VTL Tetrode/Triode

Update: I just replaced all tubes (input, output, and driver) and I must say that now, triode is a lot better than it used to be. In comparison, tetrode is a little hazy and distant. But triode is crystal clear, and now, for example, the the above mentioned Brahms trio is unbelievable, with fantastic rythm to boot. Also, piano is fantastically present. Take Richter's performance of Beethoven's Appassionata sonata on RCA. You can hear every tap, all the hidden gems, counterpoints, etc. Amazing! And now triode (with new tubes installed) has the power to preserve the waveform, as that piece has incredible dynamics, from ppp to fff.

So, maybe this is what happens over time: as the tubes age and lose power, triode loses its magic, at which point tetrode sounds relatively better. Could it be that simple?

Don't know if "it's that simple" as you say...I've never found tetrode, whatever its faults, to suffer from being "a little hazy and distant" compared to triode (whether before or after replacing the tubes). I also doubt that amps really lose all that much effective (as opposed to maximum) power in normal use as the tubes age, at least compared to how the sound changes simply from changes in the tubes' response characteristics. I do wonder if all your new tubes are broken in yet, which I find takes several days. But I'm not surprised you've already noticed improved sound overall, and I'm glad you've gotten back in good with triode, which is part of what switching to the KT-88EH's did for me. Keep rechecking the bias over the next week or so, it can take a while to settle in at a more constant level.

Zaikesman: Thanks, I'll do that. Rechecking the bias is pretty easy to do and doesn't take much time. I guess all I wanted to convey that triode mode (with new tubes) has an incredible immediacy and clarity, which I didn't experience before.

Are there any real triode amps, or even real triode tubes? When an amp equipped with KT88 or similar pentode tubes is opearated in "triode mode" it means that some extra hardware in the tube (grids) are just left unconnected. Can this be good?

Eldee: I've wondered about that before myself. In fact, VTL amps always operate with at least one element disconnected, since there is no pentode mode. I think the decision not to run in pentode is probably a good one from a sonic standpoint (I say this not from hifi system pentode experience but with guitar amps), but I don't know how a similar circuit would sound with EL-34's or 6L6's instead. Triode wiring of 6550 types has also been exploited by amp makers like ARC and CAT, so VTL is not alone in considering the unused screen elements to apparently be no handicap. But to answer your first question, of course there are "real triode amps" (and power tubes, which were around before tetrodes and pentodes were invented) - for instance, not even counting all the SET designs, VAC offers high-powered push-pull amps using paralleled 300B's, and the 6AS7 output tube used by Atma-Sphere is a true triode, along with the Russian 6C33C-B used by BAT.