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Converting practice combo to head only?

Discussion in 'Amps and Cabs [BG]' started by BassWombat, Jun 7, 2005.

  1. I picked up an old Randall 35w practise amp a while back. I took the grill off the other day and one of my kids accidently put their foot through the speaker cone (don't ask!).

    What I'm thinking of doing is making a new housing for the head as a standalone and wiring up a 1/4" socket on the back panel so I can connect it to an external speaker box.

    First off, the combo doesn't tell me the ohms of the speaker it had - anyone want to have a guess?

    On that basis, would it be as simple as plugging in an external PA monitor with say a single 10" speaker be okay?

    I know I could just replace the speaker but it seems like a fun project to do it this way.

    Out of interest, could I even plug my single 15" speaker box into it? It's an 8 Ohm, 200 watt SWR. What would be the impact on the sound of a 35w head driving a 200w speaker?
  2. PhilMan99


    Jul 18, 2003
    US, Maryland
    I'm weak on that "ohms" thing, but the old torn speaker may have the ohms stamped on the back. If not, maybe there's a model # you can lookup on the Internet (I like google).

    I'm assuming there's no "tweeter" or cross-over, just a single speaker - that would change things...

    Note: Driving the big speaker should be OK (ignoring the "ohms" thing) if you don't drive your amp into distortion. Big hairy distortion (think: Big Muff) outputs near square-wave which can fry a speaker. Guess how I learned that...
  3. ESP-LTD


    Sep 9, 2001
    What you're describing would be fine, but maybe a lot of work for a 35w amp.

    No guarantee what impedance it was but 8 ohms is a good guess.

    You might consider trying some other 10" speakers. MCM Electronics used to have an aluminum cone 10" for about $20 that did well in a sealed box; looks very cool as well.
  4. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    If you measure the dc resistance (yes, I know that's the only way to measure resistance) with an ohmmeter or multimeter and it's more than 4 but less than 8 ohms, it's an 8 ohm speaker. If less than 4 ohms, it's a 4 ohm speaker. Make sure the amp is disconnected from the speaker terminals when you do this.

    Be careful if you intend to install a jack in the back of the amp case. Not all solid state amps connect one speaker lead to the chassis. If you use a metal bodied jack you automatically connect the sleeve to the chassis. A plastic bodied jack (ala Marshall, etc.) would be a good bet here.

    Also, when drilling a hole, make sure that you either remove the circuit board or protect the board from metal shavings from drilling a hole...it would be a shame for the amp to start leaking "magic smoke". The magic smoke holds all the tone and if it leaks out, your tone will suffer ;)

    Have fun!!!!

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