VTL Tetrode/Triode

Zaikesman: Isn't that a pain? Flipping a switch on the back of the amps is one thing, but unscrewing the covers, pulling out the two input tubes, and replacing them? Or do you leave the covers open? Maybe, in that case, it is not too bad.

Well, anyway, that's an interesting thought. Enjoying the music is what this is all about.

he probably leaves the tube cages off as I also do

Yep, I leave'em off, if for no other reason than I like to check the bias more often than I want to deal with the covers. But playing with input tubes is a more fun reason.

I'm also just paranoid enough that I prefer an unobstructed view of, and access to, the tubes in case one should give a hint it's beginning to act up; a few times in the past (not always with these amps) I've caught stuff out of the corner of my eye which likely saved me bigger headaches within moments - including smoke wisps emanating from around transformers - that you're not going to notice as easily with covers on. Have you ever done the jump-up and across-the-room-sprint when a tube started in with a light show? :-) And although I think the amps look better with the covers in place, I assume that heat dissipation - and therefore tube life - probably gets slightly improved running naked.

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.