Triode vs. Pentode

Wow. Don't take much to amuse some of the lurkers, do it?...

Cool JB, thanks for clarifying that. Maybe JW would say the same, maybe not. Either way, I'm sure we'd all welcome getting a real take on this subject from each of you...

FWIW, comments on a few things written above:

KT-88's are beam tetrodes (as Ecclectique alludes to), so there is no pentode connection available as with EL-34's for example. I was under the same misimpression until very recently, and there are good reasons to be confused, because the terms have gotten tossed around somewhat indiscriminately since the time beam tetrodes were first manufactured as an alternative to pentodes. (These technical developments were under patent at the time, and from what I can infer it seems the tube company marketers might have desired the positive association customers had with pentodes).

The connection Eldartford describes actually sounds to me like Ultralinear, rather than straight pentode. I do not know whether any VAC amps employ Ultralinear connection.

As for the sound and theory regarding mode vs. mode, allow me to offer my basic take from my experience with VTL amps using 6550C and KT-88 tubes:

As I see it there are two essential phenomena in play. One is the difference in power: roughly double in tetrode what's available from triode. This changes the sound all by itself, and taken on its own, if no other properties were to change, more power from the same number of otherwise identical tubes ought to sound better, or at least more accurate, given a power supply that can keep up adequately with demand.

But this leaves out the second essential property - the presumable reason for the sonic positives associated with triode mode, despite its lower power. This has to do with how the tubes diverge from linearity, otherwise known as distortion. As I understand things, when compared within comfortable power margins, triode-connected tubes offer lower overall distortion than when run in pentode, and what distortion remains is more benign in nature, i.e., lower in order and more weighted to the even harmonic series. This greater purity is what you trade off to some degree in order to obtain higher power from the same tube connected in tetrode or pentode.

These two characteristics - output power and harmonic distortion signature - taken together, pretty well explain what I hear comparing triode and tetrode modes with my amps.

Higher power sounds like...higher power. Better speaker control, which can translate into a host of audible qualities ranging from increased soundstage size and image separation to more bass tautness and less overhang. Greater dynamic capability, which can translate into less compression, more impact, and better microdynamic expression. The ability to play loud music louder and still maintain dynamic contrast and overall authority. Greater power, considered in isolation, even seems to be able to allow for greater transparency - but the degree to which extra power will benefit the sound has to do with many other variables, including speaker sensitivity and impedance, room size, type of source material played and at what volume, etc.

Experience, though, tells us that in actual practice, many of the positive qualities associated with the higher power of tetrode in theory, are in reality somewhat compromised in their ability to convey musical enjoyment, due precisely to the less agreeable harmonic structure of the more powerful mode. Who wants to hear music played louder, with less compression, if that music has taken on more of an agressive edge? Who needs the clarity of better driver control if a good part of what it lets you do is hear more clearly the extra, higher-order distortion the amp in full-power mode is injecting into the music?

There's another variable at work as well. When we talk about push-pull amps, we're talking about a circuit that inherently works to cancel even-order harmonic distortions. This serves to leave the odd-order distortions more nakedly exposed to the ear. (This fact, rather famously, also goes to part of why people may like single-ended triode amps - despite their very low power - which leave the triode's naturally low-, even-order harmonic products intact, instead of canceling them. The lower power does mean that total distortion levels rise much sooner and higher before clipping than with higher-power push-pull amps [even if SET clipping does approach more gracefully when it comes], but the low-order, even harmonics which the ear finds more musical predominate.)

Thus, a push-pull circuit operating in triode mode gets a double benefit on the distortion front: triode generates mostly low- and even-order harmonic artifacts, which the output stage architecture then tends to cancel out, leaving both less residue, and less objectionable residue. Tetrode or pentode by contrast creates a double-whammy applied in push-pull: higher- and more odd-order harmonics are generated than with triode, and then on top of that the output stage configuration tends to cancel-out what lower- and more even-order harmonic content does exist - content which would otherwise help to mask this less-pleasing timbral quality.

These reasons explain the continuing efforts - necessarily at high cost, since more tubes are required to achieve comparable output - to build high-powered push-pull tube amplifiers wired in triode, instead of simply going for the bigger bang for the buck possible from tetrode or pentode. (Witness, for instance, Atma-Sphere, which uses only triode tubes and many of them; CAT, which uses beam tetrodes but wires them exclusively in triode for sound quality; or of couse VTL, which makes mode-switchable amps so high-powered, you can run them in triode and still have juice to spare. VAC has even gone to the expense of making high-powered push-pull amps using multiple pairs of one of the most expensive triodes, 300B's, which we normally only see used singly in SET's.)

So, especially when played at lower volumes or with music that is not as dynamically demanding, triode will often sound best, this being manifest in a variety of ways, and again dependent on several other circumstances (including the particular output tubes used). But in general, it is possible to say that triode will usually sound more timbrally pure and natural, texturally cleaner, and with less artificial emphasis on silibants and harsher overtones.

A lot of ink is spilled in the above posts trying to make generalizations about the sounds of the two modes, but I think Trelja finally comes closest to the truth when he hits upon the term 'sweetness'. (I don't believe he means to connote the word with the modifier 'cloying' implied, as in added 'sugar', but rather more an absence of bitterness, as with the way really good mineral water can taste 'sweet' in the absence of the usual contaminating pollutants, but with traces left intact that our evolutionary history informs our senses are healthful.) This is cutting to the chase of what's most advantageous about triode and weakest with tetrode, though many other qualities, some of which may be more important with any particular musical program, will always pertain. What I've tried to do here is hopefully help explain why this may be so.

It is also important not to oversell the differences. The same amp, with the same transformers and power supply, fitted with the same tubes, and playing at moderate volume - especially with music of a not too-demanding nature, or not with highly specifically-flawed or extremely revealing recorded sound - will in reality probably not sound revolutionarily different played in one mode vs. the other, and that only makes sense.

As the volume gets turned up though, triode may tend to suffer more audible compression and loss of control, although only because of the lower power available (unless maybe you have one of the monster triode amps and/or easy-to-drive speakers), and not because of any inherent shortcoming about the mode per se. So frequently you're presented, as with so many things in life large and small, with a choice, when playing an average (or averagely bad) recording: to have that kick drum really pound you in the gut the way you know it can, or to keep that high hat from cutting off your head at the ears with what feels like white noise. Or just turn it down and play lute music.

In my amps, for whatever reason, I have found that KT-88's made by Electro-Harmonix tend to thrive more, relatively speaking, in triode mode than did the Svetlana (SED) 6550C's they replaced. So I think it is possible that there are tube-specific differences as to how amps will respond in either mode, and some tubes may do better in one mode than the other, or better in one mode than another tube type or brand will do in that same mode.

Trelja: I agree that triode is preferable - and also think that higher power is preferable, all else being equal. That's the essence of what I try to analyze above: It's not triode vs. tetrode or pentode - to me triode wins that contest. It's triode vs. higher power obtainable from the same amp. In that there can be some tradeoffs, but as always depending upon many other variables.

As you indicate, the benefit of the extra power from tetrode or pentode is not mainly about higher volume for the same preamp setting. My Levinson preamp's volume control is calibrated in tenths of decibels, and switching modes seems to require only about a 2.5dB adjustment in order to maintain a matched level (but I can only go from triode to tetrode, not pentode).

"Triode injected into the midbass that extra muscle (not plumpness or lack of control)..."

I too have heard the fuller quality in the bass and lower mids you talk about with triode. Sometimes things can sound a little lean in tetrode. However, I do think that this can be explained with the model I describe above. Two characteristics combine to produce the effect. One is that tetrode or pentode is going to sound brighter, due to the different harmonic emphasis. The other is that triode is going to let the woofer have its way a little bit more. Whether the overall result sounds more accurate or not will depend on the speakers and room, the rest of the system, the program material, and I'm sure the listener.

Although I cannot agree with the theory that there could literally be "extra muscle" behind the lower frequencies with the lower-powered mode - and in fact I believe it was in large measure precisely the sound of less muscle that you heard - I can see the possibility that different modes of operation might not only have different harmonic structures and different power outputs, but maybe also different tonal balance tendencies. But it's hard to know this for sure, separately from the twin factors of harmonic structure and output power, because both of those properties can affect our perceptions of tonal balance.

"Although the volume measured the same, subjectively, listeners swore it was a good bit louder."

Why triode would sound more dynamic is harder to intuit, but the above comment is revealing. Often a little compression actually makes the music sound 'fatter' and subjectively 'louder' if we tend to calibrate our volume-match to the peak levels. This can be an enjoyable effect. Higher dynamic contrast can actually sound 'quieter' at the nonimal average level. Impulses that are slightly compressed in amplitude, get spread out slightly in time as a result. The ear can perceive the extra overhang as being 'more', whereas the tautness of the uncompressed peak, leaving little trace in the time domain, can actually sound like 'less'. Or maybe in certain situations, just backing off a little on a preamp's volume control in order to match levels for tetrode or pentode causes a slight dimunition in the perceived dynamics. It's tough to isolate all the variables. Just better to listen and enjoy.

No, never heard KT-77's. I guess the only reason you don't use them all the time is you don't want to wear them out?

Speaking of C-J's and converting to triode, someone there once told me that the company - whose amps are available wired either for triode or Ultralinear, but permanently, without a switch - finds that around half of their customers who get their amps converted to triode eventually pay to get them converted back again to Ultralinear. As Trelja said, it can be tough to lose that power.

Which reminds me of today's listening session: with the mono's still down to 1/3 of their normal tube complement and in triode, I played a couple of albums at more room-filling volumes than last night. I wasn't as pleased with the sound - in a nutshell you could say it was 'slowed and rolled'. Not overly distorted, though I could push it there, but sleepy.

Then I reinstalled all the tubes, rebiased, and listened again. Problem solved - the speed returned, the full range returned, and fine detail reemerged. Wake up! Then later on, I really cranked things loud playing James Brown, and the bass got a little mushy and the soundstage a little smoggy, so I flipped over to tetrode, and whoop, there it was, clarity and authority that pulsed the room. Tetrode still has its purposes for the time being.

>"Some people are more concerned with test tones, some prefer music"

>"Of course the ultimate test is how it sounds with music"

Glad we all agree on that. Now that we have nothing to prove to one another, what say we get back to the issue at hand...

Trelja, I don't think of anything you've said here as "flying in the face of conventional wisdom" or likely to "spark a tremendous amount of controversy". Subjectively, I've agreed with almost all of what you've stated about what you hear (about the only item I couldn't quite relate to from my own experience was your description of triode as having more "sparkle"). What I'm trying to do is offer some possible explanations for what we hear. (BTW, my comments about the volume compensation related to doing this by ear - I don't own a meter, although I suppose I ought to.)

There's another, older thread on this same subject that basically illustrates the evolution of my views on the tetrode/triode subject.

Reading over it again, it occurs to me that it really took me over one year and a change in output tube type to come to the conclusion I hold today - namely, that it was naive of me to ever suppose that by merely flipping the tetrode/triode switch, I was conducting a test that could inform me which mode was 'inherently' superior. Getting to that requires digging a little bit deeper.

When I started my comparisons, I essentially disregarded the fact that the output power increases 100% in tetrode, treating it as just an incidental factor, which one had to be mindful of in terms of compensating for matched volumes at the listening position, but no more than that.

That thinking was a mistake, as I eventually came to realize. The difference in power is an integral, confounding, factor - one which cannot be written off as beside the point when comparing modes. It so happens that in my system, through my speakers, tetrode does hold some advantages, and not just with certain music or at certain volume levels. But I now no longer ascribe those advantages to the mode itself, but rather to the increased power, an important distinction.

It's interesting to note that a good portion of the advantages demonstrated in tetrode (again, due - I believe - to the additional power) do seem to be largely independent of absolute volume level. Even at moderate listening levels, and despite the superiority of triode regarding most *musical* qualities, the extra power of tetrode still holds some of the cards regarding what I call the "physical" qualities of the reproduction.

This observation gets to the question of why higher power is desirable. We are always reminded that it's the first watt which counts the most, and I can't disagree with the fundamental astuteness of that aphorism. But the reason they make amplifiers way above 200 watts isn't only to play at 115dB in rooms the size of a small hangar.

I don't have the kind of wide audio exposure to say that I've done conclusive research in this area, but my suspicion is that you can almost literally never have too much power - even for playing average-sized, average-sensitivity speakers in a normal room, and even at moderate volume levels. Whether obtaining that power presents other problems which can compromise the theoretical benefit is a separate question.

Last night I did some fresh comparisons between tetrode and triode with my VTL 185's running KT-88EH's. The amps are rated at around 100w in triode and 200w in tetrode into 4 ohms, the nominal impedance of my Thiel 2.2's.

Even at volumes quiet enough not to wake my gal sleeping upstairs - and even though triode was unquestionably superior at presenting a natural-sounding broad middle of the audioband - tetrode was still superior for bass control, dynamic range, extension at the far ends of the frequency spectrum, and clarity of the space around images. I think if I had a similar amplifier available which doubled the power again to 400w, I would have continued to hear some improvements in those areas - at the same low volume.

Unfortunately, tetrode also carried its penalty along with the extra power: a less-organic, more 'electronic'-sounding portrayal, with the timbral balance tipped by an artificially bright scrim that stripped some meat off the bones and made images seem 'pointier'. It was less naturally compelling and believable, even though the tom toms did pop out of the mix a little more, the bass was more defined, and the soundspace air was a bit more 'see-through'. These qualities may not be terribly *musically* important in many instances, but improving them would constitute higher fidelity, were it not for the trade-offs.

So the downsides of tetrode I attribute to the mode, the upsides to the additional power. If I wanted mo' better than I've already got, I would need an amp that developed higher power in triode.

Exactly *why* doubling already fairly high rated power should make any improvements even when the volume is kept low is not a question I pretend to know all the answers for. But it seems logical to assume some combination of improved driver control and greater freedom from dynamic compression, despite that the average power draw (and I stress average - momentary peak power demand might be more of a mystery) at low volume must only be a watt or less.

While composing this post, I took a break to repeat an experiment which I first mentioned on the other thread linked above. I listened in triode mode to my amps with all 6 of their output tubes per monoblock installed, and then compared the sound after removing 2/3 of the tubes, leaving just the minimum of one pair per amp, still set for triode.

There's a little time lost to the mechanical process and also readjusting the bias, but not more than a few minutes, so I repeated this several times back and forth, auditioning the same cut. Of course volume must be compensated for at the preamp, but even cutting nominal rated power from about 100w down to about 30w didn't seem to require more than around 3.5dB's worth of adjustment to offset. I listened at a low overall volume because once again it's the middle of the night.

The differences are interesting. When configured for 1/3 power, the amps' tonal balance changes. The high treble becomes comparatively shelved-down a bit, and the mids take on new prominence. Overall, the balance sounds enjoyably 'fatter', but not like I'm listening through a pillow or anything. I won't say the bass increased in level, or even necessarily proportionately, but the mid-to-upper-bass seemed a bit plusher, the lower bass more vague. The combined effect was to tilt the spectrum to be weighted more heavily in the range of male vocals, with an intimate quality which was quite attractive replaying same.

Reinstalling the full tube complement (still in triode) produced sound which by comparison seemed more 'hi-fi' - the upper treble was more highlighted, the bass became a little more defined and correspondingly less cushiony, and the heart of the midrange was comparitively demphasized. Notice how closely this seems to resemble what we hear when switching from triode to tetrode! How much is the responsibility of the mode alone, and how much of the additional power it brings?

The sound reacquired basically the similar set of 'physical' attributes I described above in comparing higher-powered tetrode to lower-powered triode - more dynamic contrast, more extension, tighter grip, more explicit deliniation of space with a clearer atmosphere. But listening as I was to a rythym & blues song recorded in the early 60's and remastered rather brightly for CD (it was what happened to be perched in the diskolater at the moment - what can I say?...I'm just lazy, and I dug it), in some ways I actually prefered the lower-power rendering, which sounded perhaps more appropriate for the material. Maybe this is part of why people go gaga over really low-powered SET's.

Trying to set aside the balance differences between the two presentations, it did almost seem to me that possibly the 2-tube rendering had an extra bit of 'purity' vs. the 6-tube version.

This could appear to be so for more than one reason; the first that comes to mind is that with less power and the balance changes I've noted, I could no longer hear the problems with the CD mastering job quite as well.

Some might suggest that a single pair of output devices will function with better symmetry than multiple-paralleled pairs (probably the same people who would say that just a single output device is even better than a single pair).

Another possibility is that there was some advantage in the fact that at 1/3 power, the power supply was now highly overspec'ed, relative to how it is called upon to deliver at full tube strength. Certainly my ExactPower's WRMS readout showed the reduced heater filament demand dropped the total quiescent power consumption by about half. Remember, this amp's power supply is designed to handle all 6 tubes in tetrode for 200w, now only dealing with 2 tubes in triode good for 30w.

That's extra beef without doubt - but who knows what consequences can be traced to what causes? There are too many variables to be sure. For instance, the combined output impedance of the power tubes will presumably grow higher in the less-massively paralleled 2-tube configuration. Since VTL optimizes their power transformers to the power tubes' source impedance for each amp model, this ratio will be sub-optimal with 2/3 of the tubes removed, meaning that not only may power transfer be less efficient, but also that final output impedance may change a little, with possibly audible response modification at the speaker. You just can't take everything into account to draw firm conclusions by listening alone.

But still, this experiment does appear to lend some support for my theory about what higher power means to tetrode mode and the sonic comparison with triode. Anyway, if fidelity is what one is after, then for the time being I'm comfortable with my contention that triode and higher power are what one should want in a tube amplifier.

PS - By the way, for those with mode-switchable tube amps and a good FM tuner in their system, one of the best demos you can run for hearing whether triode or tetrode sounds more real and natural in its harmonic palette is to do some comparative auditioning using speaking voice of NPR desk-jockeys. Public radio doesn't add EQ, compression, and reverb to their on-air voices the way commercial stations (or most recording artists) do, and the human speaking voice is a highly diagnostic tool for assessing the veracity of timbral reproduction. This seems to be true even if we have never met the speaker in person.

Yes Ecclectique, everything probably matters as usual, and I don't want to draw too broad a conclusion from the limited evidence at my disposal. (BTW, in my previous post I mis-wrote something: it's the VTL's output transformer that's designed to account for the power tubes' combined output impedance of course, not the power transformer.)

It does seem logical that there may be some advantage in using a beam power tube in particular wired for triode, or at least maybe as compared to a real pentode wired for triode. Makes me wonder if there's ever been a pure beam triode tube designed? But I can't speak to these musings from experience, because I've never had a power amp that used either pure triodes or triode-wired pentodes in order to make comparisons. Maybe Jkaway is in a position to comment on the relative merits between running triode-wired with beam tetrodes vs. true pentodes (although again, there might be a power differential there that could act as an additional, confounding variable in the auditioning results).

However, I did notice during my experiment last night with the 2/3 tube removal, that my monoblocks now sounded superficially much more similar to the stereo amp they replaced a couple years back, a C-J MV-55 that used one pair of Ultralinear-wired EL-34's per channel, good for about 45w. A lot of the ostensible improvements I achieved when I switched to the VTL's reverted to somewhere nearer the old sound of the system, now that the mono's were running in triode with only one pair of tubes per channel for a calculated 30w or so (and despite that the mono's still held a big advantage in the power supply and output transformer sizes). How much of what I originally heard when I replaced the modest C-J with the brawny VTL's was due to things like the beefier power supply and output transformers, and how much simply to the increased output power capability as such?

Atmasphere: Yes, I did speculate above on just this factor, and how it might have been a confounding variable in my audition results. Thanks for confirming that. I'm wondering what your take is, in light of how that impedance issue may have affected my listening impressions in these trials, on my conclusion that higher power may in effect be its own virtue, and not just for louder listening levels.

Your point about differences in the tubes' output impedance relative to the way they're wired makes me wonder exactly how VTL (or any non-OTL amp maker employing a mode switch) has chosen to 'optimize' their output transformer in the presence of such a switch. To judge by the fact that VTL eschews various output impedance taps for the speaker load in favor of utilizing the entire secondary and optimizing it for a middle-of-the-road 5 ohm nominal load, it might be reasonable to assume they've likewise optimized the primary for a value midway between the source impedances presented by tetrode and triode modes. This approach would seem to be a practical compromise, but it does imply some theoretical room for improvement were the mode switch simply to be dispensed with (particularly for those customers who spend the big bucks for the Reference models with the intention of taking advantage of the very high power to run them exclusively in triode mode).

Of course, such concerns could be of merely academic interest to a manufacturer of OTL amps, but the manufacturer of non-OTL's might argue that even a slightly-less-then-ideally-optimized output transformer will still present a more consistent load (meaning possibly less colored) to the output tubes than will a speaker. As they say, every choice in audio represents some kind of tradeoff...