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Converting practice combo to head only?

B

BassWombat

Apr 2, 2005
151
2
Sydney, Australia
www.thecavanproject.com
#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
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
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
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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