Originally Posted by HAMPAMP TUBE AMP SERVICES

"F" Would be a 1974 and it is on the end of the serial number. Also, a point I had forgotten is that the letter "B" was skipped because it ran for 18 months. The letters "I', "O" and "Q' were also skipped because of their similarity to numbers.
There are some people on here that are more knowledgeable about tubes than I am, so I'll leave that subject for others to answer. MartyStrat54 should be able to help you out with that.