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New Cabs for a Bassman 70.

Discussion in 'Amps and Cabs [BG]' started by orpheus_sword, May 21, 2010.

  1. orpheus_sword


    Mar 28, 2009
    I have a Fender Bassman 70 head in great shape: looks good and sound better. Currently I'm playing it thru a 4 ohm Peavey cab which has a 15 *AND* 2 tens in it. SO as you could imagine, this cabinet is pretty bulky and heavy. I want to get rid of this cabinet, but I don't want to lose that sound of a 115 and 210. I don't know much about this head, so my question(s) may be overly basic.

    Can this head drive two cabs? Specifically, are there 2 plugs on the back? I see an "ext. Speaker" jack, but is this the 2nd cab spot?

    Also, the head says it needs 4 ohms, so If i buy two 8 ohm cabs the net load will be 4 ohms, right? SO this will be okay? or is the head expecting 4 ohms per jack?

    Lastly, does any one have experience doing this with a bassman? And do you have suggestions on which cabs to buy? I'm looking to spend around $300-500 per cabinet.
  2. craig.p


    Sep 28, 2008
    New Hampshire
    Yes, the Bassman 70 will drive two cabs.

    Yes, the "ext speaker" jack is where you'll connect your second cab.

    This is one of those amps that play with output transformer secondary-winding selection depending on whether or not the extension speaker jack is occupied. When only the main speaker is plugged in, the full winding is used. When two speakers are plugged in, only half the winding is used.

    Although the schematic doesn't come right out and say it, it seems clear that each jack wants to see 4 ohms, for a combined impedance of 2 ohms. The main jack is denoted 068270 SPKR JACK FOR 4 OHM MIN. LOAD, and the extension jack is denoted 014143 EXT. SPEAKER JACK FOR 4 OHM MIN. LOAD. That and the split secondary winding tell me it's 4 ohms per jack.
  3. orpheus_sword


    Mar 28, 2009
    Cool, thanks.

    So what'll happen if I plug an 8 ohm cab into the 4 ohm jack?

    The bassman is only rated at 70 W, and it's impossible to find a 4 ohm, 100ish W cabinet nowadays. I'm aware of the potential hazards with underpowering a cab (I read all the stickies here), so I'll be careful to not run the head too high.

    Also, do you have any suggestions on cabs? I went to GC yesterday, and they didn't have anything worth looking at --- so I'll probably have to buy them online, which (without playing it), make me pretty nervous.
  4. craig.p


    Sep 28, 2008
    New Hampshire
    What'll happen? According to the schematic, nothing bad, since it says "minimum" load. From that, we can assume there's some safety margin in the output section of that amp.

    Don't worry about underpowering a cab. Just listen for amp distortion and then back down if you hear too much. If it sounds clean or even just a little dirty, you're fine.

    What cab to buy depends on a lot: the music you're playing, the room you're playing in, how loud the rest of the band is, whether your bottom string is E or B, and whether there's mains support for your bass. But if you were to ask me just to suggest a baseline to work from, I'd start with an Avatar neo 2x12 http://avatarspeakers.com/b212.htm which would give you reasonable bass and reasonable midrange too, assuming a 4-string bass and a small club and the rest of the band behaving itself volume-wise. If you want to get a little more exotic (and serious), try Bill Fitzmaurice's Jack Bass http://billfitzmaurice.com/Jack.html but build it with a 12. Depends on your woodworking skills. Or you can have one built. Whatever you choose, efficiency will be the name of the game in the context of only 70 watts. This is where the Fitzmaurice design would shine, the relative PITA factor WRT getting it built notwithstanding.
  5. SpamBot


    Dec 25, 2008
    St. Paul, MN
    Underpowering is a myth. Read the FAQ more carefully.

    To use two 8 ohm cabs, daisy-chain (Run a cable from one cab to the other, and one from one cab to the head) them and connect the pair to one of your amp's jacks.