Re: What kind of marshall is this?
Triode is three elements in the tube in operation, and pentode is five elements inside the tube in operation. In triode the three elements are the plate, cathode and the control grid. Pentode adds two additional grids to focus the electrons, making the tube output twice as powerful.
On most class A/B tube amps, the output tubes are almost always operated in pentode for max wattage output. Four tubes=100 watts, and two tubes = 50 watts, in most cases.
By operating the tubes in triode, the wattage output is reduced by 1/2, but the tone is also softer and less bright. Kinda suedo class A. Rockers usually prefer the more agressive, tighter, attack, of pentode, and that's probably why triode isn't that popular among rockers.
Triode can be rather nice for blues type playing, or recording, not only because it's a warmer, smoother tone, but because you can more likely get to the point of pushing the output tubes, before being too loud.
Preamp tubes are actually two mini triodes in one mini bottle.