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Cab builders input please?

Discussion in 'Amps and Cabs [BG]' started by Verne Andru, Feb 22, 2008.

  1. I picked up a "vintage" Yamaha YBA-100 bass rig a few weeks ago and am in the process of tricking it out. The head is a 2 channel preamp and the cabinet contains the power amp and 3 x 12" speakers.

    The power section is rated @ 100 watts RMS [I have the SM] but the speakers are 50 watt, 8 ohm wired in parallel - so this would mean it's running closer to 300 watts near as I can tell.

    This unit was sold in the early 70's as Yamaha's ultimate bass rig. It's survived the years very well. A buddy used to be a Yamaha tech during the era this was current, so we'll be doing a cap job and changing the trim pots to bring it up to spec.

    As far as the cabinet goes, it's a closed back affair with the 3 speakers and a rectangular hole at the top of the baffle. I'm not sure if this was meant as a heat vent for the amp, or a primitive port.

    I'm replacing the 3 x 12's with a JBL K140 and 2 Peavey Pro 10 Neo's so the baffle has to be replaced which brings me to my question. The cabinet is built extremely well [this was a premium unit at a premium price] out of plywood, but what has me confused is the baffle - it's made out of a very poor grade of MDF. Not even good stuff.

    Does anybody know why Yamaha would opt for a cheap particle board baffle? I can't see it being a weight thing, as MDF tends to be heavier than ply and it certainly isn't nearly as strong.

    I'm leaning toward a 3/4" pacific birch ply baffle, but thought I toss this out to see if there's something I'm not understanding [which is highly possible].

    And a ? about speaker placement. The 3 12's were placed with 2 along the bottom and the third around the middle - the power amp section takes up most of the top 1/4 of the cab. The Peavey 10's are the same depth as the Yamaha 12's [not sure yet about the depth of the K140, but my guess is it's deeper - it's still in its cabinet]. If it turns out I can't put the JBL in the center because of space, would there be a problem with putting the 15" at the bottom of the baffle and the 2 x10's in the center?

    Thanks in advance.

    PS - they also used these really strange bolts to hold the baffle to the cab and the speakers to the baffle. They have bare, flat heads with a twisty "nail" affair at the head-end. The idea is they are "nailed" in from the front and the twisty part is supposed to hold the bolt in place. That's the theory, but in practice, due to the cheap particle board, they simply eat away and strip the wood when even a small amount of force is put to them. Anyway - I need to find something similar, or a replacement type of bolt that will let me attach the new baffle like the old.
  2. will33


    May 22, 2006
    Couldn't find anything on it on google, is that the correct model? Anyway, you need to take internal measurements of the speaker cabinet portion to see how big of a cabinet you have to work with before making any decisions on drivers, configuration etc. That will set your limits as far as how loud or low you can go with it and take it from there.
  3. The head is marked BE-200 and the powered cab is BS-100. When I called Yamaha to order the service manual they told me the 2 pieces sold together as model YBA-100. The only place that is marked on the actual unit is the chassis for the power amp, but I wasn't able to read that until after I pulled it out of the cab.

    These were sold around '72 and were very expensive, so they didn't sell that many. Mine has a serial number around 1000. The were built in Japan and from what I can tell from the service manual localized for every nation on the planet. My guess is only a handful actually made it to North America and this is probably one of the few that's survived the years. It's in very good shape for it's age.

    The cab dimensions inside are 34"H x 27"W x 12"D. There is a shelf in the top back quarter for the power amp that is approximately 27"W x 7"D x 8.5"H which I have to work around.

    I have the K140 and the Peavey Neo 10's. I had a buddy over yesterday who was in service for Yamaha around this period and he said the cab size and speaker placement probably had more to do with what looked good than sound acoustic engineering. My thoughts are that the new speakers certainly shouldn't sound worse than the stock and if I'm not happy with the results, I'll simply put it back the way I found it and punt it along to someone else.

    That said, it's got great potential - just needs a bit of help to handle modern instruments with a low B. At the moment the speakers fall apart when pushed.
  4. Here's a pic.

    Attached Files:

  5. K - finished this puppy off yesterday. Made a new baffle out of 3/4" birch ply with the 15" on the bottom and 2x10's about 2/3rd's of the way up. The original baffle had a small port at the top, which I emulated. Got some black Marshall grill cloth of the 'bay to replace the cheesy blue stripes. I stuffed the cab with insulation - the original was bare.

    Was a lot more work than I thought, but it pulled together very nicely. After tracking down a few inevitable rattles and buzzes, I concentrated on tone. Even though the 3 speakers [8 ohm each] were putting the same load on the amp as the originals, it was distorting pretty badly. After trying different combo's of speaker wiring, I settled on the 15 and 1x10. Seems the amp is happiest with either an 8 or 4 ohm load - anything above or below caused distortion. Had I known this going in, I would have either looked for 16 or 4 ohm 10's, or just cut the cab for 1x15 and 1x10. I'm leaving the other 10 in there for the moment as a passive radiator. May take it out and plug the hole, use the hole as a port or replace the 10's - but that's for another day.

    As far as tone goes, this thing just pounds now. Holds the low notes rock solid even when the amp is wide open. Not quite as loud as it was with the stock speakers, but still very loud with more than enough volume to gig with and tone for days. Well worth the effort. Here's a rough BOM [Bill Of Materials]

    Yamaha YBA-100 head and cab - $75
    JBL K140 15" speaker - $40
    2 x Peavey Neo Pro 10's - $97
    Grill Cloth - $39.50
    Plywood, screws and insulation ~ $100
    Total project costs ~ $351.50

    Gotta love Craigslist for making stuff like this happen.
  6. Here is the "after" picture:


    Sitting beside my bass gives a bit better idea of how big it is. As chance would have it, my bass is a Yamaha as well but the pickups are neodymium's [Q-Tuners]. The Peavey Neo's + the Q-Tuners give me an end-to-end neodymium rig!

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