TM 36 combo ext speaker jack

No idea why you would build a combo amp with only one output and the whole 8-16 ohm bit in the manual. Thinking maybe I should assault it and install one , not really an amp mod guy but I generally figure things out and succeed when I try to improve gear. Just wanted to hear what you folks that actually know things have to say besides I should have gotten the head.

Assuming you just mean to add an additional speaker socket you're going to have to be careful with your impedance.  The original speaker in your combo will most likely be 8ohms, (I would check whether that is definitely the case just to be sure).  To add an additional speaker and share the power equally between the two of them you must set it up so the extension speaker is also 8ohms and is in series with the existing one. At 8ohms you must be increasing the impedance to stay in the 8-16ohms range and that means adding impedance as with a series connection only. If your existing speaker should be 16ohms then you need to go down in the range with another at 16ohms and it must be put in parallel in order to drop the impedance.

The switching on a standard 6.35mm jack type socket will allow you to set up both of these so the cabinet is as normal with no jack inserted but adds the extension correctly wired when it is inserted.  It's not rocket science but it is important to get it right.

(Oh, did I say, you should have got the head!  Greenbacks are so much better with this amp. )