Are you talking about small signal tubes or output tubes for power amps?
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Your amp has output power tubes which can be hooked up as pentode or triode by flicking a switch. The pentode choice will give you about double the power of the triode hookup. If your speakers need the power you might hear better dynamics in the pentode mode. Sometimes the triode mode will give you a sweeter sound. Since it is easy to switch just try both settings and see which you like better. On amps like this that I have owned I usually like the pentode mode better. The triode mode is usually a design compromise
When you have a setup that allows you to switch between triode and pentode, you should understand that the amplifier is likely optimized for pentode rather than triode.
Triodes have a lower output impedance and so will need a lower turns ratio on the output transformer. Its really unlikely that is being dealt with properly in most switchable designs.
To answer the question though, triodes are the most linear form of amplification. Pentodes are not so linear, and usually require feedback to keep distortion to a reasonable level. Feedback is not perfect and always causes the amplifier to sound brighter even though it may not affect bandwidth. So a lot of triode amplifier designers don't use it because they can get smoother sound.
So there is a lot more here than just operating a pentode in triode mode!
I think the most pronounced differences you will hear will be in midrange/upper-midrange tonality and, if you have a well set up system, the imaging, which in pentode should be more forward and pronounced in the mid-range. Triode (in those amps I have which allow selection) usually is more recessed in the mid-range, and has a recessed soundstage and (in my stuff) more narrow as well. FWIW.
For what it's worth, in all but a very few cases, when high-end audio amplifiers label themselves pentode, they're actually ultralinear in operation.
Ultralinear is a combination / compromise between triode and pentode. You could argue it's the best of both worlds. Or, you might argue it's the worst of both worlds.
Thank you for the kind replies. After reading your replies, I have paid closer attention to each setting and found the pentode brighter, a bit more forceful. I alternate listening to jazz and small classical ensembles on that system using Gemmes Katana and Audio Physic Avanti speakers.
On triodes the Gemmes are sweeter, whereas on the AP Avantis seem more clinical; both, however, provide a very good soundstage. On triodes the sound stage appears to be better on the Gemmes.
Time will tell if the triode/pentode compromise is advantageous to that of a single format. Again, thank you for enlightening me.
Rolando (AKA Penonome)