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1972 Bassman 100 - Ohms Question

Discussion in 'Amps and Cabs [BG]' started by lustersilk, Jun 11, 2012.


  1. OK, I was reading an old thread and was learning that tube amps only want to find the same ohms they are rated for. In my case I have a 1972 Fender Bassman 100 which is a 4 ohm amp and two 8 ohm SWR cabs (2X12 & 4X12).

    Since these two cabs in parallel will equal 4 ohm I should be good to go, right?

    As always, thank you for you exepertise.
     
  2. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    should be good to go. The old bassman amps have fairly robust output tranformers and can usually tolerate a mismatch, but it is best to have the proper impedance.
     
  3. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    They want the same or lower impedance than the tap rating. That's the opposite of SS, which want the same or higher than the minimum rated impedance load.
     
  4. jumbodbassman

    jumbodbassman Supporting Member

    Dec 28, 2009
    Stuck in traffic -NY & CT
    Born Again Tubey
    By tap you mean cab rating correct?? I remeber reading fender can go double so a 4 ohm head can run 2 4 ohm cabs. is that correct?
     
  5. will33

    will33

    May 22, 2006
    austin,tx
    "Daisy chain" your 2 cabs together and plug the one speaker cable in the "main spkr" jack and you're good to go.
     
  6. jumbodbassman

    jumbodbassman Supporting Member

    Dec 28, 2009
    Stuck in traffic -NY & CT
    Born Again Tubey
    My converted super twin head is smiling... Will be a monster.
     
  7. will33

    will33

    May 22, 2006
    austin,tx
    Sorry, noticed you're not the OP. My advise was for running two 8ohm cabs. If you have two 4ohm cabs, plug one in the "main" jack and the other in the "ext" jack. No daisy chaining in the case of 4 ohm cabs.
     
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The jacks on the amp are parallel so it doesn't matter which way you do it.
     
  9. will33

    will33

    May 22, 2006
    austin,tx
    Yeah, I'm confusing the OP and jumbodbassman here.

    Not sure if the supertwin/studiobass is like my 135 with the ext. jack switching and putting them in series.
     
  10. sweet - thanks for the tips my friends. I found a re-cap kit and tube kit on ebay. My amp dude is going to love this.

    Can't wait :)
     
  11. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    AFAIK all Fenders except the 300 used a switched main output jack. The switch shorts the output if there's no speaker plugged into the main speaker out. The extension jack is parallel wired, but not switched, so if you mistakenly plug into the extension jack when using a single cab the output is still shorted and there's no sound, something virtually every Fender owner found out the hard way at one time or another.
     
  12. will33

    will33

    May 22, 2006
    austin,tx
    And one thing that is wrong with my 135. I can plug into the ext. jack and play just the same as the main. It's a good thing these are as robust toward mismatches as they are or I would've killed mine long ago. First due to sheer ignorance of their operation (cranking them into 8 ohms), and again for not realizing I shouldn't be getting any sound at the ext. jack that way...and cranking some more.:)
     
  13. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The 135 schematic shows a normally closed jack on the main speaker out that shorts when there's no plug inserted. But it also shows something on the extension jack that seems to switch taps when the extension is plugged in. As best I can tell it uses a 4 ohm tap with one cab alone plugged into the main jack, a 2 ohm tap when a cab is plugged into the extension. Perhaps the manual would shed light on exactly how it's set up.
     
  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    As was explained the baseman 135 extension jack has a switch that changes the output transformer tap. Cabs rated for 4 ohms should normally be connected to each jack. When an extension cab is plugged in, both the main and ext connections are connected in series and are switched to an 8 ohm tap on the output transformer.

    It is important to check that the ext jack switch contact is kept clean. It loves to oxidize, I use Deoxit. Also, the ext jack needs to be isolated from the chassis. They use a pair of isolation washers to do this. Sometimes the old ones cause a problem and need to be replaced.
     
  15. will33

    will33

    May 22, 2006
    austin,tx
    Thanks for all the info. I'll take care of that when I get the rest fixed. It's since developed a tube sovket that won't heat the tube. First and only problem I've had with it in the 10 years I've owned it. Still has all the original caps, etc. Time for a general overhaul, then play it for another 30 years. This has been one of the most reliable amps I've ever owned, despite user error, etc.

    Yeah, I've since learned the ext. jack is supposed to switch to an 8ohm tap. You can run an 8ohm cab in the ext. jack and a dummy plug in the main jack if I understand that correctly, or chain 2 8ohm cabs together, then plug a single cable in the main jack for it's 4 ohms.
     

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