A triode tube is a three element tube with cathode, grid, and plate. A pentode is a five element tube which adds a screen and suppresor to send secondarily emitted electrodes back to the plate. As can be imagined, a pentode can be converted to a triode by not using the screen and suppressor, hence triode connected EL34s etc. It gets a bit confusing as there is a four element tube the beam power tube, as well. The ultralinear connection of a tube is simply some point between triode and pentode, or partial triode, as it is sometimes called. Most preamps use triode tubes exclusively in the signal path. The small, nine pin, tubes that you commonly see, such as 12AX7 and 6DJ8 are actually two seperate triodes contained in one glass envelope. Rarely, pentodes are sometimes used in preamps. And combinations of triode and pentode, or triode and beam power tube, in a single glass envelope, are also sometimes used. Examples would be the 6AN8 and EF86. Solid state circuits can never be referred to as triode of pentode. Your hybrid amp may have a triode stage feeding a solid state output. I hope that this helps a bit, as I am not an expert on such things. If you are genuinely interested, purchase an RCA Receiving Tube Manual from Antique Electronic Supply and all will be revealed to you.
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