How to run my ampeg b25b?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by edwardsforest, Sep 30, 2009.

  1. edwardsforest

    edwardsforest Member

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    I don't know exactly what to run my ampeg b25b into (what cab at what ohm). I tried it through an 8 ohm guitar cabinet and the amp was very quiet. I then tried it through a 1x15 cab with an ohmage I do not know and I was a little bit louder but still very quiet. Ok ok, give me all the crap you want about plugging into random things, I knew it was a bad idea, but I turned the amp off immediatley after I knew it wasn't working. :dunno
    Anyhow, Can anyone give me a final verdict on how to run this, or maybe something is short in the amp itself?

    Here is what I am working with...


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  2. FrankieSixxxgun

    FrankieSixxxgun Member

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    It's a little newer than my bassists' B-25B, but he runs his into a pair of Ampeg SVT-CL 15" cabinets. It's anything but quiet, so I'd imagine something has gone awry with your amp. As always, check the tubes first. Luckily for you, that amp doesn't have an adjustable bias, so you can just throw a set of 7027's in the power section and see what happens. Don't forget the preamp tubes.
     
  3. AL30

    AL30 Member

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    The amp wants to see a 16 ohm cab. I think you can also use a dummy plug in the 16 ohm Speaker jack and run an 8 ohm cab into the Ext. Speaker jack.

    If you're not running a dummy jack make sure you're plugging into the correct speaker jack. I know that's common sense and I don't mean to be insulting but I do stuff like that all the time. It happens.

    By the way if you're running a guitar into that try jumpering the channels together. That is a rock n' roll machine of an amp. :aok

    AL
     
  4. edwardsforest

    edwardsforest Member

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    well, all the tubes light up when its on. And I don't know how to really check the tubes and I don't want to open the thing up because really I don't know my way around the inside of these very well at all.

    As for the dummy load, can you explain how I can do this. Someone else reccommended this to me, but I don't know HOW to put a dummy load in with a 'plug'? You just plug a 1/4' cable into the out and leave the other side of the cable not plugged into anything? Help.
     
  5. AL30

    AL30 Member

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    It's not a dummy load just a dummy plug. You're suggestion should work. You may want to ask around a bit more just to make sure (I'm not expert) but that's how I've been running mine.

    The volume shouldn't drop like that with a mismatched cab though. Are you absolutely sure you had it plugged into the correct speaker jack? Hate to harp on this but you don't want to spend any unecessary $$$. If you're certain then you should take it to a tech. That's a great amp you've got there. A little bit of maintenance will keep it happy.

    AL
     
  6. jay42

    jay42 Member

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    I wouldn't do that. You don't want a short on the dummy. If you don't have a spare 1/4" plug somewhere, they're usually easy to find at radio shack.

    The B25 is wired more intelligently than some amps of that era. When you plug into the Ext. Spkr. jack, it switches the output transformer taps. Do not extrapolate this to other amplifiers without consulting the schematics.
     
  7. pdenmark

    pdenmark Member

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    Hi - I've got a similar amp and (no cabinet) situation, and just wanted to be sure I've got this right: you can use an 8-ohm cab with the EXT speaker (with the dummy jack thing), even though it says 16 ohm right above the jack in the picture? Just want to make sure there isn't more than one version of the amp, or something, with different setups.
     
  8. bigdaddyforge

    bigdaddyforge Member

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    having researched this myself quite a bit as it's extremely confusing (frankie from a few posts up's bassist) the speaker out is 16ohm. as jay42 mentioned ampeg is notorious for using switchable jacks on their gear though they don't all work the same so definitely check the schematics first. on the b25b when something is plugged into the EXT speaker it switches to the 8ohm tap so you can run two 16ohm cabs for an 8ohm load.

    as such, you can use a dummy plug in the ext speaker out and an 8ohm cab into the speaker out, or just plug an 8ohm cab directly into the ext speaker jack in a pinch should you not have a dummy plug handy. using a dummy plug is the proper way so you don't inadvertently plug in two 8ohm cabs and let the smoke out, but either method works as long as you're paying attention.
     
  9. Cubsworth

    Cubsworth Member

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    I also have a b25b head and i am currently running two 8ohm cabs and it just isnt quite loud enough. Is running both of these cabs a bad idea? i am completely lost. I just had new tubes put in the head by a tech so it should be loud. any information on this head would be greatly appreciated.
     
  10. brad347

    brad347 Member

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    Yes, the amp is designed to be used with one or two 16 ohm cabs. One 16 ohm cab in the "speaker" jack, and you can add another into the "ext speaker" jack. The switching OT taps make this possible while keeping matched impedances.

    A side-effect is that you can do as AL30 suggested-- plug an empty 1/4" plug into "speaker," and a single 8 ohm cab into the jack labeled "ext speaker 16 ohm." This will give you a matched 8 ohm impedance.

    Great amp for guitar or rock bass. Medium-low volume with bass and a single 15" speaker (maybe not quite enough to get above a pounding drummer and stay super clean). Kind of like a "baby SVT." Anywhere you could use a B-15 Portaflex, the B25B and a single cab should be more than enough. With two cabs, it should be plenty loud for most situations.
     
  11. Cubsworth

    Cubsworth Member

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    Another question, does anyone know what the ext. amp port is for?
     
  12. snowman910

    snowman910 Member

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    Don't stop with the schematic when considering the output jacks - there have been lots of mods on the jack arrangements on these amps and what you see printed on the chassis and schematic may not be match what you see inside the chassis - sure didn't on mine!
     
  13. Stu Blue

    Stu Blue Member

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    :agree

    That will usually be a line level output to/for a power amp (or PA)......
     
  14. steadygarcia

    steadygarcia Member

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    Reviving an old thread here...but my B25B has a built in speaker cable which I was told was for an 8 ohm load, or I can use the speaker out jack for a 16 ohm cab, is that correct?
     
  15. grey67

    grey67 Member

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    What jack do you put the dummy plug into,the regular or extension on the b25b
     
  16. rf7

    rf7 Member

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    It's the other way around typically. The cable should be for 16 ohm loads, while the jack will switch the tap to an 8 ohm load.

    Into the regular jack. Plug your 8 ohm cabinet into the extension speaker jack.
     
  17. Mars Rover

    Mars Rover Member

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    Sorry for the thread resurrection but I'm desperate. After much searching this thread comes closest to providing answers. I've read that the transformer can fry pretty easily if you don't pay close attention to the impedance. I only paid $75 for this rig. The cab is roached but the amp is nice after I changed the filter caps. On the first try, I got the worst shock of my life so I took a look inside and the caps were dust with leads shorted. I want to use the amp mostly for guitar with other cabs and it sounds pretty good. I think all it needs at this point is a re-tube.

    This one has the hard wired main speaker cable (as one poster mentioned). Labeled B-25B, serial number is not legible but I think it's early - it has the toggles on the front instead of the rockers. Maybe 1969 or 1970.

    I've read all different things but what may the most sense (maybe) is that both speaker outputs are for 16 ohm (they're not marked). If so:

    Does that make the extension jack 8 ohm, with nothing plugged into the main because the plug on the built-in cable would technically be a dummy?

    Would it be better to replace the built-in cable with a jack, then then just go with the most common way to use this "dummy plug" to get an 8 ohm output?

    Maybe the dummy plug could be used in the extension jack, making the main output 8 ohms?

    This dummy plug is open, the pin is not shorted to the base, correct? It's just a switching device?

    Thanks for any help!!!!!!

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  18. AL30

    AL30 Member

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    I would take the hard wired speaker cable out and put in a jack. Then you could run the dummy plug without a cable hanging off the back of your amp.

    AL
     

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