Recently, I have considered purchasing guitar humidifiers. In addition to my new 660/12, I have some acoustic guitars, and some old Gretsch electric hollow bodies. I have never had any issues with cracking due to dryness, but with forced air heat running, and no additional humidity added...are these case humidifiers needed? Are they effective?
Thanks for your response.

I use one in my acoustic case during the winter. When the apartment humidity can drop to 5-6%, it keeps the in case humidity level about 40+%. Well worth while, in my opinion. On the other hand, I have never used one for any electric guitars and have never had a problem.

If they are needed depends on where in the world you live... Rickenbacker recommend that RH (Relative Humidity) in the air must get no lower than 36% - which keep the humidity content inside the wood a about 6% - which is fine for maple. But at many places around the world RH gets much lower than 36% (right down to zero!) and the wood will dry out, crack and split.

The best way to keep it up is to use a humidifier in the room/house - an electric device. Another way is a humidifier in the case (as mentioned) - but that's not possible in the newer molded Rickenbacker cases where there is only room cut out for the guitar and two other small fitting rooms which get isolated from the rest of the case when the case is closed... I don't think the humidity will spread to the hole interieur of molded cases under these circumstances. We do need an advice to place in the dash sound hole of the thinlines.

I use the Oasis in-case humidifier for my 325C58. I comes with a small metal clip to attach to the case. I cut a small piece of cardboard to fit tightly in the case and clipped the humidifier to the cardboard - works great.

That'll proably work as you got an old style case (right?)... I don't think that the vapour will reach quite far away from an in-case humidifier in the new style cases...

That said, the winther will soon be over here in Scandinavia - with an RH average % around 32 (over three/four month). Soon it will rise to about 45-50% - but maybe next wither I'll build myself a guitar enclosure with a perfect humidity controlled atmosphere.