Triode mode vs pento mode

Triodes are some of the most linear amplification known to man. Pentodes are not and generally need negative feedback correct to be useful for audio.

Good bass has nothing to do with the amp being triode of pentode; good bass results from having the correct relationship to the speaker (see: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php and proper bandwidth to play bass in the first place. That is not an issue between tube types.

Atmasphere, there might be some issue since triodes have lower internal anode resistance leading to higher damping factor and perhaps perception of less bass.

Pentodes are less sensitive to supply voltage variations meaning that large change in supply voltage cause very small change in plate current. Triodes require better power supplies. I also read somewhere that pentodes are noisier.

Atmasphere points out exactly what my experience has been. With my Manley mono's, with speakers that don't need more than their 100 triode watts, triode always sounds better; including in the bass. Speakers (my Stax F81's) that suck up as much power as I can give them, pentode "sounds" better; but, only inasmuch as not sounding limited in volume and dynamics. As far as refinement, staging, and all that other good stuff, triode still sounds better; albeit at limited volume.

Not sure if it was clear to everyone that the question was not about pentodes vs. triodes, but was about pentodes operated as pentodes vs. pentodes operated as triodes.

The initial answer by John (Jmcgrogan2) pretty much said it all in a nutshell, IMO. It is common for triode mode operation to sacrifice something like 40% or 50% of the power capability of pentode mode operation, while providing some improvement in sound quality, mainly by providing an approximation of the greater linearity that a pure triode can provide in comparison to a pentode or other amplification device (as Atmasphere indicated).

See this Wikipedia writeup for a technical overview. Note the section toward the bottom headed "Triode-strapped pentode circuits." Basically, connecting the middle of the three grids of a pentode (the "screen grid") to the plate, usually through a low value resistor, causes the tube to exhibit triode-like characteristics. A triode being a tube having only one grid, corresponding to the "control grid" shown in the article.

The amplifiers I have had that could be switched between the two modes were two pairs of the classic vintage Marantz monoblocks, the Model 2 and the Model 9, which I owned during the 1990's. Using them in conjunction with 90 db speakers having easy to drive impedance characteristics, in both cases triode mode provided a considerably richer and more natural sound that was clearly preferable to pentode mode, as long as I didn't run out of power. Which in turn only happened on a limited number of recordings having exceptionally wide dynamic range, such as some of the Telarc symphonic recordings with their notorious bass drum beats.

Regards,
-- Al

I would add to some of the excellent responses above that if you happen to be using 101dB or even more high efficiency speakers, the power will not be an issue at all.