Anytime I hear of bitter tasting meat of any kind, I can almost always chalk it up to 1 or 2 single problems..
- Using wood that is green (wood should be dried for 4-6 months for best results)
- Smoker is not getting enough airflow.. i.e. vents are closed off and air is not getting into and out of the smoker properly.
You say you are using pellets which should not be giving you bitter tasting meat. The only other option is that the smoker is not getting enough airflow. I went and took a quick look at the smokers on the Smokin Tex website. I don't know which model you have but it looks like they all have a drain hole on the bottom and a vent at the top. I don't see any other venting.
Be sure to keep the drain hole open at all time.. completely open. The vent at the top should be completely open as well until you get a little practice and success under your belt. This should give you enough positive airflow through the smoker so that the air will be drawn in, mix with the smoke then exit out the top.
With time you may opt to close the top vent slightly to increase the smoke flavor on the meat.
You might also try using wood chips instead of pellets. Use them dry and try some milder woods such as cherry, apple, pecan, etc.. Replace the wood chips/pellets with fresh ones once the smoke stops and keep up the smoking process for about half of the cook time for best results however, if your smoke is right and the airflow is right, theoretically speaking, you should be able to keep it going the entire time with no bitterness or "over-smoked" taste.
As others have already said, 3 hours is just not enough time to produce any kind of tender ribs at 220 degrees F. Ribs are not smoked to temperature or time in most cases. We estimate but ultimately, they are done when they reach that perfect level of tenderness. Foil helps tremendously with that process. A very common practice around here is the 3-2-1 method for spare ribs or 2-2-1 method for baby backs as outlined below:
|Baby Backs (hours)
||Spare Ribs (hours)
||Hours on the grate, no foil, smoke is going
||Hours wrapped in foil, 1/4 cup of apple juice in foil, no smoke needed
||Final hour unwrapped and back on the grate, smoke is optional.
Be sure to remove the membrane from the ribs before they go on the smoker and you should be good to go. The covered pan you mentioned could easily work in lieu of the foil wrap in the 3-2-1 process just be sure to use a little apple juice or other liquid at the bottom of the pan.
To make sure they are done to your liking, grab a couple of bone ends and pull them apart in opposite directions. Use this to judge the tenderness of the meat and if you like how they look then it's time to eat.
You can always use my own personal method too.. I just slice off one of the ribs and give it a real "chew" test for tenderness. My wife to this day still thinks there are only 12 bones (and sometimes 11) in a rack of spare ribs
Let us know if you have further questions..