I need to replace the reverb tank in my Ampeg 212 reverberocket. I can't find the specs for the original tank so I don't know what to replace it with. I found a post where someone said they replaced it with Accutronics 4BB3C1B and it sounded "lush". the nearest I can find is 4AB3C1B. Can anyone tell me what's the difference between 4AB &4BB? Anyone tried something different?