VTL Tetrode/Triode

VTL doesn't specify if or how NFB is effectively changed between the two modes, though from what I've gathered about the method in which the switch is implemented vis-a-vis the output transformer and the tubes it does seem plausible that this parameter will not remain exactly constant. But I'm not technically qualified to know or judge about this, and since VTL only specifies "6dB" NFB without further elaboration, I'll assume any difference is not large or intended to be sonically consequential in and of itself (as opposed to the change in the tubes' mode of operation). FWIW, 6dB is considered to be quite a low amount of NFB, and many electronics designers - in contrast to many audiophiles - would not be so quick to condemn the usage of *any* NFB, citing tradeoffs both ways, in which design skill and execution in NFB application are more predictive of resulting sonic quality than pronouncements of absolutes. However, it can't be said whether your listening experience in tetrode is a confirmation of that philosophy, due not only to our not knowing what the actual NFB difference - if any - may be, but also because of the other factors involved (obviously) like output power and tube mode.

Update: I left off this thread around the time I switched from running Svet (SED) 6550C's to Electro Harmonix KT-88's in my MB-185's. The latest report is that I am listening more than ever in triode mode these days. The KT-88EH's seem to largely ameliorate some of the relative weaknesses of triode vs. tetrode that I outlined above.

Tetrode definitely still retains the greater bass control and wider macrodynamic envelope. But now triode suffers much less by comparison in terms of soundstage transparency and extension at the frequency extremes. Triode still retains its advantage over tetrode in naturalness of midrange tonality and the harmonic structure of the overtone series. (Let me not leave the impression that the switch to the KT's didn't also improve the sound of tetrode mode - it did.) At this point, I think the advantages that tetrode does retain are probably due mostly - maybe even entirely - to that mode's higher power.

Owners of higher-powered VTL models than mine, like Alberporter and Mejames (who also use bigger speakers and rooms than mine), have reported their unwavering preference for triode mode. Soon I plan to audition a pair of MB-250's, the one amp VTL makes which is permanently wired for triode mode only. It roughly doubles the triode power available from my 185's (meaning it equals their power in tetrode mode). If I get these, I will fit them with the KT-88EH's and test them against the 185's in tetrode to see if macrodynamic range, microdynamic detail, and bass control/punch are comparable - hopefully combined with triode's superior tonality and harmonics (at a higher cost, of course).

Oh well, my highly anticipated (by me) MB-185/tetrode vs. MB-250/triode showdown will have to wait...I decided not to buy this pair of the latter amps for other reasons. (Didn't get to audition them in the seller's system either - just his replacement Halo JC-1's.) Don't cry for me though; my amps still sound great! (In either mode :-)

On my MB-450 (and they have plenty of power), I now only listen to Triode with solo instruments (or solo voice). For everything else, Tetrode is far superior in overall satisfaction. Even with small ensembles (duos, trios, quartets, etc.), Tetrode gets the timing right, the individuals come together and really play as a group.

As an example where this is easy to hear, try Brahms piano trios, e.g., Trio No.3, op. 101, Fontenay trio (on Teldec), first movement, when the piano gets really hammered. In Triode mode, the piano is just too loud, reverberates, is simply overbearing, and the overall melodic structure gets lost. With tetrode, what a difference. Sure, the trio is farther back in the room, but the melodic structure is intact. And it puts a smile on my face!

Happy listening.

Hgabert - This is what I wonder: could tetrode mode be inherently superior in some ways, or are your observational results mostly a manifestation of the power issue? I have a feeling that all inherent superiority may actually lie with triode mode, but that for certain music - played back beyond a certain volume, through certain speakers in certain sized rooms - the power advantage of tetrode will out, even with 450's. The comparison in your case would presumably be your 450's in tetrode vs. a pair of 750's in triode. I realize that the 200+ watts available from your amps in triode seems like plenty of power, but I can sometimes reach the limits of those same watts with my amps in tetrode, and I think there's a reason VTL makes the 750's and 1250's.

As mentioned above, I was more of your frame of mind before I swapped the 6550C's for the KT-88EH's. The 6550C's didn't quite seem to thrive in triode the way the KT-88EH's do. And as of very recently, the amps are now powered through an ExactPower EP-15a voltage regulator/waveform correction unit (along with the rest of the system, followed as usual by the balanced power isolation tranformers in my API Power Wedge Ultra for the sources and pre), and there's even more physical specificity and consistent clarity on tap in either mode.