There are two people in the world with whom I willingly speak on the phone on a regular basis: my daughter and SoundStage! Solo senior editor Brent Butterworth. My little girl makes the cut for obvious reasons. Brent, on the other hand, is my mentor and sounding board and one of my best friends, but since he lives on the left coast and I live in the armpit of Alabama, I get to see him once or twice a year at most. Hence the reliance on that damned infernal contraption.
At any rate, during one of our recent telephonic tête-à-têtes, Brent asked what I was working on. When I told him that I was deep into a review of Vincent Audio’s SV-500 integrated amplifier, his immediate response kinda said it all: “Are the tubes in that thing bullshit or what?”
To put your mind at ease, we agreed in short order that Vincent’s application of tubes is not B.S. But how? And why did the question even come up in the first place?
That last question answers itself. This is audio electronics we’re talking about, so a healthy dose of skepticism is warranted right up front. And since vacuum tubes are an older technology experiencing something of a resurgence in popularity in recent years, they’re the subject of all sorts of cockamamie hogswallop.
But let’s make one thing clear from the giddy-up, just in case it wasn’t clear already: I’m not saying that tubes are inherently flimflam. I’m merely saying that, as with most technologies, the application of tubes in audio electronics falls on a spectrum that runs the gamut from “legit” to “just quit.” But if you’re new to the hobby, or new to vacuum tubes in particular, it isn’t always easy to tell the difference.
Unfortunately, to get your bovine-excrement detector in good working order, you’re probably going to need to do a good bit of research about how tubes work, and that’s beyond the scope of this rant. The seriously condensed explanation, though, is this: at their most basic, the vacuum tubes (or valves) employed in audio electronics consist of three electrodes—the cathode, the control grid, and the anode—sealed in a glass tube with all of the gases removed. Hence the name “triode,” a term you may have heard used to describe the number of electrodes within this type of valve.
The way these tubes amplify an audio signal is pretty simple. Voltage is applied to a heater, which warms the cathode and causes it to emit electrons. Those electrons are drawn toward the anode (also called the collector by some manufacturers, although it’s more commonly called the plate) surrounding the cathode. In between the two is a control grid—a permeable electrode that, when negatively charged, prevents the flow of electrons from reaching the anode. In audio amplification applications, it’s the input signal that controls the grid. As a result, the signal that reaches the anode is an amplified version of the signal being fed to the grid.
You may have heard of some vacuum tubes being referred to as pentodes, which is effectively a triode with two additional electrodes—both grids that serve slightly different functions. But again, the above is really intended more as a glossary of terms, not a technical explanation of how tubes work. For that, I recommend EJ Jurich’s excellent book, Vacuum Tube Amplifier Basics (many thanks to Brent for the recommendation on that one).
The main thing I want you to get from the above description is that, due to the way tubes work, more powerful tubes are generally much larger tubes. And this is the first tool in your B.S.-detection toolbox.
To understand why, let’s look at one of the most popular tube amps for sale on Amazon.com, the Rockville BluTube, which promises to deliver 25Wpc into 8-ohm loads or 35Wpc into 4 ohms. Here’s the problem: that amp employs a pair of 6P1 tubes and a pair of 6N1 tubes, and although it would be difficult to calculate exactly how much power those valves would crank out without knowing some additional specifics, a bit of quick back-of-napkin scribbling tells me that this configuration of tubes would likely net you less than 10Wpc. And that’s being generous.
By the way, if that amp looks familiar even if you don’t know the brand Rockville, your eyes do not deceive you. This is actually a quite common amp design sold by a variety of brands under different names, with slight cosmetic differences. Monoprice sells what looks to be the exact same amp. So do Nobsound and INFI. All of them have minor variations—some simply to the chassis, some in the power supply. And all of them have a bogus input VU meter that serves absolutely no purpose other than decoration.
But here’s the biggest difference: all of those other brands legitimately advertise the amp as hybrid designs, with the tubes in the preamp section and a solid-state power amplification stage. Rockville, meanwhile, says in its product marketing on Amazon, “Tube Amps do not hard-clip even at high volumes and are extremely efficient. Due to this the amp sounds more like double the rated power!” And look, I’m not here to argue the relative merits of tube amps versus solid state. I’m merely saying that this assertion is a non sequitur in this case, as this amp is not using tubes to drive your speakers. This is akin to touting the benefits of petrol direct injection in an ad for the Tesla Model S.
Mind you, designing a hybrid amp that employs vacuum tubes in the line stage and solid-state circuitry in the power amplification stage is totally legit. It’s actually my preferred application of tubes, since it gives you the sonic characteristics of tube gear without all the operational and maintenance downsides. Tubes handle noise differently than solid-state circuitry. They generate distortion differently. You might dig that difference (if it’s even audible—oftentimes it isn’t).
But when used in high-power applications, tubes can be fragile, they require regular replacement, and they can be fussy about what kind of speakers they’re paired with. So, in my opinion, a product like the Vincent Audio SV-500—which uses tubes in the line stage, with solid-state circuitry employed in the power amplification stage—gives you the best of both worlds: whatever sonic characteristics the valves bring to the table combined with the reliability and flexibility of transistors.
So the fact that this amp sold by Rockville (and others) relies on tubes in the preamp stage and solid-state power amps isn’t what makes it suspect. It’s the fact that Rockville advertises it as a tube amp.
But hold on tight for just a second. A teardown of the Nobsound MS-10D MKII (again, pretty much the same amp as the Rockville BluTube et al.) by YouTube user Oleg Grounine gives pretty solid evidence that, at least in this iteration of the amp, the tubes aren’t even functionally involved in the line stage. The most charitable interpretation of all this is that one of the tubes functions as a tube buffer and nothing more.
If you’re not familiar with that term, basically what a buffer does is change the impedance of an electrical signal. But that’s not why tube buffers are generally employed these days in audio gear. They’re often used to add some of the flavor of tubes to the audio signal without actually involving tubes in the line stage or power amplification stage. They can also sometimes introduce a graceful roll-off of high frequencies, which I kinda dig.
Or sometimes they’re used just for show. I’m convinced that’s what was going on in the “tube” soundbars and powered speakers Samsung sold a few years back. And again, I’m not saying that’s entirely illegitimate, but if you’re buying a product with tubes because you want to experience the purported sonic advantages of tubes, and all you’re getting is a weak lightbulb on top, you’re being had. And that’s doubly true if the tubes are supported by an LED to compensate for the fact that the cathode isn’t receiving enough voltage to make it visibly glow.
Another thing to keep an eye out for if you’re shopping for a tube power amp (not a preamp, not a hybrid amp, but an amp or integrated amp that legitimately uses tubes to power your speakers) is the presence of separate speaker-level connections for low- and higher-impedance speakers. The reason for this is that traditional tube amps require output transformers, since they output a high-voltage, low-current signal that must be converted to a low-voltage, high-current signal to power your speakers. Since these output transformers are impedance specific, you’ll generally see separate speaker-level connections for nominal 4-ohm and 8-ohm loads. (This isn’t the case for output-transformerless—aka OTL—tube amps, but if you’re shopping around for one of those, the odds that you’re reading a remedial-level introduction to tube woo-woo are slim to whatever.)
By the way, those big boxes on the back of the aforementioned Rockville/Monoprice/Nobsound amp? They’re there to give you the impression that there are output transformers inside. They’re actually empty in most cases, although I think Monoprice’s design does move the power-supply transformer up into one of those boxes to give the impression that it’s there for a reason, which indicates that there might be something different going on with that amp’s design, even if it looks almost just like the others.
In short, though, if you find a $200 or $300 “tube” amp with only one pair of speaker binding posts that promises to deliver 25Wpc from four tiny valves, just know that it ain’t actually a tube amp and adjust your purchasing priorities accordingly. These amps may sound perfectly fine. I don’t know, since I haven’t auditioned any of them. But best I can tell, the tubes in them do little to nothing.
. . . Dennis Burger
dennisb@soundstagenetwork.com