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ampeg b15

Discussion in 'Amps and Cabs [BG]' started by jbass67, Jan 5, 2004.

  1. jbass67


    Nov 17, 2003
    does anyone out there know if i can use my
    early 1960 b15 as a preamp going to a separate
    power amp? or does it need to see a speaker
    load to operate safely? the b15 is a tube
    model rated @ 25w.
  2. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    OK, you found an Ampeg product that is before my time!

    In general, you can use it as a preamp IF

    1) you have a line out from it (I don't actually know if that one does), or modify it to have one.

    2) you provide some sort of load for the power amp same as the speaker impedance

    If you don't provide some load for the output it will very likely oscillate and overheat the tubes. If it doesn't do that, running a signal in with no load can develop very large damaging voltages in the output section of the power amp.

    If you just pull the output tubes out, that prevents some problems. But the internal voltages might change (go up) quite a bit, and I don't know what would happen to sound or other components as a result.

    You can get an 8 ohm power resistor good for at least double the rated power, and hook that up in place of the speaker. If you aren't using the power amp and don't want to, you could have the unit modified to be a preamp only.

    I assume you don't want to haul the whole cabinet, otherwise you could just leave the speaker etc hooked up, those units sound pretty good.

    Most of them have a power interlock on the speaker cable, so you can't turn it on unless you have the speaker plugged in. That's to prevent to problems from running no-load.

    So you may have to provide some sort of "cheater cable" to allow the unit to work with no speaker.

    I'd try not to mess it up doing anything permanent to it, like drilling holes, etc. It could have some serious value if it isn't too beat up.
  3. Have you got pics you could post? There are a few variations of the B15 all with slightly different features. Specifically, does it have the four pin speaker cable? If it does, you're going to have to get an adapter made for a dummy load, the "cheater" Ampeg Insider referred to. Most of the amp's magic is a combination of the preamp AND the power stage compressing under stress due to having a sagging tube rectifier, so using it a preamp won't sound the same as just playing through it.
  4. jbass67


    Nov 17, 2003
    thanks ampeg insider & psycho. the amp was modified before i bought it. the 4-wire connector
    wire was replaced by a 1/4" jack on the head & cabinet. the other out is an external speaker out that doesn't work. the set-up works great as a stand-alone but with obvious power limitations. it sounds so nice i might try miking it since that should not effect any internals.
  5. PB+J


    Mar 9, 2000
    arlington va
    I have exactly the same question and the same amp--modified so it no longer has the xlr

    I wish someone made a preamp based on the B-15!

    I'm going to try it with a Weber MASS attentuator hooked up to it, so I have a dummy speaker load and can run the "EXT AMP" output to my QSC power amp