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vintage tube amp tech question, please help!

knewjakknewjak 1,231 Posts
edited April 2006 in Strut Central
Who knows old electronics? I am trying to make a extension cabinet with a old tube amp. My plan is to hook up a speaker to the amp, and hook the amp's input line to the output of a late 50s Wurlitzer piano. The Wurly already has a 40watt internal tube amp, but it only powers its internal speaker. The 1/4" output jack on the piano supposedly is low output so it needs to be amplified, which is why I am bringing in the new amp and speaker. Make sense? ok. so how do I hook this shit up properly without blowing myself up? if anyone has a clue, please get at me. it is much appreciated. thanks.here are some photos of the amp...

  Comments

  • 4YearGraduate4YearGraduate 2,945 Posts
    The question here would be if the output from the Wurly is pre or post internal tube amplification and what the output impedance is. Regardless, you can probably get a variety of flavors by trying the different inputs on the back there. I would try a 1/4 inch to XLR (or make one) cable which, I think should drop the ground enable you to plug into the mic input. This is assuming two things: that the XLR input is pin 2 hot (and that the cable is pin2 hot) and that the output of the wurly is pre amplification. If it is post, then you need to go right into that "program in" RCA jack from the wurly - so you'ld need a ts male to rca male cable.

    Make sure, though, that you are confident the amp actually work correctly before you try this out, that it passes signal, amplifies, etc. Tube stuff can be finniky.

    There is really no chance of killing yourself unless you actually open up the amp.

    good luck and let us know what happens..
  • asstroasstro 1,754 Posts
    Find what impedance the speaker output is and make sure that the speakers you use match it, you could fry the output transformer if it isn't matched, especially on an old amp with old tubes in it. If you kill the transformer you have pretty much killed the amp unless you do a very expensive repair.

    Also, make sure to unplug that thing and drain the power before you go poking around inside, tube amps store a pretty powerful jolt of electricity even after you unplug them.
  • knewjakknewjak 1,231 Posts
    tube amps store a pretty powerful jolt of electricity even after you unplug them.

    I did not know this, good to know.

    ...also when you speak of impedance are you reffering to ohm's? Regardless, I cant figure out even where to attach the left and right speaker wire on the back of that thing, got any ideas? I know that I could just keep trying a bunch of different combinations, but I want to be on the right path first. And for whatever its worth, Im only trying to hook up one speaker, so do I have to worry about bridging the speaker wire?

    And Thes, thanks for the tips I appreciate it. Im not sure if the piano output is pre or post amp, Im guessing it is 'pre' as you are supposed to have a secondary amp when hooking up an additional speaker. I could be wrong though.
  • asstroasstro 1,754 Posts
    Yeah, I was talking about Ohms, but there's no clear indictaion that I see. I'm used to looking at guitar amps though, not hi-fi gear, so I might be missing something obvious.

    I would go a forum for hi-fi geeks and ask for advice there before doing anything if I were you. The amplifier section here looks like it might be a good place to start:

    http://www.head-fi.org/forums/
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