How to add center channel to stereo

     I use my all-tube stereo for the tv, too.  It sounds great, but the way movies are mixed, not to mention my aging ears, I could use a center channel for clearer dialogue.  Could care less about surround sound - or actually, I sort of dislike it.  Is there a good, reasonably priced way to add a center channel to an existing stereo? 
      I've considered just getting a sound bar (all those tubes cost money and run hot).
I'm not aware of any way to steer dialog to a center speaker without a processor. If you start down that path, you're going to be adding either a HT receiver or a processor and amp, as well as the speaker itself.

If you just want better dialog, you might try something like one of the ZVOX Soundbase or Soundbar models. They make 9 or 10 different models at different price ranges. They seem to be effective and very well built. Go to the ZVOX website for information on all their models.

REUBENT nailed it succinctly ..... full stop.

If you want to introduce a stand-alone CC discrete speaker, then it  is on a discrete encoded channel requiring a multi-channel AV preamp/ processor to decode. 
Accuvoice processing on thé Zvox soundbar makes a very noticeable difference to the resolution of dialog.
I 2nd the ZVOX. I just added one to my television in my living room due to the poor sound quality. I'm really happy with the results.