VTL Tetrode/Triode

I'm just curious if any VTL amp owners (I have a MB-450) with triode/tetrode switchability have any preference for one or the other mode, depending on the type of music one is listening to.

Even though some music is a no-brainer (e.g., a Mozart piano trio sounds much better in triode mode, and a Mahler symphony sounds better in tetrode), sometimes I'm hard pressed to choose. Small-scale jazz or blues can sound good in either mode.

Any thoughts?
When I first got my MB-450s, I played around with it. I managed to make it a difficult lean to get around to the back of them so I have since usually switched only once every couple of months - usually when I'm listening to large scale symphonic works or opera. The rest of the time it stayed on triode.

Recently I got a pair of MUCH more efficient speakers so have no use for the added "stomp" that tetrode provides.

FWIW, I agree that piano and almost all single instrument classical or chamber music, jazz, and vocal is better done on triode. Tetrode really shines with music with "impulse" dynamics - things like Holst's Planets, Beethoven's Fifth, above-mentioned Mahler symphonies, and good feisty operas (Carmina Burana, Don Giovanni, etc).
I agree with your assessment. There are some surprises, however. For example, certain Reggae pieces (Bob Marley, Rastaman Vibration) sounds better in triode, not tetrode (as I originally thought it would). Vocals are a lot clearer, and it sounds more real, live.

On the other hand, take Vivaldi's Four Seasons with Salvatore Accardo on Philips, and it sounds better on tetrode (more liquid, smoother, soaring highs on the massed strings), at least to my ears. Weird.
Hi Hgabert

In my experience (Manley 200/100 Mono's, very similar circuit to your MB's)), in absolute terms, the amps always sound better in triode: much more refined sound, better detail, less grundge, etc., etc. Tetrode becomes advantageous when your system, wether because of your speakers' efficiency, or your preamp's gain characteristics, benefits from the extra power. If your speakers are efficient enough to meet all of the needs of every type of music that you listen to, at every loudness level that you listen at, with the amount of power that triode has to offer, triode may always sound better to you. It sounds like they are not; few are.

I found that in my system, with many speakers, tetrode not only allowed the speakers to play louder, with more bass, but more importantly, it let the speakers unravel complex orchestral passages, it let a string section sound like many individual players as opposed to one large mass of "string" sound; probably why the Vivaldi sounds better in tetrode. Most reggae, while "bassy", is not particularly complex music; so will probably tax the amps less. The problem became that I missed the purity and sweetness in the highs that triode offered. The solution for me was two-fold: get used to listening at slightly reduced volume; this taxed the amps (and my hearing) less, in triode mode, and experiment with the input tubes. I found that with the right NOS 12at's (for me it was W.German Amperex or Tele's), there was enough of an improvement in the detail and treble extension departments, in tetrode, to bring the sound close, if not quite, to the level of triode sound with run-of-the-mill 12at's. This allowed me to enjoy tetrode mode when I absolutely had to have the extra watts. Of course, the sound in triode became that much better also; and this is why it is called "audiophilia nervosa" :)

Good luck.
Hi Frogman,

thanks for your response. I'll try the German Amperex or Telefunken 12AT7, that's a good idea.

But what about the 6350 tube (phase splitter)? Any suggestions there?

And the output tubes (Russian 6550C) are probably ok, right? (I don't think you can change them anyway, even if you wanted to).

Right now the rule of thumb for me is: Anything from solo instrumental, up to seven or eight players, use triode. Anything above, use tetrode (this is also what T_bone mentioned above).

BTW, i use B&W N803 speakers (90 db sensitivity).
The 6350's affect the overall sound less than the 12at7's do. It still pays to experiment, though. This is an American made tube and there are no exotic versions available, to the best of my knowledge. My favorites are the Tungsol's and the earlier RCA's. The RCA's that I have (two different vintages/two different internal construction), sound a bit more relaxed and romantic. The Tungsol's sound a bit more dynamic and alive, with a more prominent top end; this better suited my system. The differences are relatively subtle; but a definite improvement over the GE's that came in the amps. Try "Antique Electronic Supply" as a source of 6350's.

As far as 6550's go, I really don't have an opinion since my amps use 6L6GC's. I will say that the Sovtec 6L6's that the amps came with sound less refined than the NOS GE's that I have. The GE's are not a particularly great tube either, but in my system, a better sound. I hear good things about the Svetlana's; maybe worth a try. By the way as far as the 12at7's go, the Telefunken CCa (a super-premium version of the 12at7) is supposed to be THE tube for these amps. Very expensive, though.

Have you experimented with power cords? My amps definitely benefit from upgraded power cords. I use the old Tiffany "reds". Recommended by the Manley's themselves as the chords for these amps. This is going back several years, however, and I'm sure there are better cords out there for these amps, by now. The improvement over the stock chords was unmistakeable, however. Gutsier sound, with larger soundstage and more refinement. My amps also like to be placed on hard cone-type footers. The soft type makes the same a bit ill-defined and tubby. And finally, VPI bricks on the transformers are a very worthwhile tweak; the tighten up the sound, and make the spaces between instruments cleaner.

Good luck.

whew! I appreciate your insights. Lots of things to try out when it becomes time to re-tube, that's for sure. Right now, I'll enjoy the sound and definitely check out the power cords, and perhaps the other two tweaks you mentioned. Again, thanks!
I'll go along with the generalization of "The less complex, the more fitting the triode works". I have VTL MB-125's and tend to keep them in tetrode most of the time... I prefer the extra gusto and sheer effortlessness. I can find triode intriguing with the right mood which usually entails late nights and some smooth jazz but I'm often dismayed by the less than agile bass if I kick up the volume to any more than moderate levels. I find the increased "3D" addicting in triode but the tetrode always gives me that extra kick. Another generality, (obvious?) : the better the recording, the more it will display the virtues of triode... tetrode can rock to much less sophistication. Oops, does that make me some kind of "Common Joe"? Oh well. Just for the record, my good friend runs his MB 450's (I've got my eye on those someday) and his Wotans (2 separate systems) in tetrode virtually all the time. "Too syrupy", and "not tight enough" being his opinion... . It's sure nice having the option though isn't it ?!! Good listening.
Hi Lissnr:

All larger-scale works that I have played on my VTLs do sound better in tetrode, I agree there. For instance, with a Beethoven symphony (try Karajan's 1963 cycle with Berlin Philharmonic, 9th), triode just can't match tetrode's steady grip, with a totally steady soundstage, all instruments staying in their places, just like you would hear in the hall, especially when the chorus comes on in the last movement.

But listen to some small-scale jazz or classical, and triode just is more real. I attend live music performances quite a bit, and to my ears, there is no contest.

In fact, with tetrode on smaller-scale music, one loses the inner detail, the beauty and air around instruments, and the ambience of the recording venue. It sounds flatter and less involving.

But to each his (her) own. I just wished VTL would make the switching less of a pain. Always having to turn off the amps before switching is inconvenient, to say the least. And they should either have the switch on the front panel, or have an indicator light for triode mode. That would be nice.

I understand the new Siegfried amp does automatic switching.
Try listening at reduced volume, and I think you will find that triode has a decided advantage over tetrode, even with large-scale, complex orchestrations; the issue is simply that to unravel and amplify a complex signal, more power is needed. It's not that tetrode ever has an advantage, per se, it's that when the music is more demanding, the extra power makes up for any loss in transparency that triode provides under ideal circumstances (higher speaker efficiency). Conversely, try turning the volume up, higher than normal, while playing small cale jazz or classical. I think you will find that you lose the detail and purity.

I think that we can all agree that filet mignon is a better cut of beef than sirloin; but if the budget only allows for one pound of filet, and we are trying to feed six adults, would it not be more satisfying to buy two pounds of sirloin, than one pound of fillet. Poor analogy, perhaps, but the point is that I think that if you were to compare tetrode to triode, using something along the lines of a Klipsh corner horn with it's extraordinarily high efficiency, it's unlikely that there would be any advantage to tetrode mode; unless, perhaps, if you were trying to fill some incredibly large space with sound.

By the way, it is not necessary to turn the amps off to switch between the two modes. You simply have to be sure there is no signal running through them: mute the preamp.
Hello again, I agree that turning the amps off is a definite inconvenience... I'm always pondering how long to linger (off) before flicking the switch and going on again (I do the whole procedure in less than a minute) but there's always that hesitation in my mind when I click back on after running them hot for awhile... So far they've been flawless every time. I try to choose my mode and music at the beginning of my listening session just to avoid doing it although it sometimes can't be helped... I honestly think that because you must turn it off for switching modes that it actually makes sense keeping the switch on the back... This will keep unknowing fingers from flipping a mistake. In fact, when I saw the recent review of those big Neo classic Manleys in Stereophile it was seeing the switch on the front that actually brought this concern to mind. A small pilot light [on the front] indicating which mode would be a clever and convenient feature. I actually keep a small index card with "triode" on one side and "tetrode" written on the other which I leave near the amps to remind me where I left the switch positioned during its last use.Anyway, nice talking to you, maybe I'll spend a little more time hanging out with the virtues of triode a bit more... it really is fun playing with each type of sound.Happy listening.
Oops, looks like my answer didn't get posted quickly enough. Is it really true we can switch simply without a signal going through? That would be a lot easier... mute the pre and we're all set? BTW, I am driving 95 db efficient speakers (into a fairly stable 8 ohms which never drop below 6.5 or so):Silverline Sonata 2's. What my MB 125's are putting out into them is probably close to 100 wpc or so in tetrode and I'd guess about half of that in triode?... What do you guys sit your amps on? Mine sit on 2 navcom silencers [in the back] and a few sorbothane isolation feet in the front. This is all on a couple of sandwiched marble slabs (about .75 inch thick ] which sit on a pair of 6 inch speaker stands (not spiked) on the carpet. Do you think tip toes would be better than the sorbothane/navcom silencers combo?
Yes, it is true that you don't HAVE to turn the amps off; according to Manley. However, by turning them off, I suppose the problem of forgetting to mute the pre becomes irrelevant. NO SIGNAL THROUGH THE AMPS WHILE SWITCHING!

Again, I found that if the goal is the purity and detail of triode mode, that any kind of compliant isolator sent the sound in the opposite direction. For me, hard cones between the amps and sandstone slabs(on the floor) gave the tightest and cleanest sound. Sorbothane tended to blur images and make the amps even more "tubey" sounding.

I'll stick to my comments about triode/tetrode. IMO, given the same circuit, triode operation is always superior. Does one then have enough power for the speakers being used, however?

Lastly, make sure that your tubes are biased properly! This will definitely affect the sound.

Happy listening.
first, as to the potential for switching with no signal, i wouldn't trust it. At the dealer i saw a sales person do juet that with a vtl 150 much the the chagrin of several tubes. I just switch the power off on one mono amble over to the next switch it off then go back to the first, switch mode and power up and then go to the other. about half a minute total switch over time with no problems.

I pretty much concur with the above opinions. I always found triode nice for acoustic blues, small scale jazz with an emphasis on acoustic music, vocals. To me it gives the instruments better 3-d presence. Once the music gets complex or needs a boost in the bottom end the tetrode is a must, even for small 3 piece blues bands that still rely on a solid thump to make it all work.

Recently a friend and i sat down with a couple of bottles of good cab and proceeded to test the limits of my MB185s and neighbors. He has listened to tons of live music but relatively little high end audio. His unsolicited opinion of triode vrs tetrode was that the triode tended to sound more like the music was being beamed at his head while the tetrode had a more spacial ambient sence about it. I had switched modes based on my preferences and recieved that responce. We then spend the rest of the listen session doing a lot of switching. Overall his preferences tended to be very recording specific rather than music type. It was interesting to get a non-audiophile biased opinion. I also tried to get his feel on vinyl but we were to far into the cab by the time that experiment rolled around
Piezo: It would be interesting to know if your evaluation of tetrode vs. triode sound changed over time (as the cab got depleted). Perhaps it wasn't recording-specific, but cab-specific, what do you say?

Just kidding, thanks for the input.
I own MB-185's, powering Thiel CS 2.2's (not the demanding load that some associate with the brand - 8" woofer and fairly flat 4 ohm impedance with well-behaved phase angle, average sensitivity of about 87dB).

My general observations about triode vs. tetrode - the inevitable variations in recordings that cause disk-specific preferences aside - go like this:

Triode: Superior in only three areas, but three of the most important areas - tonal balance, harmonic structure, and audible distortion (within its dynamic limits) on transients.

Tonal balance: Triode has a more neutrally-balanced tonality throughout the heart of the audio range. Tetrode is is brighter and less evenly-balanced in comparison, with a relative lack of lower-midrange/upper-bass fullness combined with an overabundance in the 'presence' range of the upper-midrange through the low-treble.

Harmonic structure: Triode operation reduces odd-order harmonic production, while the push-pull operation takes care of reducing even-order byproducts. Tetrode produces more odd-order byproducts, which push-pull cannot cancel, so they will heard predominantly over the more pleasing even-order series. This skews the amp's harmonic signature to yield a 'harder'-sounding emphasis with tetrode, while triode gives a more natural descending harmonic distortion with asceding harmonic order, and lower total distortion overall. This is entirely independent of the tonal balance issue noted above, which can be addressed to some degree through choice of input tubes, cables, etc. Triode will always have the advantage over tetrode when it comes to natural sweetness of harmonic presentation.

Transient distortion: Although triode won't give the dynamic impact of tetrode, and its limits are lower due to the reduced max power, transients are cleaner distortion-wise, even though they are slightly compressed by comparison to tetrode. This means that triode will deal better with sibilants and sound smoother-textured overall, while tetrode will give some false emphasis to fast-rising impulses that produces a slighly 'edgier' sound, with more emphasis on 'detail' than is strictly natural. BTW, this is just my listening impression, not something I know from a technical standpoint, and I surmise it could be the result of either intermodulation or temporal distortions.

After those three areas, I believe tetrode holds the rest of the cards, including those relating primarily to all qualities physical:

Greater extension: Tetrode has more extended response at the top and bottom of the frequecy range.

Bass definition: Tetrode maintains greater control over woofers for more bass-range detail and tonal differentiation with less overhang, although the bass often won't sound as full-bodied in tetrode as in triode.

Dynamics: As mentioned, triode will compress earlier and more than tetrode on dynamic peaks. But tetrode is also better at eludicating subtle dynamic shadings of expression on the micro end.

Soundstage: Tetrode presents a clearer, and more defined and separated soundspace than triode, which by comparison sounds somewhat homogenized and foggy. Tetrode also has greater scale.

Imaging: Tetrode presents better-focused images, triode more amorphous ones.

Resolution: Tetrode is better at illuminating decays, and combined with its clearer atmosphere and better micro-dynamic tracking, the result is more transparency to fine detail and more precise articulation.

In sum, I think the amps pass more information in tetrode, but don't necessarily present it as nicely as in triode. (Of course, all my observations are drawn by me in my system, and are therefore not to be taken as universal truths.) I do end up using the switch as a tone control sometimes, dependent on the recording. If a recording is overly-aggresive, I'll likely go to triode regardless any impact or definition I give up. Likewise, if a recording is muddy or hooded-sounding, I'll use tetrode to help pry it open, harmonics be damned.

With well-recorded material, its a little more disconcerting to me to have an amp with two distinct sounds, neither of which I know is absolutely correct. I'd love to be able to combine the purity and naturalness of triode with the control, power, extension, and precision of tetrode. But since no amp is totally correct, I guess it can't hurt to own that at least gives you an option. So on decently-recorded disks that won't penalize you too much either way, my preference is mostly determined by the type of musical material, where its inherent priorities lead me.

BTW, I often do my comparisons on the fly, sometimes with music playing. Over the past year, switching with impunity has possibly cost me two output tubes and one output fuse, but I can't be entirely sure of the causes. One thing I know is that if you are going to do this, it is vital to flip the switch as quickly and positively as you can - no half-assed indecisive or scared-acting switch-flipping. Just do it. Or don't do it - this has got to be a personal call. I've flipped hundreds of times with no problem, but who knows what'll happen next time? Oh, and to make valid comparisons, you MUST compensate for the 2-3dB volume differential between the two modes (something that's easy with my Levinson preamp's programmable muting function).

As a final thought, it's worth saying the change in modes is not a night-and-day affair - it can be pretty subtle, or pretty insignificant with certain sources. After all, most of what makes the amp sound like the amp that it is, remains the same in either position. Right now, I'm considering a coupling cap upgrade and some experiments with input tubes in order to try and bring tetrode closer to my ideal tonal balance, which for me is its biggest flaw at present.

So far, I'd say that I run the amps 80% in tetrode over triode, but when I first got the amps, the switches had been removed by a previous owner and left set on tetrode, so for almost the first of a year I had them I never heard triode. As I've gotten used to the restored switch and its effects, I find myself increasingly willing to forego some of the qualities I had become accustomed to before the repair and listen in triode.
I feel like a complete fool, but I recently plugged each mono block into its own dedicated outlet, and man, what a difference! Now tetrode and triode sound a lot more alike, and I actually prefer tetrode for 80-90% of my listening.

Tetrode sounds perhaps "grittier," but more real! I certainly agree with Zaikesman that with tetrode there is more extension, and that the frequency spectrum is flatter. This is important to me, as I can lean back, listen, and there is nothing missing (highs, mid-range, lows, everything is there, and nothing jumps at you - - just like in the symphony hall). The soundstage is incredibly linear and extends well beyond the speakers to the left and right (but is also deep).

Conversely, with triode, I feel that the midrange is accentuated. Vocals are clearer with triode, yes, but over the long haul, it's not as involving (it get's a bit "glassy" and the soundstage is smaller than with tetrode. Actually, the soundstage with triode stays beween the speakers, and it's more "curved."

My experience is that the db difference for equal loudness is maybe 1.2 to 1.5 (i.e., triode needs to be set at 1.2 to 1.5 db higher than tetrode, which is also easy to do with my preamp (Levinson 380s).

I agree that it would be nice to keep tetrode the way it is, but also have triode's better harmonic structure and inner detail. Why can't VTL do that? Hopefully, they will at some point in time.
Interesting result on the outlets - are they separate dedicated lines too? I guess your 450's in tetrode were pulling too much from the wall to share nicely. My 185's probably don't - I have them plugged into the same Power Wedge Ultra PLC I use for all my gear combined, with its lone cord feeding from the single available outlet. I haven't minded, but I may find an improvement if I ever re-situate to allow plugging into multiple lines or outlets (then again, maybe not - hum can often become worse in such installations).

Freaky that you have the same preamp as me - how many guys could be running Levinson pre's into VTL's? Our slightly different impressions of the relative tonal balances between triode and tetrode could be because of our different speakers, and also because of different tube and cable choices. I've recently done some switching in the tube department, you can read about it on this archived thread.
Yes, the outlets are separate, dedicated lines @ 20 amperes each. It is funny we have the same preamps. My speakers are B&W 803 Nautilus, hooked up with Transparent super Bi-cable speaker wire. I also use Transparent super interconnects, and my digital source is Mark Levinson 390s. Good sounding system, very lively, I like it now (but it took me a while to get there)!

BTW, your thread is interesting, thanks!
Zaikesman, you description of the physical qualities of tetrode verses triode are exactly what i would say...only completely opposite. I agree with you where you say tetrode has a greater dynamic range and has more purity and naturalness. But i find triode to be more detailed, have a more focused images, tends to reveal more high end detail with a little less body, all characteristics you attribute to tetrode. Interesting.
I think I need to slightly amend a couple of the (mis)impressions I seem to have left from above.

Hgabert refers to agreeing with me that "with tetrode...the frequency spectrum is flatter". Yes, tetrode could be called 'flatter' in that it sounds more extended, both top and bottom, but I said above that triode sounds more neutrally-balanced to me through most of the audioband, including all of the midrange. To me, tetrode is less flat than triode here, which is where it counts most. Questions of ultimate tonal balance, however, are quite dependent on partnering gear and choice of input tube.

Piezo says he agrees with me that "tetrode...has more purity and naturalness". Actually (except for the part I just omitted about tetrode having greater dynamic range, which is true, and which he stipulates to), that is what I said about triode, not tetrode - which generally seems a bit less 'pure' and 'natural' than triode, for reasons I speculate some about above - and in that respect we really do agree.
Doh! I did indead mis quote you, but i do agree, as you noted that triode is more natural and pure. It appears that I forgot to add "triode" at the appropriate place.

As to our other observations being 180 out, that's strange because we both are apparently hearing the same things but in the opposite modes and have a similar overall impression of the two modes (ala triode being more natural). The question that arises is: are there system dependancies that are driving our observational differences or are we describing the same thing differently. If the latter it would seriously cause one to take all reviews with an even larger grain of salt due to the subjective listening impressions being overprinted with imprecisly defined descriptions. Just some philosophical head scratching.
This also reminds me that we audiophiles are using a particular definition when it comes to the word "natural" - one where we are mostly talking about matters of texture and tonal and harmonic balances, and here the word applies best to triode. If we expand on that definition a bit to consider other aspects that could also be described as being more so, then I would say that tetrode has the more 'natural' spatial presentation, dynamics and articulation, and ability to resolve fine detail. But we normally use other words to denote realistic performance in these areas.

I'll also mention an analogy I've used before to sum up the different impressions I get from comparing the two modes: With triode, I often feel as if the performance is less 'electronically' rendered than with tetrode, but also that I'm listening to it at some remove, sort of as if I were in the next room - while in tetrode, the performance sounds slightly more like an 'electronic' artifice, but also as if it's a performance that I'm more in the direct presence of, in the same room with if you will. Or put another way, with triode I more feel like I'm hearing the real thing but also that I can't 'touch' it, and in tetrode I feel more like it's embodied right there in front of me, but that it's less quite like the original thing and more of a slightly 'off' clone. The choice is like looking through a window at your very own mother (triode) vs. exchanging a hug with her replicant (tetrode). I guess people buy even better amps (presumably - hopefully, for me - at a higher cost) so the two qualities can be combined to an even larger degree than these amps can manage strong glimpses of in the respective modes...
I think where we are seeing things different is in the presence part of our descriptions because we both agree that triode provides the most realistic rendering of the music.

For my system and ears the triode mode conveys much more detail and recorded room ambience, more direct palpable presence of the instruments than in tetrode (small group, preferably acoustic based music is forming the basis of this comment). Listening to Keb mo i feel that i could walk around him while listening to triode, not so defined in tetrode. For me tetrode tends to provide the wash of sound that you attribute to triode, though it tends to have more body/fullness, just not as pinned down and accurate..the slightly off clone that you describe.

Seeing that we agree on a number of our observations my guess is that the way our speakers (and rooms) deal with the different modes (and resulting drop in available power in triode) may play a significant part in our differences
What are your speakers/cables? To reiterate, I'm listening through Thiel 2.2's and either Satori or Au24 cable (though these impressions were formed only with the Satori, as I'm still evaluating the newer-to-me Audience cables).

Anyway, I'm not sure I can put a tag on which presentation is 'most realistic' so easily. I think it depends a lot on the recording and type of music. As I said above, in general I find tetrode more realistic in terms of qualities I would call 'physical'. But I think triode is more 'accurate' in its tonal and harmonic presentation. I've had some recent success in transforming the tetrode tonal balance more to my liking by both changing input tubes (the previous ones had become microphonic, and were brighter) and by placing tube dampers on them. However, I'm not quite done sorting this all out yet, because I've also gone from Svet 6550C's to EH KT-88's, and on top of that I haven't yet carefully evaluated again the tetrode/triode question with all the new tubes in place. Not to mention that I'm auditioning these speaker cables, which sound perhaps more transiently accurate but less warm than my reference, and I also just had a tweeter and crossover resistor replaced and that speaker is still breaking back in...it never ends :-)

Anyway, yes, my physical impressions are pretty much the opposite of what you describe in your second paragraph above, so apparently our 'clones' are 'slightly off' in different ways. Where you say triode gives you more detail, room ambience, palpability, walk-around transparency, and is more pinned-down in space, I feel tetrode does all those things better, with triode being the mode giving more body/fullness (if less extension and tautness). It's enough to make me wonder if our mode switches are wired-up the same way or if one's reversed! But I guess I envy anybody who's come to the comfortable conclusion that one mode is usually best for them...at some point here I'll revisit this question with the new tubes and post my findings.
I'm using Dunlavy Athenas, a VTL 5.5 pre, ARC CD-2 with harmonic tech pro silways between the CD and the pre and truth links from the pre to the amps (i needed 22 ft so budget had to fit in there somewhere).

I re-read your original post and noted where your switch had to be reinstalled...Putting it in 180 out could be the reason our interpretations are opposite.

As to the conclusion which is best it depends on the music, my mood and whether i have the ambition to get off the couch and monkey with the switch. If i had to live with only one it would be tetrode though...got to have that power when the application calls for it.
I was kidding about the switches being reversed - that's pretty unlikely with two monoblocks, and besides you can't mistake which mode has less power. Last night I did my first listening tests between the two modes with the KT-88's installed. I'm not ready yet to report with certainty, but preliminary results suggest that my impressions will be different with these tubes...

I'd be curious to get your report on the KT-88 tubes vs. the 6550s. I called VTL a couple months ago and they advised against any tube-rolling (at least for my amp). So long.
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.
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.
Hmm . . . good points, Zaikesman. I've never listened to the MB-750 in triode mode in my room, and they could certainly sound better than the 450 in tetrode. Who knows?

But the comparison here is tetrode/triode for a given unit, and yes, maybe that's unfair to triode (with only half the power). However, listening to small-scale ensembles shouldn't tax the power requirements too much.

As far as your swapping of tubes is concerned, do you need to do anything special before you swap KT-88s for 6550s in your MB-185? I'm wondering if I could do the same, if I had the urge to do that at some point in time. And what did you do with your input/driver (phase-splitter) tubes? Did you change them as well? Just curious.
Zaikesman is absolutely correct in his response above what your describing in triode is power limitations problem definitely had the 450 just wasn't quite enough power in triode sometimes so now have 750 no more power limitation issues. The 750 is sonically better than the 450 and it's not just because of the added power its just better in every way possible. If your VTL doesn't have the infini caps currently I would highly recommended you get them installed they are a rather significant improvement in my system for sure.
I'm sure Mejames is right about the 750's, and the comparison is a little unfair not only because of the added power, but also because the Reference amps have an improved input amplifier circuit topology and power supply, not to mention the better parts. Upgrading the coupling caps in my 185's is probably the next step, although going to 250's or 450's could also be in the picture eventually (the Ref amps are too big and too much money for me - and besides, I like to think I can and should be happy without having to have the best :-)

The tube swap to KT-88's requires nothing more than installing the new tubes and biasing as normal. (This is not the case for KT-90's, which need resistor changes.) The driver 6350's are the same as always (there's not much to choose from with this tube, NOS being tough to come by and limited to just a couple of options I believe). I've played with several different input tube varieties of 12AT7's - maybe I'll do a thread on the results sometime.

About your comment that "small-scale" music shouldn't present very taxing power demands, just today I did some comparisons between tetrode and triode using Sonny Rollins' "Way Out West" MFSL CD. Jazz doesn't get much more intimate than this: just a sax, bass fiddle, and a small trap set with a little percussion. No chords at all, and the tunes are taken at easy tempos and volumes, with plenty of space left in between the notes. Still, there are differences to be heard when listening between the two modes - even at moderate levels - that could relate to power, such as bass control and dynamic range. As ususal, those catagories go to tetrode, while overall naturalness of timbre, particularly from the low midrange up, goes to triode. Which presentation sounds more 'real' is a bit of a toss-up, depending on what sonic qualities you prioritize.

I think that to approximate reproducing the true sound of any acoustic instrument or voice, no matter how delicate the music or spare the arrangement, still presents an enormous challenge to any power amplifier, and I tend to agree with VTL's position that a lot of the inevitable compromise is simply related to a lack of available ultimate power. The task, of course, is to make a power amp that can deliver the needed level of clean power and yet do it instantaneously and with good musicality. I guess VTL probably does as good a job of trying to give us this as anybody, but there's a price to paid both monetarily and in size/weight/heat/appearance, and besides most speakers can't take full advantage of dynamically unlimited amps anyway. Most audiophiles will never attempt this, and the SET/horns crowd would pooh-pooh the notion, but my suspicion is that a home 2-channel system utilizing several thousand watts of multi-amped power would not necessarily be overkill strictly from a sonic accuracy standpoint. (You here Sean?... :-)
horsepower wars in sound reproduction, man how reality changes. I remember in the early 80's when i played in a loud rock band (we're talking marshall half stacks running 3/4 by the end of the night)and all we had driving the vocal mains was a lowly crown dc 300 that need a fan blowing over pitchers of ice to keep it cool enough to run. The last band i was in was a pretty quiet blues band and we used almost six thousand watts a side for the mains. My home system is the same deal, i'd love a mb750 for my 14 by 22 room and i'd probably play it quiter than my high school system of klipsh cornwalls fueled by a crown d 150. Headroom is a good thing.
My MB-450 have the MIT caps, I'm not sure if they differ a lot from infini caps in terms of sound reproduction.

I agree, power reserves are crucial. (I just find it astonishing that 220 watts for small-scale music might not be enough). However, all of the clues certainly point in that direction. And I'll give the Sonny Rollins "Way out West" a spin as well in both tetrode and triode. That should be interesting.

So, what to do? Get the MB-750s, or more efficient speakers, or keep everything the same and listen to tetrode mode? The audio hobby just doesn't appear to have an end to it.
Hgabert wrote My MB-450 have the MIT caps, I'm not sure if they differ a lot from infini caps in terms of sound reproduction.
my reply was "If your VTL doesn't have the infini caps currently I would highly recommended you get them installed they are a rather significant improvement in my system for sure." my amps had MIT caps they sound like completly different amps now much better everywhere with infini caps.
Mine don't even have the MultiCaps (MIT caps), they just have the yellow "VTL" caps. I'm trying to decide whether to go with the InfiniCaps, as VTL uses, or the DynamiCaps, both of which are made by TRT. I described where I was sonically and where I wanted to go to both Peter Moncrieff (the manufacturer) and Michael Percy (the retailer) and they unanimously recommended the slightly more expensive DynamiCaps, but that was before I switched to the KT-88EH's and made some changes regarding the input tubes (including adding tube dampers) which went a ways toward addressing the brightness I was hearing in tetrode. VTL officially prefers the Infinis, while Moncrieff's comments seemed to imply that they were flatter in their response than the DynamiCaps, which from his description sound like they're probably a bit more 'tailored' in their response, but in a way that he apparently thinks better serves the music in many circuits and systems...scary choice ;^)
Thanks. I will, for sure, look into the infini caps. This seems like a logical, first step to take. I really appreciate the feedback from everybody.

the infini caps VTL uses are custom made to different specs for VTL they aren't off the shelf infini caps
That's what VTL says, although I couldn't find out exactly how the parts are supposed to differ. I also couldn't get TRT to either confirm or deny this assertion of proprietary specification for me. VTL seems to charge around twice as much for these parts as they're generally available for - assuming they'll even sell them to you at all; I've found the company is very loathe to assist in any way owners wanting to do mods on their own or hire outside tech assistance that's qualified and local to them. That VTL charges $200 more for the InfiniCap upgrade than for the MultiCap upgrade raises my eyebrows; that suggests a parts cost increase of over $16 per cap for my amps, way more than the actual retail difference - in fact, it's slightly above the InfiniCap per-piece retail price itself!

They are insistent that the cap mod be done at their CA factory, which I can understand for reasons of quality control and warranty impact, not to mention their own business. Plus, they will perform other updates if needed - such as to the fusing and certain resistors - while the amps are in, and retest them generally and provide you with updated test documentation. But for those of us who don't have a warranty with VTL anyway and live on the opposite coast (and who've already had the factory updates performed), this means shouldering 2-way shipping charges considerably north of $200 (you must ship the way VTL says) for the privilege paying the factory almost double what is commonly charged for this same job done independently. Tell your local tech that the manufacturer wants $1K (not including shipping) to replace 12 (in my case) capacitors that cost about $15 apiece retail (at $50 an hour, this would imply around 16 hours of labor, but since the job should only take about half a day for a skilled tech, well, you do the math) and see what kind of reaction you get... ;^) Add in the associated expenses, risk, and hassle of shipping, plus the extended time the amps will be away from the system, and you can understand why I'm inclined not to worry about VTL's vague claims concerning the InfiniCaps they use.
Yeah, that is somewhat strange. But everyone needs to make a living, I guess. I emailed VTL about prices but haven't received a reply yet (maybe because I bought my amp used, so I'm not exactly a priority customer for them).

On another note, I listened to Sonny Rollins "Way out West." Here is my take: Everything sounds more similar in tetrode vs. triode than on the Brahms piano trio No.3, mentioned in a previous post (maybe because that piece has more of a dynamic range?). However, track 4 (Wagon Wheels) does sound better in triode (at the same loudness settings), even ravishing, especially at the beginning, but maybe it is almost too pretty. On other tracks, especially track 1 (Old Cowhand), I can make the bass lines out better in tetrode. But from the midrange on up, triode preserves the harmonic structure a little better, and there is more air between the players (if I don't crank up my pre-amp too much).

It's almost as if triode makes the music slow down a little, and you can hear more into the internal fabric of the instruments. In tetrode, everything appears a little faster, with better rythm. I noticed I foot-tapped more in tetrode mode, and listened more closely in triode.
One thing I've been doing lately is to use different input tubes for each mode with certain types of recordings. For triode mode, the brighter, more incisive Telefunkens make a good match, while in tetrode the darker, fuller Mullards complement nicely. I agree (I think it's agree) that the Shelley Manne intros to those SR tunes generally sound more natural in triode, but when Ray Brown kicks in the pendulum swings toward tetrode...space is also presented differently in each mode, but there I tend to prefer whichever mode I'm listening to at the moment ;^)

Even if your input tubes display no audible microphonics, I urge everybody to try some sort of tube dampers here (I'm using Herbie's); for a nominal investment, you'll probably hear the amps cope better with stuff like vocal sibilants, plus truer tonality and more stable imaging, with 'quieter' spaces between performer images. Everything gets a bit less edgy and a bit more listenable, especially at higher volumes (not that transient impact is smoothed out - actually, HF transient tracking fidelity is increased, because there's less tizz and overhang). The improvement isn't huge, but neither is the cost or hassle.
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