VTL Tetrode/Triode

Hgabert: I have been posting my ongoing findings regarding the output tube question on this archived thread, but my comments there do not touch yet on the differences between tetrode and triode with the KT-88's installed. VTL has told me that they've auditioned their amps with KT-88's and liked the results, especially in the bass, but preferred to stick with the 6550C's for the vocal-range reproduction. I don't know what KT-88 versions they've tried to date, but I suspect the EH's were not yet available at the time of the trials Bea described to me, and assume they were Svet's. In my auditioning with the KT-88EH's, I have detected no penalty on vocals compared to the 6550C's, and in fact am now hearing more vocal detail, but as I will shortly go into more specifics about on the other thread, I am reluctant to draw any final conclusions since my 6550C's are over a year old and my KT-88EH's are almost new. Nevertheless, I am tempted to wonder whether VTL's preference for continuing to employ the 6550C might not have something to do with the cost factor - though it's a good enough tube, I am thinking the KT-88EH is better in this application (particularly in the bass), at least in my system and to my ears. I also am currently investigating whether it may possess comparitively greater significant advantages over the 6550C when the two are compared running in triode... :-)

So many variables: your speakers, cables, impedance matches, recording quality, musical genre, your personal tastes, I could go on and on. I own the 450's with the 5.5 Pre. At first I kept switching back and forth. Audiophile friends with strong opinions indicated triode was better (without even listening!). I finally decided that tetrode matches my Nautilus 802's (better grip), and the images are more precisely focused. That's it. You can sonically tailor your listening sessions by warming up the sound on harsh recordings in triode, or you can experience the incredible speed and headroom 450 watts a side give you. But trust your ears, man. And remember, If you stop the music to switch modes constantly, your actual listening time is reduced. Hope this helps.

Cedhz: RE your experience with your audiophile friends - In this hobby I have found that the stongest opinions often seemed to be formed in the absence of listening... ;^)

As I stated above, for me it also Tetrode now (90% of the time), it just sounds better. As an aside, I understand that there is more negative feedback with tetrode (about 6dB or so), and some people (whose opinions I generally respect) claim that negative feedback is BAD, no matter how it is applied in the circuit. So I'm a little baffled why Tetrode would sound more real in my system, but it does. Go figure.

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.