Could someone tell me the difference between a single-ended triode amp and an output transformerless amp?

Is it true that despite its operational inconveniences, a good OTL (eg Tenor Audio) will always sound more "natural" than a good SET (eg a Cary 300SE)?

To add to that I have an Audiovalve RKV OTL amp that sounds really wonderful, but 3 watts per channel was never going to be enough so am looking to upgrade soon to a little bit more power, hence the question.
The guy perhaps most equipped to answer questions about OTL is Ralph of Atma-sphere (apologies to you other lurking OTL experts), who sometimes finds the time to post on Audiogon. Until he gets here, I will attempt to not embarrass myself unduly.

SET means Single-Ended Triode. A triode is a three-electrode amplification tube, and a single-ended triode amplifier is one where only a single triode tube acts as the gain stage. This is in contrast to a 'push-pull' amplifier where the signal waveform is 'split' into two parts, and one half of the signal is amplified by one device, and the other half by another device.

OTL stands for "Output-Transformer-Less". This means that while most tube amplifiers are coupled to the speaker by an output transformer (that big hunk of steel which makes the amp so heavy), the OTL amplifier is not burdened by the hunk of steel and the way that it might affect the signal. OTL amps are coupled to the speaker load either by (to my knowledge - and I dearly hope Ralph of Atmasphere can jump in if I miss something here) direct coupling (which prevent DC through circuit topology (and tube choice?)) or capacitive coupling (which does so with large-value capacitors).

Reading between the lines, this means that SET and OTL are not necessarily mutually exclusive by definition. It would be possible to have a SET OTL, however, the Cary 300SE is not one of them.

The Atma-sphere website has a section for white papers and there is some interesting reading to do there. From a quick look, it appears there was a 300B OTL project.

I also know that David Berning also has developed OTL and ZOTL (a variation on the OTL theme) technology and he probably has some interesting info on his website. At one point, I think he made a SET OTL amp for someone (if I remember correctly, it was an Audiogon member).

As to your question:
Is it true that despite its operational inconveniences, a good OTL (eg Tenor Audio) will always sound more "natural" than a good SET (eg a Cary 300SE)?
I don't have a clue. I expect that implementation counts for more than topology, and besides, I do not have enough experience comparing OTLs (which when I have heard them have been wonderful) with 'regular' SETs.
With regard to your follow-on post, several of the extant OTL mfrs offer higher-power OTL-topology amplifiers - many of them a LOT more than 3W. If OTL is what floats your boat, it can be found with enough power to drive somewhat insensitive speakers (as long as their impedance is high enough and the load is relatively flat across the frequency spectrum). In fact, if you wanted 50W, it would likely be a lot cheaper to do this with OTL than SET (because with SET you need to find a single tube which can put out 50W (there are a few, but not many, and the tubes themselves are expensive, and the OPT (output transformer) will generally be really big (and expensive)).
I like both SET and OTL amps. Both kinds of amps can deliver very clear sound and very good, natural dynamics. OTLs, in particular, sound extremely lively and dynamic.

OTLs can be easily scaled up in power by adding more output tubes, though heat and power consumption can be very high. The more tubes in parallel, the lower the output impedance, an issue which some people are concerned with when it comes to both OTLs and SETs (high output impedance can be a concern with speakers with a low nominal impedance).

I like the Atmasphere and Joule OTLs. The big Joule I heard was richer sounding and had more midbass than the Atmasphere. Perhaps it is a matter of tradeoffs, but, the Atmasphere sounded more nimble. The OTL I did not particularly like was the Tenor. It was a nice sounding amp, but, it completely lacked the kind of explosive dynamics which is the raison d'etre of OTL designs.

A well designed SET can also be very clear, fast and dynamic, but, the designs that have that quality tend to be extremely low powered. The better SETs, while simple in design, put an extreme demand on a quality, air-gapped output transformer. These things can be really costly. Personally, I think that some of the clarity and low level dynamics of SETs are lost when higher powered tubes are employed in SET designs (845, 211), though some of these are still terrific sounding.

I own a SET designed around 2a3 tubes. It is a parallel SET (two output tubes per channel). The amp can deliver almost 6 watts per channel. I like the sound of 2a3 tubes, but, compared to 300B designs, for example, this is a leaner, and faster, sound. Like everything else in life, parallel output tubes in a SET is a compromise. Purist will say that multiple output tubes muddy the sound and cause a loss of low level detail (don't tell that to the OTL crowd).

Overall, I like the SET sound, but, most SET amps can effectively be used with an extremely narrow range of speakers. My speakers are 99 db efficient and I probably could use a bit more power. OTLs are WAY more practical when it comes to matching with common speakers.
Larryi, I don't agree that the low level dynamics and clarity of my 845 SET are lacking in the least. In fact these very things are improved vs. 2a3 monoblocks, push/ pull tubes and SS amps I own or previously owned.

I would agree that my 845 amp can sound rather dull and flat at lower volume levels, but that is not the same as saying it lacks in microdynamics and clarity. Perhaps this is what you meant?

I also love the Joule OTLs and Atmasphere certainly is well respected. OTLs are fast and immediate as they don't have the liabilities of tons of wire in output transformers. My 845 amp shares this trait despite it's output transformer, in fact, I would say that is it's most distinguishing characteric vs. my push/pulls and SS amps.

I actually do like a number of 845 and 211 SET amps. For speakers needing more power than what a low-powered SET can provide, good SETs using either tube are contenders. But, when driving extremely efficient speakers, I don't think they are, at lower volume levels, as detailed sounding or deliver small scale dynamics (sound lively) as 2a3s or 45s. Of course, I've only heard a small number of any kind of amp/tube combination in my own system, so I am only roughly generalizing here. When it comes to SETs, the output transformers are really hard to design and build properly so a LOT depends on getting that part right. I bet that is particularly the case with either 211 or 845 amps because of the high standing current in the primaries.

For the vast majority of applications, that would be an academic argument because a low-powered SET would be unsuitable. I believe that first and foremost, one picks a speaker system that sounds great and then looks for a suitable amp. If I needed more power, I would certainly consider 845/211 amps (e.g., I really like the Wyetech 211 amp). I've heard some really strange sounding systems where someone "fell in love" with a particular low-powered amp and then went looking for suitable speakers (some single driver systems and horn systems are truly weird sounding to me).

It is a matter of picking the best compromise. The SET purist out there would certainly have a lot a bad things to say about the parallel single ended approach employed in my amp. I am not convinced any particular design is inherently superior. One of the very best amps I heard was a pushpull amp that is over 50 years old (a huge, rack-mounted Western Electric amp that currently sells for something like $70,000 per channel).
A single-ended triode amp will always have an output transformer. The transformer is both the boon and doom of the amp- it allows the tube to drive the speaker, but the bigger you make the transformer, the more limited the bandwidth will be.

As a result, the best SETs are the smaller ones (2A3s, 45s, that sort of thing) assuming you have a speaker that is efficient enough to work with them.

OTLs have no output transformer and so don't have the power vs bandwidth issue. Instead you have the opposite problem- its harder to make a practical low-powered OTL because you have to be more careful about choosing a speaker for it.

Usually SETs are zero feedback. OTLs normally have feedback (although ours tend to use little or none). A triode zero-feedback OTL operating class A, like SETs, will generate primarily lower-ordered harmonics, with very little higher-ordered harmonics. This will give either one a relaxed quality.

The ear uses odd-ordered harmonics to tell how loud a sound is, so if those harmonics are not emphasized by the system, the system will have a sense of ease and a lack of hardness.

OTLs are normally push-pull, if built single-ended will produce very little power for the number of tubes involved! Since push-pull operation allows for even-ordered harmonic cancellation, OTLs generally lack some of the romantic lushness of SETs as they will also lack the 2nd harmonic. This is an advantage if you are looking for neutrality and transparency: anytime distortion is present, detail is obscured.

Normally push-pull transformer-coupled amps will have an increase in distortion at very low power, robbing them of that 'inner detail' detail that is part of the 'magic' that SETs are known for. OTLs, like SETs, produce less distortion as power is decreased, giving them a 'magical' quality that they share with SETs; good 'inner detail' and good low level resolution while being very smooth at the same time.

I apologize for the nutshell quality of this post- I'm glossing a lot of things over as this can be a detailed subject!
Thanks guys this is absolutely fascinating!
3 specific follow on questions:

1. Other things being equal, the choice then is deciding what hurts naturalness more... (a) the lushness or "smearing" that occurs as the tube interacts with the transformer in an SET, or (b) the fake neutrality coming from the even ordered harmonic cancellation of push-pull in an OTL. Correct?

2. I am not looking for a lot of power as my listening room is small. As such, the choice could likely be between, say, a CARY 300SE (15W per channel) as the SET contender, and the GRAAF GM20 (20 Watts per channel) as the OTL. Not that much difference in price...Theoretically which amp should sound more natural?

3. "its harder to make a practical low-powered OTL", why is this? I have a low powered OTL which sounds great:

Thanks again.
Just to add to the above, my speakers have a very flat impedance curve (ASA Monitor Pro). Looking forward to clarifications on my 3 questions. Thanks

I've owned both types of amplifier and, before you can describe either, you have to bear in mind that the sound of these amps are entirely speaker match dependent. So, assuming that we've got the right speaker match, I'll paraphrase Ralph;

SETs amps tend toward candlelight (warm & romantic)

OTLs amps tend toward sunlight (neutral & revealing)


Aarif, removing distortion never causes an amplifier to have fake neutrality.

OTLs have an economy of scale- the bigger you make them, the more efficient they can run. A big OTL that can make 200 watts will very likely be comfortable on 4 ohms, some smaller ones that make 150 watts might be able to do that also. A 30-watt OTL will not like 4 ohms much at all.

IOW the smaller you make the OTL, the higher you will want the load impedance to be. Many OTL headphone amps are designed for 32 ohms or more, plus they do not have to make more than 0.2 watts. Headphone amplifiers also are a special case because most headphones are designed to operate with amplifiers that have a much higher output impedance than their own impedance.

Without more information about your speakers, it would be hard to make a certain case for either amplifier technology, and we have to include your listening preferences somehow too. Although I have heard very good things about the Graaf, I've not actually heard it, so the following is my opinion only. I think it would have a chance at being a more neutral, musical presentation if the speaker impedance is linear and 7 or 8 ohms at least. I don't agree with some of the tube choices in that amplifier, as I feel that they tend to make the amp sound brighter, so I do have reservations at the same time.

In the end, like so many other things in audio, you will have to audition to know for sure. Remember- its all supposed to be fun :)
Is it possible that the reason SET tends to sound harmonically richer than a push-pull OTL amp is that the latter has a phase splitter that causes information to be lost?

After all, when we speak of 'SET vs OTL', what we mean is single-ended transformer-coupled vs. push-pull direct-coupled.

Ralph, I like your amps and have a lot of respect for you & your company, but to say that the <1% of 2nd-order harmonic added by a SET at moderate volumes on appropriate speakers, when studies have shown up to three times that amount is not audible, just doesn't make sense to me.

A good SET just gives me more 'chills of realism' than an OTL push-pull amp.

Mind you, I'm no musician and don't have perfect pitch.
I meant to say "but to say that the <1% of 2nd-order harmonic added by a SET at moderate volumes on appropriate speakers *is what causes this harmonic richness*, when studies have shown up to three times that amount is not audible, just doesn't make sense to me."
Paul's last question is one I share - does the phase splitter itself ever create
distortion issues (i.e. something like 'hysteresis') when the signal is re-

Second, I have always had questions about 'parallel' SET circuits, and
wondered whether or not one could, with the right tube, do a single-ended
OTL circuit which was 'accordion amplifier' circuit style (and if done right, use
more than two tubes). I expect this will be revealed to be a stupid question as
I am surely overlooking something obvious, but... I'll ask anyway... :^)

Has anyone ever used/made/heard a singled-ended 'accordion amplifier'
circuit? I read about this a while ago (TubeCAD Journal) but have never seen
anyone talk about one, though my gut is that this exists under another name
and I do not know about it.
When driven from a balanced source, I thought the dual differential Atma-Sphere circuit required no phase splitter?

regarding your point on fakeness and removing harmonic distortion... I understand that in an OTL / push pull amp, some of the harmonics will cancel-out showing lowered (measured) harmonic distortion. But if it is the even order harmonics that are being cancelled then the odd-ordered ones are left to dominate, the sound will be much worse/less natural even when measured harmonic distortion. Is this not an issue with OTLs?

As for SETs, the problem is (correct me pls) that even order dynamics can be accentuated by the tube's interaction with the transformer, making the sound more lush/"pleasing" but less natural/neutral.

Regarding my speakers they are Asa monitor pros (,
The monitors have a very flat impedence curve and are 8 ohms/ 89db.
They come with a pair of active subs that sort of make up on frequencies below 40hz (cut off point) for where my small OTL amp was lacking.
My listening room is not big and I usually listen to jazz.

Limited budgets mean that one usually buys used equipment from overseas, so it is often not possible to audition... (very hard to find used GM20 Graafs, but did see a Navison Audio OTL on Agon recently). Hence my question on whether a good SET (eg the Cary 300SE) would sound more natural than an OTL amp of similar price/quality.
Sorry, the sentence in first paragraph above should read:

"But if it is the even order harmonics that are being cancelled then the odd-ordered ones will be left to dominate, and the sound will be much worse/less natural even when measured harmonic distortion is lower"..

It is true that there are studies indicating that a 'natural spectrum' of harmonics - which includes even-ordered components - sounds better/more natural to our sound processing systems (brains) than the same signal with only the odd-ordered components - despite the fact that THD is obviously higher in the former case.

Re: "As for SETs, the problem is (correct me pls) that even order dynamics can be accentuated by the tube's interaction with the transformer, making the sound more lush/"pleasing" but less natural/neutral." I've never heard that.

I would say that good SETs on the appropriate speakers do *not* sound 'lush' - ever. They sound crystal-clear, just right, completely natural, etc. That's how a 45 sounds, for example. Or a good 211. Other tubes probably do have some intrinsic 'character'.

Of course, good push-pull OTL tube amps sound a lot like that too. They sound even leaner, or 'whitish', as I would describe it.

What's closer to reality might be impossible to say.
The harmonic structure of a good OTL will be the 3rd harmonic, which the only harmonic that the ear considers musical and thus does not object to. SETs have that and the 2nd harmonic.

Paul, what happens with these harmonics is not that they are not audible! It is that they are not **objectionable**. When GE did their experiments in this regard, they found that people will not *object* to nearly 40% of even-ordered content. That is not the same as 'you can't hear it', you certainly can, and audiophiles have terms for the presence of even orders (just like they do for odd orders) in small amounts. From small amounts to larger amounts: Warm, lush, rich, bloom, thick, murky, woolly, muddy. FWIW these terms refer to even ordered harmonics that are in greater supply than the terms that apply to odd ordered harmonics: hard, harsh, brittle, clinical, etc., where the odd-ordered enhancement may well be less than .001%.

With regards to the phase splitter- some OTLs that is an issue, others it is not. In our amps, there is no 'phase splitter' as a stand-alone circuit- that function is integrated into the single gain stage that exists in our amps.

Most SETs have more than one stage of gain- its my contention that if they were able to have only one, they would be a lot closer to the neutrality that our amps routinely express. I've got a 45 SET and a 2A3 SET- they sound great within their dynamic limitations, but they also sound veiled compared even to amps we were making 10 years ago, despite using the best parts available. As far as I have been able to experience, transformers always limit bandwidth, add distortion and rob the music of detail.

Given the example of the speaker above, I would recommend something with more power than the Graaf, or any SET for that matter. The speaker is simply too inefficient for an SET to strut its stuff. You would really need at least 10 more db for that. There are OTLs that can easily drive that load though. IMO 15 watts would not be enough power, unless you listen near-field and at moderate volumes.

Yes, there are studies that show even-ordered harmonics as musically consonant, but I do believe some of them or other also showed lower amounts - like 3% - to be inaudible.

It is very easy to keep THD down well, well below 3% - more like .1% or better - with copasetic speakers at moderate volumes. That's why I can't accept that the differences in HD spectra explain *everything* about why single-ended sounds different than push-pull.

As for the number of gain stages, most low-power SETs have one plus the output stage - just like your amps - right? I'm quite admittedly not an engineer but your OTLs do have two stages of gain counting the output tubes, right?

As for the lack of a phase splitter, that sounds like a plus, but please tell me, does crossover distortion still exist? I've always thought another of the big benefits of single-ended operation is that there is (obviously) no such thing as crossover distortion - because there's no 'crossover' - and distortion is *proportional to power output*.

Push-pull amps, all of them if I'm not mistaken, do suffer from the fact that crossover distortion is *inversely* proportionate to power, meaning the tiniest nuances are lost.

Not that your amps sound like they are missing nuance - they don't.

They also, incidentally, have some of the best low-bass control I've ever heard, even on 4 ohm speakers - an OTL myth shattered.
I do listen near-field and at moderate volumes and am looking to maximise neutrality.
It seems your advice Atmasphere would be to either get an SET with a single gain stage (who makes these pls??), or an OTL.
... and here is the famous SET that generates 150w per channel and a bandwidth of 20Hz-100,000 kHz. Too bad it costs $350,000...
The Wavac SH-833 :

I don't think there really is any standard for "neutrality," or any other subjective quality. What I would consider neutral (neutral what? -- high frequencies, low frequency, etc.?). I am not familiar with the Audiovalve RKV that you like, so it is hard to say what it will be closest to in terms of tonal balance, or any other quality.

I don't think that the theoretical discussions are that helpful either. Sure, Atmasphere is a designer of OTL amps and honestly believes that they are inherently superior (transformers are FAR from perfect in their conversion of high voltage/low current to low voltage/high current output, and arguably SET transformers are WORSE than pushpull transformers). The other camp can point to theoretical disadvantages of OTL vs. SET (need to use multiple output tubes in parallel for high output and sufficiently low output impedance, need for a servo circuit or blocking capacitor to prevent passing high voltage DC).

Also, there is quite a bit of difference between specific models for each type, plus the different way each model may interact with the specific speaker it is used with.

So, as a very rough generalization, I would say that almost all of the OTLs I've listened to (Graaf, Fourier, SAP, Atmasphere, Joule, custom designs) were exremely fast, dynamic and exciting (the Tenor was the exception). Once you've had your ears pricked up by an OTL, it is hard to listen to more listless and dull amps. I liked the tonal balance, so I would call it "neutral," but, I could see how someone else might find certain of these amps a bit lean. The "excitement" also comes with a bit of rough and tumble quality that someone might describe as crude or "not refined" (someone who wants a smoother sound).

The SET amps I've heard vary FAR more than the OTL amps I've heard. The better amps deliver a pure, natural sound, dense and complete harmonic structure, and natural sounding attack and decay of notes (the notes seem to bloom into space like they would in a real hall). But, depending on the design and type of tube, one can get SET amps that sound lean and fast and dynamic (within their restricted power output), and others that sound warm, sluggish (smooth ?) and woolly. "Neutral" or not, there is something seductive about good SET amps that is harder to describe (you know it if you hear it), while the special qualities of good OTLs are pretty evident.

I would put the Cary 300 SE you mentioned well into the warmer, smooth side. If you think the Graaf is neutral in balance, the Cary may not be for you. I actually liked the Cary even though it is radically different sounding compared to my SET amp, but only after the tubes had been swapped (the stock amp can be substantially changed in sound by tube selection).

I only heard the Graaf briefly, but, I recall being favorably impressed. It sounded similar to the SAP OTL I heard and like quite a bit. You should also seriously consider the 30 watt Atmasphere OTL. A friend had that amp and I thought it sounded terrific (he traded it in for something else because of noise -- a bigger issue in his system than it would be in almost anyone else's system because his speakers are 105 db/w efficient).

By the way, I've never heard the WAVAC, but, I heard an amazingly good Audionote Sogon amp (211 tube) that cost something like $250,000. I know someone who has two pairs of those amps. Crazy world.
Paulfolbrecht, since you asked- in our amps there is only one stage of gain, not two. Most SETs I've seen have either two or three. This is because there is usually some gain in the output, while in our amps there is none.

Crossover distortion, FWIW, is not a function of the 'phase splitter', it is a function of the push-pull operation of the output section. A number of crossover issues stem from output transformers, and some of them stem from class of operation. In a class A push-pull amp, carefully designed, you will not get any crossover distortion at all.

You are right in that if there are crossover issues, they can often appear at low power only, masking low level detail. This is something that simply does not happen in our amps, and it is easy to hear how they have plenty of low level detail, actually more than SETs running at similar power levels do.

You are also right in that harmonic spectra is not the defining issue regarding sound of OTLs and SETs. I should point out here though that most OTLs employ negative feedback, often fairly large amounts, while hardly any SETs use feedback. Our OTLs use little (1 or 2db) or none, so in a way they tend to have more in common with SETs sonically than they they do with a lot of push-pull amps.

Larryi, just for the record, an OTL that has a direct-coupled output does not have to have either a coupling cap or a servo circuit. They can be built to be so stable that the DC Offset can always be very small and only need adjustment occasionally, once every 3-6 months.

The multiple power tube issue is shared by SETs with multiple tubes, Push-pull with multiple tubes and of course transistor amps with multiple output devices. In all cases, it is possible to build the amplifier so the multiple output devices are not a defining characteristic.

I agree that multiple output tubes is really only a theoretical source of less than perfect sound (my SET has parallel output tubes). I am merely pointing out that "theoretical" concerns can be raised about any design choice.

A friend of mine has a direct-coupled OTL that has performed flawlessly for years. I would worry about some kind of catastrophic failure, such as an output tube shorting. Can that happen and is it a realistic concern? Wouldn't a servo circuit be the best approach to maintaining good sound while assuring safety?

Thanks, your posting are very informative.
I'd strongly disagree with Paul's comments re: SET amps not sounding "lush" vs OTLs.

While my earlier comments re: the "romantic" sound of SET amps were primarily intended to describe 300B SETs (and more specifically the Cary monos I own), I find some elements of this "lush" quality in all of the SET amps I've heard. I would specifically include the Cary 805s I also own, and more specifically include the 805s when used with the 211 output tube.

I have never gone the "flea power" route, so I can't comment on those, but I've heard 300B based amps from several manufacturers and ALL of them sounded warm tonally, so it ain't just Cary. As to OTLs sounding "whitish", I've heard that, too. My early production S-30s could sound this way when paired with the wrong speaker.

Incidentally, nothing wrong in my book with "lush" sounding amps. That's precisely the reason I own two pairs of SETs.

Martykl, no properly built single-ended 45 amp is lush in any sense into appropriate speakers.

Have you heard any? Which do you call lush?

As I noted in my post, I've never gone the really low power output route, so I wasn't commenting on any of the 45 tube designs. OTOH, you mentioned the 211 tube as well. I use this as the default tube in my Cary 805s specifically because the slightly "darker" top end (vs. the 845 output tube alternative) emphasizes the warmer tonal balance of the amp. When the 845 is in service in the Cary, it, too, is warmer/lusher than most amps I've used, but closer to neutral.

I'm sure that tonal balance of most SET amps varies with the impedence curve of the speaker being used, so my observations are qualified by that fact. In actual use, my SET amps have produced pretty consistent results, regardless of the speaker; deep bass is diminished, mid-bass performance is strong, midrange lovely, highs are airy but a bit rolled off. (To be honest, I no longer hear the roll off at the high end - probably one more 50th birthday gift). Dynamics are surprising, almost always exceeding expectations for the power output. However, IME (which -to be clear-excludes the 45 and similar very low output designs), SETs definitely tend to be warm and lush.
Are OTLs more accurate to the source? More linear across the frequency spectrum?
Martykl, if there is one maker of SETs that could be called LUSH, it's Cary. Even their 2A3s are lush, which I'd have thought impossible to do.

You're description of SET bass is especially off in regards to the 45, by the way: very, very tight low end, NO exaggerated mid-bass at all. In fact, they're rather like Ralph's OTLs in that aspect.

No triode is as clear and direct as the 45 IME. I may have mentioned the 211 in the same breath - it's still not nearly as see-through - not in amps I have experience with anyway. (Ongaku? Maybe. Cary? Even less than the Consonance 211s I owned.)

Like you said, 'lush' is not necessarily bad.

Ralph, I didn't realize your OTLs have only one gain stage. I learned something. I may need to try one again.
Paul, I like the type 45 tube a lot too. Not many speakers that will work with a 45 SET though. I am using a set of Coral Betas, which are fine as long as you don't expect any volume.

Larryi, We've been doing this over 30 years now and all our amps have had a direct coupled output. We've seen many tubes fail in that time. So far, we can still count the number of times that a power tube failure damaged a driver on one hand. In all the cases but one, alternative power tubes had been installed in the amp. So it appears that the amp is quite safe. All of our amps except the big 500-watt unit do not use a servo circuit (we installed one in the big amp, but for convenience only). They are quite stable, and simply don't need a servo. The key is controlling the power tubes, something that you would **think** would be common sense, but it is an issue in a lot of designs. That is one of the reasons why our amps have been so reliable over the years (the other being that the amp does not need negative feedback to work).

Gotta side with Paul on this one. My Wyetech Sapphire monoblocks use two 300Bs in parallel per side (18 wpc) and sound exactly like Paul describes: "crystal-clear, just right, completely natural", and certainly not lush. And these are 300Bs! And they drive monitors with a wild impedance swing as well.
Agree with Paul. The 45 is the triode gold standard.

Followed closely by 50, 2A3, and PX-25.

The 300B is a distant fourth and IMO, highly overrated.
I agree with Bill's ordering of the triodes except that I've never heard a 50.

300B can be done well (I learned recently I really like the JE Labs 76/6SN7/300B circuit) but bad implementations outnumber the good, it seems! The thickened midbass and blunted transients is a show-stopping for me.

I'd also throw the PX-4 in there either before or after the the -25. I had a Kurashima PX-4 - amazing amp.

People always talk about how much difference the circuit, driver, power supply, and output tranny make, yes the various tubes almost always retain their basic sonic character no matter what the amp IME.

However, I've heard those Wyetechs really break the 300B mold. I've also heard that about the Welborne DRD circuit.

As for volume and dynamics, yes, for a 45, you need a back horn, front horn setup, or highly efficient widebander on OB. But it's amazing how dynamic the good SETs are within their capabilities.

Any decent 300B SET will drive 90 dB/W dynamic speakers that are a fairly easy load very well. I'm thinking DeVore, Green Mountain.. lots of others.
Its the amp design,construction as much as the output tube thats responcible for sound quality. Having owned over 10 -45 SET. Probly arround 25 others SET many OTL SS PP etc. I can say that I did not find 45-50 SET to be the best in anyway. I look at total design not just tubes used and make dam sure it matchs loudspeakers or you SET yourself up for falure which is what most 45 2a3 owners do. Wavac, MasterSound, PAD, John Hogan have owned these 300b amps and they sounded much better than my 45 SETs and I have owned most of the great 45 SET amps that get raved about. With SET match your loudspeakers to amp or forget about it. Would say the same about OTL designs.
Given a speaker can be driven by an SET amp of comparable build quality/design the 45, 50, 2A3, and PX-25 will outperform a 300B all day long, due mostly to superior linearity. Don't even mention the 845 and 211.

I've heard a boatload of these amps with appropriate speakers and I stand by my previous post.
I can't argue with any of the above as I've never heard any amps built around those output tubes. I've always been concerned about speaker matches - as per Atmasphere's observation. Personally, I love most of the 300B designs I've heard and some of the best sound (though maybe not best deep bass) I've achieved came from 300Bs with slightly lean speakers

The original Merlin VSM SE with the Carys was a great match. I remember having a turntable set-up guy -from a very, very high end dealer that carried only SS electronics at the time - install a new arm. When he was done and we listened to the system, his eyebrows literally
arched up. He was stunned.

My point is that a good 300B set up with appropriate speakers can be very satisfying. Strictly neutral sounding amps have never IMHO been any guarantee of good sound. The right amp with the right speaker has always been the trick for me. After reading these comments, I'm sure I'll check out some of the lower power SETs you guys are so enthused over.

"Don't even mention the 845 and 211."

Don't mention the 845 and 211 because they are more linear or less linear than the 300B?
The Merlin needs a little more power than most SETs can produce, if you really want to hear what the speaker or the SET is capable of. With that speaker, hands down an OTL will do better.

I used to use the Cary 300B monos with the original VSM SE in a loft (2 parallel walls & 2 open sides terminated only by railings) that was app. 20' across and app. 12' from open railing (speakers) to open railing (listener). SPL capability was completely adequate for my needs and dynamics were just damn stunning.

I also used these speakers with an early production S-30. The combo worked well, but I preferred the 300B by a fair margin. The speakers sounded warmer with the Cary monos and -For My Taste- that delta easily trumped whatever benefits accrued to the S-30. I'm not suggesting that everyone would share my preference.

BTW, I auditioned the speakers with the S-30 and bought both speakers & amp, so you know that I really liked the combo a lot. It's just that, on direct comparison in my home, I preferred the Cary amps for these speakers. I also understand that later runs of the S-30 were pretty different sounding, so my opinion must also be qualified by that fact.


PS The speakers have since been modified to current spec by Merlin and now seem happier with more power. My space has also changed, so an "apples to apples" comparison isn't possible, but the modified VSMs sound warmer than pre-mod. Therefore, they don't benefit as much from the Cary's tonal signature and I've generally paired 'em with PP amps from Prima Luna using either KT66s or EL34s. In fact, I sometimes pair them with SS amps. At some point, I'll probably revisit your OTLs again since I find playing with these combos endlessly fascinating.
We all have personal tastes. Bill likes 45-50 as do I but I still stand by my post and Bill still using the mastersound you sold me;)
It's undeniable that every triode has its' own characteristics. A reputable manufacturer and/or experienced hobbyist knows this. Simply reviewing the electrical properties of each tube is sufficient proof.

Given a speaker is capable of being properly driven by a variety of SET amplifiers, the sonic signature of each triode is is clearly demonstrable.

To deny that is disingenuous.
Thanks Atmasphere for clearing up the issue of safety/reliability of direct coupled. A friend has direct coupled OTLs hooked up to Sonus Faber Stradivari speakers. When asked about what might happen if a tube shorted, he just shrugged his shoulder and said: "I guess you just buy another speaker."

On the which-low-powered-triode-is-my-favorite debate, I like 45s, they are sort of in between: not as dry and lean as the 2a3, not as warm, and lumpish as the 300B. 45s have a big low end, but that low end is also tight. The other nice thing is that a lot of decent older 45s can be had without spending crazy dollars. The PX25 is a nice tube too. I am not a big fan of 300Bs, but some amps using them can be quite nice, particularly when equipped with Kron 300Bs.
A question for the 45, 50 2A3 crowd:

What's a good speaker choice to evaluate the bottom end capability of a low powered SET like the 45/50/2A3.

Given that

A) every 300B SET amp I've heard (granted, I haven't heard all of them) lacks power & control < app 50hz when paired with a speaker with sufficient bottom end to reveal that shortcoming,


B) the way I reached the above conclusion is by pairing said speaker with a solid state amp and comparing the delta in bass response

It raises the question as to which low power SET friendly speakers go low enough in frequency to allow such a comparison.

In particular, if there's anything in reasonably wide distribution that not crazy expensive and fits this description, it would be great to know of. Similarly, if there's a particular low power SET amp that's specifically recommended for that speaker, such info would also be appreciated.

Thanks in advance.

Martykl, one thing about SETs that makes them quite different from OTLs is the way their distortion signature interacts with the human ear/brain system. SETs tend to have very low distortion at low power, but on transients, where the power occurs, the distortion can be quite a lot higher (10% at full power is quite common).

Thus there is high distortion on transients, and while normally the odd-ordered harmonics are kept at vanishingly low levels, at high power they have an effect. Of course, they are masked by the presence of even-ordered harmonics, but despite that the brain reacts to their presence anyway.

That reaction is a sense that the amplifier somehow is a lot more dynamic and can play louder than it should be able to given its actual power output. You commented about that quality in describing your amplifier above.

OTOH if the amplifier actually has the power to make real spls, and if the distortion is kept low, then the result is that the system is can be more relaxed- that the volume you pick is not based on perceived 'loudness'. IMO, a good system will not have a sense of volume or loudness- it should act that way real music does in that regard.

I know what you mean. I also wonder if the steep impedence spike of the VSMs (@ app 1Khz) is audible via SET and contributes to the sense of dynamics from the SET/VSM combo. Nevertheless, the cumulative effect of pairing this speaker with these amps was very, very satisfying in many (not every) way(s).

While I wouldn't particularly agree with your description of the dynamic behavior of a "good" system, I will acknowledge your point. There were times when the dynamics felt a bit more "juiced" than strictly natural, but with most of my recordings, I'd call this quality more realistic sounding. There were recordings, however, which would substantiate your view.

When I described OTLs as more "revealing" earlier in this post, that's partly what I was getting at. Shortcomings in speakers and source material was usually more evident via OTL. I know some folks find this the holy grail of this hobby, but I don't.

Marty, I understand what you are saying. I want the amp to play what is good in a recording, and with the poor recordings, not get upset by artifacts in the recordings. In other words, forgiving? -but without giving up anything to resolution. Its a fine line to tread.

Bobby P. has been making certain over the years that Merlins are tube and particularly OTL friendly. OTLs and Merlins are a very common and successful combination!

Couldn't have said it any better. I guess I found those early iterations of both the S-30 and the VSM SE slightly less "forgiving" than I'd prefer. Not "ruthless", or even "unkind", but maybe a touch "strict"?


PS I think my S-30 was one of the very first you guys manufactured. I was wondering which current Atmasphere model you typically pair with current VSMs? After this thread, I'm (uh-oh) starting to get that urge to expand the amp collection (again!). Let's see, we got 45s, 50s, 2a3s, OTLs,....
Hi Marty, most people with VSMs usually go with the M-60. They have enough power to really make the speaker sing.