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Ampeg B25B Question!

Discussion in 'Amps and Cabs [BG]' started by Funkster, Apr 30, 2002.


  1. Funkster

    Funkster

    Apr 6, 2000
    Wormtown, MA
    Hi all, I just acquired a Early 70's all tube Ampeg B25B. The Amp is in mint condition and works awesome but I have a question about the ohm's it is rated at.
    The speaker out's in the back say it's rated for a 16 ohm cab, will running it into a 8 ohm cab temporarily do any harm to the head or cab?
    My plan is to hunt down a matching 16 ohm B25B 2 15 cab to go with but I want to use it with my 8 ohm Eden D 115 while hunting for it!
    Thanks ahead for your reply's!
     
  2. Yes, another B25B fan!

    I wouldn't recommend running a tube amp without the proper load, and in your case, you can do just that with a few simple config changes.

    OK, there are (2) taps on the output tranny, 8 and 16 ohms. The original cabs are 16 ohms, and you hook (2) of them together to get an 8-ohm load. The speaker cable / jack setup is kind of weird. If yours is still in its original state, just tape up the cord end and don't use it, and plug in your 8-ohm cab into the EXT SPKR jack. That will switch to the 8-ohm tap of the output tranny. Don't use the attached speaker cable at all, and your amp will see an 8-ohm load on the 8-ohm transformer tap.

    On mine, I removed the freaky isolated jack and cord, and put in a switch and normal jack. So I have a small switch on the back to select 8 or 16 ohms, and I have a normal 1/4 shorting jack, but no attached speaker cable.

    Any more questions, fire away. I finally found a cab for mine, pretty miserable condition, but original. I love it. The coolest and most impressive jamming / recording rig ever.

    [​IMG]

    Chris
     
  3. Funkster

    Funkster

    Apr 6, 2000
    Wormtown, MA
    Thank you Chris! I might have a few more questions when I get into it a little more! I just got her last night!
    I'll post a pic this weekend!
    Mine has two regular 1/4" speaker input jacks in the rear and the I don't seem to get anything out of the Ext jack! I got to play around with it a little more. I plan on bringing it to my Amp Guru and have her cleaned, re-tubed and re-biased, It looks just like yours and it is a sweet little amp that will be perfect for recording...
     
  4. Alrighty, if you've got (2) standard jacks, what I said above is not true for you. Probably, the first jack is a shorting jack, so if nothing is plugged in it, the output is shorted out to save the transformer from nasty voltage spikes when running with no load. The second EXT jack may be a switching jack that switches from 16 to 8 when a plug is inserted. But if you don't have anything plugged into the main jack, then the output is shorted out anyway.

    So yeah, let your tech clue you in on the output stage wiring. You may want to do like I did and install a toggle switch to let you simply select 8 or 16 ohms, and make sure the Main speaker jack shorts out the output tranny when nothing is plugged into it, but don't hook up a shorting configuration on the EXT SPKR jack. Then always plug into the Main jack first to un-short the tranny.

    Just for information, you want the output shorted-out on a tube amp when there is no load. This saves the output transformer from voltage spikes that occur when running with no load. You wouldn't want to play it shorted out for any length of time, cause it's beating the hell out of your tubes, but the shorted output saves the tranny from getting an internal short circuit between windings.

    Chris
     
  5. Funkster

    Funkster

    Apr 6, 2000
    Wormtown, MA
    Ahhhh! Totaly makes sense Chris!!! I think!
    That's exactly how it acted.
    Plug into the ext and you get very low or no output.
    Plug into the main Spkr out jack and it worked great.
    Are you saying that if I have two 8ohm cabs that if I plug them into both jacks, the tranny switches automaticly to 8 ohms? If so that's cool!
    Wow complicated little head! that's why I love the real Ampegs.
    I like the idea about having a switch to change ohm's, I'm going to inquire about that when I bring it to my guru.
    I think I found a cab also! It's a late 70's model but it's a B25B 2 15 cab from Daddy's Junky Music, It just came in on trade. I put 50 bucks down on it but I got to wait for there 30 day hold on all trades policy, Arghhhh!!!! I can wait. Thank you Chris forn all your help.
     
  6. Cool! I'm looking for a second cab so I can have the ultimate 50 Watt stack.

    Just to clarify what I think is going on, if you plug in one speaker into the main jack, it should be 16 ohms. If you plug in a second cab to the EXT jack, it should also be a 16 ohm cab, and it will be hooked in parallel to the first 16 ohm cab, making the amp see an 8 ohm load. When a cable is inserted into the EXT jack, it switches the outputs to the 8 ohm tap of the transformer.

    At least that's what I think it is. Your guru will tell you for sure.

    If the above is correct, you can get yourself a 1/4" plug with nothing wired to it, plug it into the EXT speaker jack to change the impedance tap from 16 ohms to 8 ohms, and then plug your single 8 ohm speaker cab into the Main jack. Don't do this until your guru appoves it.

    Chris
     
  7. Funkster

    Funkster

    Apr 6, 2000
    Wormtown, MA
    Chris I just got pics developed, Yes no dig cam!
    Here's my rig a 72 cab and 76 head.
    [​IMG]

    Petty Sweet pair! Thanks for all your help.
     
  8. Hell yes. Tone to the bone.

    Chris