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

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?

Trelja, I read carefully your comments on these tubes in the archived posts. In those threads there were some dissenters who seemed to be expressing basically the opposite view, but many seemed to agree with your take, although this variability could just be an accurate indicator of the importance of any particular circuit in question. This is the most part of why I started a new thread, in hopes of finding a VTL owner or two who've done this dance.

The way we are characterizing the 6550C sound here is reminiscent of the criticisms leveled at the earlier 6550 versions, which I only ever used in Marshall guitar amps. The rep of the "C" is that it is supposed to correct these tendencies. Then again, I've listened to VTL's small stereo EL-34 model in direct comparision to my old EL-34 equipped C-J and not heard any brightness, hardness, coolness, or lack of body in either, though neither possessed the overall fidelity of these mono's.

It is quite possible that the manufacturer's preference for the Svet 6550C is based largely on dependability. I found that I preferred the Svet's to the JJ's when it came to EL-34's (with Sovtek last), both in sound and dependability, but I note you reverse this preference for the KT-88's. I like the sound of EI small-signal tubes but have never heard a power tube of theirs. It would be great if I could find someone in the DC area who's got either KT-88's or 90's and who's curious to do a temporary swap if they've never run the 6550C's as an experiment, but I suppose that'd be a real long-shot.

Zaikesman, if you ever travel up the road to Philadelphi, you are welcome to give my 4 KT90 tubes a spin,
Joe

Trelja, thank you (I do get up to Philly from time to time, my bro lives there), but unfortunately each one of the monoblocks requires 6 power tubes. We could swap 4 of my 6550C's into your Jadis, but of course that wouldn't tell me what the KT-90's would sound like in mine...

Since I've taken the conversation (what there is of it - it seems like maybe I'm not going to find a VTL owner who's done the swaps) away from just a discussion of the various tube merits and into the realm of triode vs. tetrode in the VTL, I want to come clean about something I've glossed over in my response to Mejames. I mentioned that the only two aspects of the sound I thought were superior in triode mode were evenness of tonal balance throughout the range (excepting the frequency extremes where response falls off in comparision to tetrode) and smooth texture (mostly this makes itself apparent on transients such as vocal sibilants).

Things would be simpler if this was really all there was to tell, but that is not true. I left out what is arguably the most important advantage triode has over tetrode, but I did so for a reason. This advantage is in the way triode presents the harmonic structure, which is subtly but distinctly more ear-friendly than the emphasis given by tetrode mode. In this sense, think of flipping the mode switch as somewhat analogous to the idea of changing the nature of the light illuminating the subject of a photograph, its angle and diffusion. The cast lent to the harmonic series of overtones for any sound when in triode mode is perceptably more consonant, tetrode more dissonant. The result is that timbrally, triode sounds more natural.

The reason I chose to omit this distinction above - and it is a separate observation than the simpler question of overall tonal balance - is that I don't expect that this difference between the two modes will fundamentally change even if I install alternate power tube types. Rather, I suspect this phenomenon is inherent to the mode of tube operation. Don't get me wrong - the amp is still plenty listenable in tetrode mode, the slight harmonic hardness (compared to triode) not being obtrusive by itself, and less than other amps I've heard, tube or SS. I do think possibly a different tube type could alter this quality for better or worse - just not change the basic relationship between the two modes relative to one another, but again this is just what I suspect until I do the experiment.

If you haven't already guessed this by now, the total result of my auditions involving the mode switch have yielded paradoxical findings. I can summarize them this way: In all qualities 'physical', tetrode mode is the more convincing of the two. If, when you play a good recording, you try to imagine to what degree the sound reminds you of what it must have been like to be present at the recording session, triode will sound more like you're in an adjacent room to the music-making, listening through a fairly large aperture in the wall, while tetrode by comparision sounds more like you've walked through the door and entered the main room. BUT - and this is the paradox - the tonal and timbral quality of the music you hear in that main room (tetrode) actually sounds a bit more 'electronic' and less 'acoustic' than it did when you were on the other side of the wall (triode). Life is complicated...