Wiring 5e3 combo to use as a speaker cab

[mebond11]

Hello all, I recently completed a 5e3 build and a Marshall 18 watt head.  The 5e3 has a great speaker in it and I would like it to do double duty as a speaker cab.

I’m thinking of installing a jack plate in the back and simply connecting the jack to the speaker lugs with speaker wire.   I can disconnect the speaker from the 5e3 chassis by pulling the jack out, but curious if there a problem with leaving the speaker hooked up to the (unplugged) 5e3 combo chassis, and plugging in the amp head to the speaker?  Does the simultaneous connection to the unenergized secondary of the output transformer change the impedance?

I can also just connect the speaker jack to a female/female connector, but thought a jack would look better.

Sorry to ask this simple question but couldn’t find anything exactly on point with a google search. 

[pdf64]

Measuring the resistance of the 5E3 secondary may answer your query

[sluckey]

I would probably do this. When nothing is plugged into the jack, the 5E3 chassis is connected to the speaker via the switching jack. When you plug your 18W head into the jack it will connect to the speaker, automatically disconnecting the 5E3 chassis. Be sure to turn off power to the 5E3 chassis when doing this!

[mebond11]

Thanks, elegant solution.