How to wire speaker in amp to be used just as a bench test speaker

[wsscott]

I'm sure this is simple, but I don't want to screw anything up.  I am a novice amp builder, I've built 3 tube amps with help from folks on the Forum, and there are times if I'm working on one of my amps or a friend's, that I want to connect that amp to an external speaker to hear the output.  Putting the amp up on the workbench isn't feasible because of size and weight, but I have to have a speaker plugged into the tube amp's output for obvious reasons.  So I have a cheap Fender Champion 20 that has an 8 ohm speaker, and I'd like to be able to install a separate speaker jack onto it's cabinet that would then be wired to the Fender's speaker.  Then, when I want to use it for testing purposes with another amp, I can just plug a 16 gauge speaker cable with 1/4" phone jacks on each end into the testing amp's speaker output jack, and the other end into the jack I've installed on the Fender.  I was thinking of using a self-grounding jack (I figured when nothing is plugged into it, the jack would just go to ground and there wouldn't be any noise caused by an unplugged jack) and just wiring it directly to the speaker, but I didn't know if adding that additional jack to the speaker would cause any problems when the Fender was turned on and being used.  The Fender wouldn't be on when I'm just using it like this, since I'm not using the amp output of the Fender.  Hope my question isn't too confusing. Thanks.

[sluckey]

Use type 12A switching jack. Like this...

[acheld]

I used to use a setup like that (tapping into an existing amp's speaker), but I found it to be a pain to use in the shop.   It does work, and the wiring is simple as explained by Sluckey. 

Yet, it was easier to get a cheap ($25) 8 ohm guitar speaker, make a small cabinet, and voila,  there you have it -- a shop speaker. 

[wsscott]

Thanks.  I said self-grounding jack, but I was thinking switching jack, just couldn’t recall the proper name.  Thanks again.

[sluckey]

It's probably a good idea to put a caution note beside that jack. "Disconnect power cord before connecting to this jack."