Upright bass bridge adjuster knob stuck

Lower the tension on the strings first, my man.

 

That's a good idea, there's probably old sticky bass rosin in there causing friction too. Be careful to put the bass on it's back and lightly outline where the bridge feet are placed with light pencil in case the feet move. Don't loosen the strings too much or else the sound post will come loose and end up rolling around inside the bass. You might want to bring it to a bass luthier if you can.

 

The threads in the bridge (feet or top depending on the placement) need to be re-tapped. Its a very simple procedure. Just turn the appropriate tap in there a few times.

 

Yeah, actually removing the bridge is addressing the problem rather than the symptom.

Mark the location with masking tape so you put it back in the right place.
Lay the bass on the floor, on its back, to reduce the chance that your soundpost falls.

Bar of soap works too.
Please notice how I have purposely skipped the joke about, 'You do have soap, don't you?'
(The appropriate response would have been, 'No, but I have plenty of candle wax.')

The best solution to most problems!

 

Simple if you know how to take your bridge off and put it back correctly, and can use the correct tap without messing up the threads. If not, you could easily ruin your bridge. A luthier could do the job quickly and inexpensively.

It’s also possible, as others have mentioned, that the threads are fine and just need lubrication. My preference is graphite powder.

 

Soap tends to draw moisture. Candle wax does not. Just a thought.

 

One person can pull off the pliers trick. I’d cover the belly of the instrument with a heavy towel in case you slip. If you can get the adjuster to move it should be fine.

 

My luthier lubricated the threads on my bass with graphite powder. This was at least seven years ago and they still turn easily.

 

Just for future reference, it’s best to keep WD-40 as far as possible from all of your instruments. It is a silicone spray and it has potential make repairs to your bass impossible because finish repairs will not stick to where WD-40 has been and glue will not stick to wood where WD-40 has been. If you need a lubricant for your tuning gears use machine oil or 3-in-1 oil and for your nut and bridge slots graphite powder or just rub a pencil in there. If you get goo or funk on your finish use naphtha.