didn't see the "ID" bit. 3.7cu.ft it is.

    The gist of my post still applies though. 2 x 12lf's would ideally need something like 4.8cu.ft. I'm off to work now but I can try running the lf's in 3.7cu.ft through Winisd later tonight.
  17. Thor

    Thor Gold Supporting Member In Memoriam

    I emailed Funkengrooven for the WinISD program , downloading executable programs from the internet at work is one of the seven deadly sins according to our IT dept.

    Guess I can't blame them after we got hit with all that Paris Hilton pr0n on the network ... :eek: ;)
  18. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Kinda ironic, given that you got it in email from a computer of unknown integrity, rather than the software developer's own website. :eyebrow:

    I had to deal with the same restriction at my last job pretty regularly, FedEx loved us. :cool:
  19. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The Delta 12LF would ideally like to see 7 cu ft per driver, which would get you an f0 of 35Hz and an f3 at 28 Hz, which is pretty much hi-fi response. But even with about 1.5 cu ft per driver volume it's quite good for bass, with a very acceptable 55 Hz f3, a response bump above that and enough excursion and power handling to get the job done, and at a very reasonable price.
  20. Petebass


    Dec 22, 2002
    QLD Australia
    Granted. I hadn't had a chance to diddle with the smaller enclosure size. It seems to measure up well enough under small signal conditions. I'm a little worried about the large signal performance though, especially in the 82Hz region. Seems it's only good for about 150w at that freq before x-max becomes a problem. I suppose two of them equates 300W, and the Eden WT300 is a 300w head, so in this case there's no problem.

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