Another 6550 vs. KT88, 90, et. al.?

what your describing in triode mode sounds to me like its a lack of power to me. Tetrode shouldn't appear to pass more information than triode also the presence should also be better in triode mode than in tetrode. Sounds to me the triode mode isn't enough power causing the resulting sonics your describing to me Is a stressed triode amplifier my 450 signature in triode sounded this way at very high levels it was running out of power which was solved by 750 reference is triode mode. Btw the harmonic technologies magic is not a good match with vtl in my experience see my review of the xlo signature 1.1 or signature 2.

Mejames, I too have thought about this factor, and it seems to me that it could well be part and parcel of why tetrode has the advantages it does. After all, one is not only switching from tetrode to triode operation inside the tubes with all other variables being held equal - doing this necessarily cuts the power rating in about half. A comparision to a similar amp run in triode, but with twice the number of output tubes to compensate, would be very interesting, but VTL's triode-only MB-250 and triode-switchable MB-450 are not this theoretical amp: They each purport to wring about twice the triode power from just 8 6550's as opposed to the 6 in the MB-185, and so must be running them quite differently. (I've never seen the 250 up close, but I know the 450's power supply and transformers, both power and output, are much larger than the 185's - and ought to be, at almost twice the price. The 450's weigh in 20lbs. heavier than my amps, but the 250's are only about the same - go figure).

When I do my listening tests between tetrode and triode modes, I take advantage of the Levinson preamp's programmable muting function to set it at only -3dB, which enables me to make level-matched comparisions without wasting time fumbling around with the volume control to compensate. (I've also found that I don't need to heed VTL's admonishment about powering down the amps before switching modes - everything is fine and calm switching on the fly, with no untoward transients at the speakers as long as I operate the switches quickly and decisively. Other VTL owners reading this account experiment at their own risk and on their own judgement when deviating from VTL's strict instructions regarding proper operation of this feature.)

My speakers - despite being Thiels, with all the power-sucking reputation that implies - are actually not a model that presents a particularly tough load to drive. The audible subtractions in dynamic contrast, for instance, when going from tetrode to triode are present even at fairly low volumes, and these amps are rated for circa 100wpc in triode mode, which based on my previous experiences with these speakers using amps rated at 45wpc and 70wpc should still be plenty of power. Another telling result is that the space around and between images is more explicitly defined and less amorphous and congested whenever tetrode is selected, again whatever the volume.

Nevertheless, it is true that the advatanges diplayed by tetrode mode seem a lot like what are often held to be - and held by VTL to be - the inherent advantages of higher power. The only advantages I hear in triode mode are in natural tonal balance (excepting the frequency extremes which are more rolled-off) and in ultimate textural smoothness. Other than those two (important) areas, in which tetrode is hardly intolerable itself, triode cedes to tetrode in every other way I can hear. Whether this finding is due solely to mode of operation, or is solely a pure power issue, or is a combination of the two I cannot say. My suspicions lie with the last scenario, but tilted somewhat toward toward the first, based both on past and low-volume experience, and on the fact that I haven't audibly approached the limits when running in triode at any volume, implying that I'm remaining well within the amps' comfort zone.

So my hope is to massage the amps' balance when run in tetrode. I figure I can look primarily at four areas (without changing speakers or signal cables - I do not hear a problem with the Magic One IC's vs. my previous Cardas Cross, but that topic is, of course, endless): Power tube substitutions, input tube substitutions, power cord substitutions (the Sidewinder's main virtues are a lot like tetrode's), and installing better AC lines and outlets. My mind for now is on the power tubes because the repair of the mode switches focuses my attention there. But this may not be the answer - Luke Manley says he prefers the Svet 6550C's, and Kevin at Upscale doesn't disagree. And VTL's low-negative-feedback design (6dB) mandates a highish output impedance (1.6 ohms) no matter what the tubes, which will cause some unpredictable tonal balance variability even with speakers, like mine, presenting a fairly flat impedance curve.

There is a fifth factor I'm strongly considering as well: Upgrading the coupling capacitors. I was planning on doing this anyway, and may still do it first, but what I've learned since getting the mode switches repaired is making me pause a bit before sinking any more money into the amps themselves. I'll probably get over it though - I was just shown that my amplification isn't quite as neutral as I may have supposed before the mode-switching was available to me. Big surprise. Nothing's perfect. It will ever be thus, and they still sound great, best I've had, even if right now I'm wishing for an unattainable combination of the best attributes of both triode and tetrode modes.

This is just my opinion, but I see no reason whatsoever to use the 6550 tube. The Legend Starlet integrated amplifier, a fabulous sounding product, designed around this tube(run pedal to the metal) excepted.

My complaints are basically everything you outlined Zaikesman. I'm not sure I wouldn't investigate a nice solid state amp over something running 6550. My money would be on the KT90 or KT88 every day of the week.

If I was after neutral sound, the Ei KT90 would be my choice, which is what I use in my Jadis. This tube is rugged, reliable, and very long lasting. Still going strong after 4 years. It is not a sweet tube, but offers a nice tone. It betters the 6550 in every area one would ever opt for that tube - bass, power, longevity, extension, neutrality.

As for the KT88, it is a sweeter sound, and sounds more like what most people consider classic tube. Will probably give them a go next in the Jadis. It's midrange is great, with a nice punch in the bass. And, I can't complain about the treble.

The JJ KT88 are my favorite, by far. After that, I guess I like the Svetlana. The JJ tubes, badged Teslovak, had a bad production run and ruined many a person's day a few years back, but recently a good friend of mine had a bad Svetlana do some serious damage to his Rogue amplifier/ Coincident speaker. So, who knows?