Replacing MOSFET with tube
Please clarify whether you wish to use a MOSFET or a vacuum tube in the final design.

If you are looking at an older tube or an old circuit then the abbreviation 14GTB may have been used for a range of vacuum tubes.
 G T B = Glass, tubular, version B (as opposed to GTA = version A)
 so GTB on the end does not tell you a lot about the tube's functionality.

Do you have a circuit diagram?
Please provide more details.  

MOSFET in place of a vacuum tube:

BUT, yes, if you want to use a MOSFET in place of a vacuum tube (in old equipment or using a very old circuit) and you want to drive it with a 555 to make a flyback driver you can do so, subject to getting a MOSFET that meets the circuit needs. The MOSFET would need a high enough voltage rating to survive the inductive peak on its drain during flyback, and it needs a high enough current rating to meet the circuit's design. You will need to use an N Channel MOSFET to get the polarity correct. 

Vacuum tube in place of a MOSFET:

If you want to use a vacuum tube in place of a MOSFET it MAY be possible but will need good design knowledge as VT's are somewhat harder devices to drive. (Correct grid biases, limited current, higher voltages, heaters, ...).



Related:

Many vacuum tubes for sale here

Some GTB suffix tubes