Yes you do - standard guitar truss-rods are around 18"max - Bass truss-rods are around 21" - you can get them through Allied lutherie.com , Stewmac.com, LMII.com etc etc - all the usual suspects. Double acting rods are useful for first time builds as they can adjust out a multitude of sins and problems and their installation is relatively simple.
Stewmac have a free instruction for fitting truss rods but all the standard rules apply: if you have no woodworking skill or equipment don't attempt this at home. If you have used some very expensive pieces of wood to build whatever you are up to and don't feel confident get someone who does to do it for you for the first time.
Good luck Zeke, welcome to the forum and please seek help here as required - there are no dumb questions - but there have been plenty of dumb outcomes (just ask me!).
At first sight I would say yes, you need a special one. But it depends on the body/neck joint design too : for example, if you plan to make a single cut body, chances are that you could use a trussrod of standard lenght because the stiffness of the neck in the area where it's glued along the body should be enough to avoid the use of a trussrod.
hi. first post here, i saw this thread yesterday, and its what made me decide to join, so i could reply. how many string bass is it and what kind? what kind of woods are you using? something very hard, or softer woods? on my 5 string and wider necks, ill use double truss rod setups, or a truss rod and 2 stiffening rods on the sides with a 5 string. 6 or more i always use 2 truss rods. being 39 inches, id suggest a slightly longer rod, maybe 24 inch.something to cover most of the area under the fingerboard. if you cant find one readily available, i can make you one. ive been making my rods for about 22 years, never had one break or have a problem. they are removeable if you want them to be (depends on how you rout the channel and install it) and can be rolled 180 degrees upside down and reinserted in case you ever have to bow the neck the opposite way. ive got about 34 years experience as a luthier. any questions, just ask.
The double action bass rods available from Allied lutherie or LMII are cheap and work. They only require a 1/4" route which includes the nut and give you both convex and concave relief adjustment in situ. They are slide in slide out if you route the channel that way (but this cause problems with peghead design). For a first time build they are my choice.
If you can't work out how to get them or whatever I have 15 of them surplus to requirements (supplier sent bass rods instead of electric rods) and they are $15 each excluding shipping. These are the same bass rods as Allied lutherie sell and while we don't sell stuff through the forum this is just an option if you get stuck. I don't build basses so anybody is welcome to these rods. Other wize good luck and be careful over the Xmas break.
no prob. i was out in the shop before, and my bass rods are normally 24", about 25 with the rod adjustment, so if you were going the scale that you stated, id make it even a bit longer. i use those for 34/35 scale. you can rout a 1/4" channel for it, and a bit wider at the end of the rod to accept the adjustment screw. shoot me a message or email preferably.