Qs on Connecting to a 1970s Ampeg B-15N

Discussion in 'Amps and Cabs [BG]' started by OldFenderPlayer, Jun 21, 2017.

  1. I recently played a televised gig using my mid-1970s Ampeg B-15N, which has been and continues to work fine. The sound engineer plugged a 1/4 cable into the Extension Speaker input on the back of the amp's head, immediately turning a great sounding amp into a no-power fuzz pedal; when the cable was removed the amp returned to its normal self. The cable was not going to another speaker, but to the television station's sound board

    This has me wondering about a gig next week with the same amp. I'll be using two passive Fender basses and was planning on keeping both plugged in for the duration of the three-hour gig, likely having one in the Bright channel and the other in the Normal channel.

    Here is the question: Since the amp didn't like having something plugged into the extension speaker, might it not like having two basses plugged in at the same time? Is there any relationship between the two sections of the amp to be concerned about? I have bench-tested and all seems fine, BUT the test was for 10 minutes and not 3 hours.

    At this age (40+ years) , do these amps mind having multiple instruments plugged in simultaneously?
  2. Without knowing which particular version you have, (tube or SS?), and without looking at a schematic, I think the two channels probably have quite a bit of resistance in series between the two inputs which would make any interaction between the instruments plugged into the two inputs simultaneously pretty much non-existent.

    Worst case scenario, is you unlplug the instrument that is not being played.

    You already tested it out with no problem. Don't worry about it.
  3. two fingers

    two fingers Opinionated blowhard. But not mad about it. Inactive

    Feb 7, 2005
    Eastern NC USA
    Speaker level DI.

    There are quite a few DI boxes that will do it. Countryman Type 85, a few Radial, etc.

    You run a speaker cable from the head to the DI. You run another from the 1/4" out to the speaker. The the XLR out on the DI goes to the mixing board. I did this for quite a while with an Ampeg V4. Works like a champ. If you don't EQ too boomy on the amp end it sends a beautiful tube sound to the board that incorporates the entire tube compliment (preamp and power sections).
  4. coreyfyfe


    Nov 19, 2007
    boston, ma
    Extension speaker or extension amp, they're very different.

    If extension speaker - don't do that. This is for a second 8 ohm cab, not a connection for the FOH. With a speaker level DI it might be okay, but you'd want it on the main output not the extension to keep the proper output transformer tap selected.

    If extension amp - this jack is essentially a preamp send for slaving a second amp, but there can be issues with impedance mismatch when using this output direct to FOH, and the result is usually exactly what you described. Try plugging the extension amp into a DI box with a high input impedance if you want to send the preamp signal. There are some tips and tricks in the portaflex wiki.

    There shouldn't be any issue running two instruments into the two channels, that's exactly what they were designed for.
    Last edited: Jun 21, 2017
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  5. BassmanPaul

    BassmanPaul Inactive

    The extension speaker jack switches the amp to 16Ω and puts what is plugged in to it in series with the internal driver. It requires an 8Ω speaker cabinet to be plugged into that socket.
    Last edited: Jun 21, 2017
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  6. lz4005


    Oct 22, 2013
    This is why it's so important to know the difference between in and out, powered and non-powered, and understand impedance matching requirements before randomly shoving plugs into holes.

    There's no such thing as a speaker input on an amp head. Instrument signal goes in, amplified signal comes out. Speakers only receive, except in the very rare case when you're using one as a microphone.

    Other people have already explained what the Extension Speaker output does. That "engineer" should consider himself lucky if he didn't damage his board.
    This is why half the house PA systems you see in dive bars have one dead channel. It's an expensive lesson, but usually one that only has to happen once.

    There's also a slave amp output on the back of your amp that would have been possible to use, but it would still require the right impedance for any connection, as coreyfyfe pointed out.
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  7. DI or mic the amp. No harm, no foul.
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  8. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    This is the correct response. I have heard Jamerson talk about going to the board using one of the ext amp jacks, but that only gives you the preamp. Which is fine, but speaker level DI gives you the entire amp, and it makes a difference.

    As for basses, that would work if you use the separate channels and not the different inputs on the same channel.
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  9. My original post had an error: it was the "extension speaker output (not input) that the engineer plugged in to. Please excuse the error.
  10. BassmanPaul

    BassmanPaul Inactive

    I think I talk for everyone when I say that’s what we all thought you meant! :)