This is something us guitarists are always looking to find (guitar to bass).....I use Jam Origin's  Midi Guitar and it works very well.......until I get carried away with soloing.
 
You can also do this in Amplitube3 & 4: https://www.youtube.com/watch?v=7WD0AkR0g-k 
 
Good tip there Chandler!!

Probably the best direct conversion of analogue guitar signal to bass sound I've found is the COSM effect(s) to be found in the Boss Micro BR-80.  I was so impressed that I went looking for any other examples of this and found it in the Roland (AKA Boss) GP-10.  Somewhat surprisingly, I think the BR-80 does it better!
 
Ross/Boland do not make a stand-alone pedal to do this, which is a bit of a shame IMHO.

ston
Probably the best direct conversion of analogue guitar signal to bass sound I've found is the COSM effect(s) to be found in the Boss Micro BR-80.  I was so impressed that I went looking for any other examples of this and found it in the Roland (AKA Boss) GP-10.  Somewhat surprisingly, I think the BR-80 does it better!
 
Ross/Boland do not make a stand-alone pedal to do this, which is a bit of a shame IMHO.

I've used the COSM effect as well to convert guitar to bass and it worked well.  The only drawback is if you are holding the bass note for a long time, as the signal decays the pitch gets a little wonky.

wouldn't be easier to simply get an electric bass and play it?

batsbrew
wouldn't be easier to simply get an electric bass and play it?

And after I finished recording that track, that is what I did.  Although my bass playing sucks.

batsbrew
wouldn't be easier to simply get an electric bass and play it?

Yep....and then you play it more like a bass as well.

After trying to replicate bass on guitar a few times, I found a simple solution: I got a bass.

Got to agree with a few of the posts above....
 
I tried this method. I don't think my Sonar 1 essential has the ability to alter the pitch so I grabbed Melodyne and used it.  Played a "bass line" on the guitar.... used Melodyne Editor to move it 12 semitones down.... and hit playback...
 
OMG that sounds like S*&@!  Tried a few things to fix it.... nope...still sounds bad if not worse.  Nothing even close to what the OP said they got going this.... Oh well,,,,, it was a fun experiment.
 
Delete the experiment and continue to use Midi with a good bass sample until I get a real bass guitar.... I'll probably be using midi for some time.

ston
Probably the best direct conversion of analogue guitar signal to bass sound I've found is the COSM effect(s) to be found in the Boss Micro BR-80.  I was so impressed that I went looking for any other examples of this and found it in the Roland (AKA Boss) GP-10.  Somewhat surprisingly, I think the BR-80 does it better!
 
Ross/Boland do not make a stand-alone pedal to do this, which is a bit of a shame IMHO.

I still use an ancient Boss BR-8 as a guitar modeller. Such great tones from that unit, I can't bear to part with it.

Some real funny stuff going on here ….
Aye Carrumba  , I thought some of you guitar players around here play well enough to play a real bass 
 
Heres a conversion I did where I converted a real Bass part that I had played …
one side is my Bass , the other side is a Melodyne audio conversion to Midi playing Dim Pro 
http://soundclick.com/share.cfm?id=13308818 
 
Kenny
 
PS my inner bapu made me do it