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Discussion Starter #1
How would I go about switching between my combo's speakers and my 4x12 cab? Its a fender twin reverb (4ohms) & a marshall jcm 800 cab (8ohms). How would I set up an ABY switch & is it possible to run both at once without something getting fried?
 

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I'd like to try and answer this but I'd need to know the impedance for each of the 4 speakers in the cab.

Someone else will likely respond to you as they might own this cab and know the best way to wire it to get 4 ohms total impedance...and/or Wild Bill will respond.

I have no idea if the Twin is able to power the cab at the same time you are using the Twin's speakers. The Twin's speakers might be cut from the circuit as soon as you plug in the extension cab. Again, wait for the responses of others that have this knowledge.

Best of luck with it.

Dave
 

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What would I give to see a schematic for that?

Any ideas as to how it works?
A configuration of power resistors with 2 rotary switches? (probably more complicted)
How would it dissipate the heat generated? (maybe a fan?)

I'd like to sort out how to build an 8 ohm to 4 ohm version.

Cool device !!

Dave
 

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Discussion Starter #5
It would be cool to match the impedence...my 4ohm combo to 8ohm cabinet is a pinch nasel-ish; however, it won't help me switch between combo speakers and cab speakers in a live setting.

The speakers in the combo are originals...oxfords I believe and the Marshall cab is loaded with Celestion G12-65's (reissue)..I'm fairly certain they're 15 ohms.
 

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Hi Dave,

What would I give to see a schematic for that?

Any ideas as to how it works?
A configuration of power resistors with 2 rotary switches? (probably more complicted)
How would it dissipate the heat generated? (maybe a fan?)

I'd like to sort out how to build an 8 ohm to 4 ohm version.

Cool device !!

Dave
It's simply an "Z-Pad" (the output is attenuated by a maximum of 16dB and the impedance changes), as opposed to an "L-Pad" (which is a continuous impedance and variable level control) by using power resistors.

Want to make your own? Here you go:
http://radiomagonline.com/notebook/passive_attenuators/

Scroll down to "Javascript Pad Calculator" and enter in your desired impedance vs. output. If you get negative resistances, you have to up the dB attenuation so it's practical.

Cheers!
 
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