Welcome to a journey through the fascinating world of electronic tubes, true gems of vintage technology that have left an indelible mark on the history of audio amplification. In this article, we will explore the EL84 family, one of the most iconic and versatile tubes ever produced. Since its introduction by Mullard in 1954, the EL84 has won over audiophiles with its excellent performance and the warm, enveloping sound it can deliver. We will not only examine the characteristics of the EL84, but also take a look at its variants such as UL84, PL84, and 7189, discovering how these tubes have influenced the landscape of amplification. In addition, we will make a surprising foray into the world of the EL86, a close relative of the EL84 that shares many of its distinctive characteristics. I am writing this article with the aim of dispelling the confusion that often surrounds the designations of these tube variants, offering clarity and understanding in the magical world of electronic tubes.
Confusion in Designations: Differences Between EL84, UL84, and Other Variants
In general, the first letter in a tube designation, such as the “E” in “EL84”, indicates the heater voltage, which in the specific case of the EL84 is 6.3 volts. According to the classical convention applied to all other tubes, one would expect a “UL84” to be essentially identical to the EL84, but with a different heater voltage. However, this expectation does not match reality.
When the UL84 was created, the goal was to develop a tube dedicated to radios in which all tubes had their heaters wired in series. These radios were generally powered by autotransformers and, for various reasons that I will not list here, required an audio output tube that operated at lower voltages than the EL84. As a result, not only was the heater modified to operate at 45 volts with a current of 100 mA, but the anode structure was also changed, making it smaller and suitable for operation at lower voltages, along with modifications to the screen grid. This led to the creation of a tube that is different from the EL84, breaking the tube nomenclature standards that were in place at the time.
Later, for various reasons, the different electrical characteristics of the UL84 also proved useful for other applications. Thus, the PL84 variant was created, intended for use in televisions with a 15-volt, 300 mA heater. Finally, a version with a 6.3-volt heater was desired, and it was named EL86. In summary, UL84 = PL84 = EL86 (with only different heaters), while these three tubes differ completely from the EL84 in all other respects.
The creation of the UL84, PL84, and EL86 variants added a layer of confusion to tube nomenclature, breaking with the previously followed standard. This departure from tradition generated uncertainty, as one would have expected tubes with similar names to be substantially equivalent, differing only in heater voltage. Interestingly, many DIY builders look for the EL86 without knowing that, apart from the heater voltage, it is the same tube as the UL84 and the PL84.
UL84 – PL84 and EL86 curves at 170 volts screen grid (also available in the datasheet)
UL84 – PL84 and EL86 curves connected as triode
6N43N-E, a little-known tube
The 6N43N-E, a Russian variant compatible with the EL86, is mainly distinguished by its internal construction: it is a beam tetrode rather than a pentode like the EL86. However, since the datasheet for this tube does not include curves with G2 biased at 170 V, which would be necessary for a direct comparison with the EL86, the curves were acquired using a u-tracer and published for this purpose.
G2 +100 V
G2 +170 V
6N43N-E connected as triode
When the EL84 Becomes a Killer: The Truth About the 7189
The 7189 represents a variant of the EL84, but with higher plate and screen grid voltage ratings compared to the standard EL84. This configuration allows the 7189 to deliver higher output power, making it a common choice in many hi-fi amplifier applications in the late 1950s and early 1960s.
It is important to emphasize that, despite the similarity between the two tube types, the 7189 is not equivalent to the EL84. This distinction is crucial to avoid installation errors and damage to equipment. Some sellers on platforms such as eBay incorrectly advertise the 7189 as equivalent to the EL84, leading buyers to install the latter in devices designed for the 7189. This confusion is so widespread that most online sellers, with the exception of a few reliable ones, tend to present them as interchangeable and offer them as if they were identical. This practice can unfortunately mislead buyers, sometimes unknowingly, resulting in situations that amount to fraud.
There are significant differences between EL84 (or 6BQ5) tubes and the 7189. The maximum anode voltage of the EL84 is 300 V, while the 7189 can withstand higher anode voltages, typically up to 440 V. The 7189 is a reinforced version of the EL84 capable of handling higher voltages, thus allowing greater output power. However, it is important to note that these two tubes are not equivalent. An EL84 installed in a circuit designed for the 7189 would be subjected to anode and screen grid voltages beyond its operational limits. This scenario often results in overcurrent conditions or tube failure, which can easily damage the primary winding of the output transformer. Let us look at the photo below:
In the center is an EL84, operating at an anode voltage of 300 volts. On the right is a UL84, a version with a 45-volt heater designed to operate with a maximum anode voltage of 170 volts. It is easy to see that the anode structure of the UL84 is narrower than that of the EL84. Finally, on the left is a 7189A, highlighted by its massive anode structure, which is significantly larger than that of the EL84.
Small equivalence table:
EL84 = 6BQ5 = 6N14N
7189 / 7189A = 6BQ5B
Until recently, 7189 tubes were mainly available as NOS (New Old Stock) items, often at prohibitive prices. However, the good news is that today some manufacturers have started reproducing them, providing an affordable replacement option. This is positive news for owners of vintage amplifiers, as people often tend to replace 7189 tubes with EL84s, with disastrous consequences. In the photo below, a quartet of 7189 tubes from my current favorite manufacturer, Thung-Sol.
Considerations on using EL84 tubes connected as triodes
In the world of tubes, the EL84 and its variants have always attracted great interest for their versatile use in many audio circuits. However, when it comes to using the EL84 connected as a triode, there are some important considerations to keep in mind. While the various EL86 tubes connected as triodes show well-defined and usable curves, the EL84 connected as a triode presents a significant challenge. In an illustrative graph, one can observe a very steep slope in its curves, compromising its suitability for this type of configuration.
Optimistically, even with a dissipation of 12 watts, it is possible to obtain less than half a watt from an EL84 connected as a triode, with a transfer efficiency below 5%. This makes it unsuitable for such applications. It is important to stress that misleading information is not uncommon on the market, with some experts promising EL84 triode amplifiers capable of producing tens of watts. In reality, practical experience often shows a real output of only 0.39 watts RMS in some cases.
In modern Hi-Fi systems it is quite common to see people wanting to “warm up” the sound of their setup, especially when it is solid-state. Others, who already own tube equipment, look for a way to further refine its performance. The most common answer? Add a tube preamplifier. Unfortunately, in most cases this solution is not only useless, but ends up creating more problems than benefits.
Modern sources and excessive gain
Today’s sources, CD players, DACs, streamers, already provide a generous output level, often higher than 2 volts RMS, more than enough to directly drive a power amplifier. Inserting a preamplifier into this chain almost always means unnecessarily increasing the overall system gain. This leads to several issues:
Difficult and overly sensitive volume control
Channel imbalance caused by standard potentiometers, often inaccurate at low listening levels
Hiss, hum, and background noise
Constant need for “tube rolling” in search of quieter tubes, even when the installed ones are perfectly fine
In short, the beloved preamp turns into a source of complications. I have seen countless cases of people struggling with unexplained hum and noise, when simply removing the preamplifier from the chain made every problem disappear. The only ones who truly benefit are the usual “gurus” ready to sell gold-and-diamond stepped attenuators or magical devices to eliminate noise.
The misconception of tube buffers
The most popular alternative among low-cost Chinese products is the tube buffer: dual triodes in cathode follower configuration, therefore with total feedback, which do not amplify anything and merely “pass” the signal… but with a glowing tube inside, which looks impressive. Often powered at low voltage, these circuits do not add any real “tube sound”: at best they introduce a slight dullness and some poorly defined distortion. They are simple solutions, but fundamentally useless, giving the impression of doing something while in reality doing very little.
The real alternative: tube audio processors
If we truly want to add tube character to our system, we need a different approach: the tube audio processor.
What really makes the sound “tubey”? It is not enough to have just any tube glowing inside the chassis. You need a tube that actually amplifies, that generates the correct harmonic signature, and above all a transformer, a fundamental component to round off the response and provide that “openness” so appreciated in tube sound.
The combination tube + transformer is what truly brings to life the warm, three-dimensional sound many listeners are looking for. And there is more: thanks to the transformer, it is possible to bring the output signal level back down to its original value, thus avoiding the useless gain increase typical of traditional preamplifiers.
What we obtain is not a preamplifier, not a buffer, but a true signal processor in class A, capable of imprinting the full, elegant, and harmonic tube signature onto the sound, without total feedback, without excessive gain, and without unnecessary complications.
But like all good things, this also comes at a price. A true tube audio processor is not just a tube thrown in there and poorly powered, where the signal goes in and comes straight out. No: it requires at least two well-designed output transformers, and careful construction to prevent them from picking up 50 Hz hum or other environmental noise. It requires a serious power supply, proper shielding, and a well-thought-out layout. It is a solution that, yes, has a higher cost, but it is also the only one capable of truly delivering authentic tube sound, the kind that enhances the system without compromising dynamics or introducing problems.
Technical demonstration
Below you can see an LTSpice simulation that illustrates the concept in detail: how a simple tube in an amplifying configuration, coupled to a well-designed transformer, can shape the harmonic content of the signal in an elegant and musical way, without altering the output level or introducing noise.
The screenshot below shows the stage of a tube audio processor based on the VT-33 wired as a triode, the same circuit that forms the basis of the Baryon Omega project. The circuit operation is simple yet effective: the signal enters the grid of the VT-33, is amplified, and transferred to the output transformer, which features a primary of 8500 ohm and a secondary of 820 ohm. In green you can see the input signal, in blue the output signal, slightly larger but already enriched by the second harmonic and a barely noticeable third harmonic, as shown in the following screenshot.
This technique is applicable to many tubes, provided they do not have excessively high internal resistance, a condition that would make the design of a suitable interstage transformer difficult or even impractical. The site already features two ready-to-use tube audio processor projects:
the Neutrino, based on 6SN7 / 6J5 tubes and all their equivalents,
and the Baryon Omega, built around the VT-33 wired as a triode.
Anyone interested in building a custom processor, or in obtaining transformers tailored to a specific tube, can contact me directly. We will evaluate together the best solution to integrate true tube sound into your system.
This article is a double feature: on one side I propose some simple but effective modifications to a small Chinese single-ended amplifier based on 6L6G and ECC83 tubes, and on the other I use this very unit as a test bench to tackle one of the most discussed, and most often misunderstood, topics in the world of tube high fidelity: negative feedback.
I chose this inexpensive amplifier as a base to demonstrate, with instrumental measurements and real audio recordings that you can listen to on headphones, what actually happens when negative feedback is switched in or out of a properly designed circuit. The result is clear: when applied with the right amount, negative feedback does not degrade the signal at all. On the contrary, it faithfully preserves the original content, introducing only minimal variations. Conversely, in zero-feedback mode, the changes in sound are obvious, especially in the low end and overall response, to the point where the difference is clearly recognizable by ear.
This does not mean that tube technology is useless or outdated. Quite the opposite: the beauty of the tube world is precisely that you can experiment, build, and shape your own sound according to personal taste, choosing one tube rather than another, one capacitor instead of another, and chasing those nuances that make every project unique. However, it is important to put an end to certain false myths that keep getting repeated without any objective verification, such as the idea that negative feedback would “erase” information while zero feedback would preserve it. In this test, with the same circuit and the same test conditions, it becomes clear that negative feedback tends to preserve the integrity of the signal better, while the zero-feedback configuration introduces additional content that is not present in the original.
By using low amounts of negative feedback (without ever overdoing it) and playing with the choice of tubes, transformers, and components, you can achieve listening experiences that are truly interesting, fun both from a didactic and a musical point of view. It is fertile ground for those who love experimenting, understanding, and shaping sound according to their own tastes.
However, I am convinced these experiments would work even better if they were approached with the awareness of deliberately altering the sound in a pleasing way, to achieve a “guilty-pleasure” effect that is personal and even creative, rather than being stuck with the belief of chasing absolute fidelity, while in reality you are modifying the signal without realizing it. Knowing how to distinguish the pursuit of fidelity from the pursuit of sonic character is essential. Only then does DIY become a form of conscious expression, and not an illusion fueled by distorted technical myths. This article aims to be a practical and transparent contribution, dedicated to those who listen with their ears and measure with their brain, not with prejudice.
Over the years, I have always shared with enthusiasm the experience and knowledge I have gained in building amplifiers and audio transformers. I have often expressed critical opinions about some common practices in the field, but I also understand that, given my direct involvement, someone might think I am simply promoting my personal ideas.
While browsing the web and listening to podcasts, I came across people trying to refute my statements, perhaps in response to the doubts raised by my articles. It is well known that it is often easier to convince people with superficial arguments than with in-depth analysis. Many sellers and TV-style pitchmen ride the wave of popular credulity, claiming that “zero feedback” amplifiers are absolutely superior. They also claim that damping is not that important, and that annoying bass is just an excuse to sell special loudspeakers (often very expensive) marketed as the only solution to a problem they themselves created. It is the classic case of selling the disease first and then the cure.
It is natural for the reader to feel disoriented when faced with contrasting opinions, especially when there is no chance to compare solutions directly. In the world of tube amplifiers, where personal taste can vary enormously, there is no single universal answer. What one person likes may not please another at all. Many factors influence the listening experience, such as the environment, individual preferences, and system matching with other audio components. Despite the frustration of not having absolute certainties, it is important to keep an open and curious mindset, carefully evaluating your needs, tastes, and budget.
Unfortunately, enjoying something does not always guarantee the highest sound quality. Often you are satisfied simply because you have not had the chance to hear something better. It can happen that one day you come across a higher-quality system, and only then you realize that previously you had never truly listened in an optimal way.
I am reminded of the experience of a customer of mine who, after years of listening to a Beatles record, had the opportunity to hear it through a higher-quality system. Only then did he discover details and sonic nuances he had never perceived before, even though he had already used prestigious amplifiers. This example clearly shows that there are quality levels that can be reached only with truly high-end equipment.
Sadly, some dealers push enthusiasts into a continuous cycle of buying and replacing equipment, fueling constant dissatisfaction. Many audiophiles even evaluate an amplifier more for its resale value than for its actual sound quality. This is a wrong and counterproductive approach. I have seen people replace excellent-quality amplifiers with products that have a disappointing sonic impact but are highly rated by the specialist press. I believe that those who behave this way have not really understood what high-quality audio means, and might not even be able to tell the sound of a fine amplifier from that of a simple intercom.
It is essential to pursue a personal listening experience, do research, compare options, and, if possible, rely on industry professionals who have deep and impartial expertise. Only in this way can you make an informed choice and obtain an amplifier that delivers a truly satisfying listening experience, beyond commercial trends.
I find it sad that some people buy audio gear just to show off their wealth. Often what gets advertised as excellent ends up disappointing expectations. There are expensive, famous products that deliver mediocre performance, just as there are cheaper, lesser-known products capable of excellent results.
This article aims to be an honest contribution, an invitation to personal reflection. It is intended for DIY builders as a fun didactic experiment that still offers a satisfying sonic result. I do not promise perfect sound, but I do guarantee an enjoyable and instructive experience. My goal remains to offer an open and transparent view of the audio world, so that everyone can discover and appreciate sound according to their own preferences and possibilities.
The Nobsound 6p3p + 6n1 (6L6G + ECC83)
The basis of this experiment is a small Chinese amplifier, with virtually no real name. You can easily find it on various online shopping sites, available both assembled and as a kit. It is easily recognizable from the photos, sold under the Nobsound brand, under other names, or even unbranded. It is probably one of the cheapest options available on the market.
This is the original circuit schematic:
Unfortunately, the original output transformers of this amplifier have an impedance of 3500 ohm, which is not suitable for use with a 6L6GB (or equivalent) tube. During bench tests I was getting limited power, about 3.5 watt, with a very high distortion level: in practice, one half-wave was reproduced well while the other one was heavily compromised.
To improve the situation, I disassembled the original transformers. Since I could not replace them with larger transformers built according to my method (because there was not enough space), I chose to rewind the original transformers using the same bobbin and laminations of identical size, but I replaced the original simple silicon-steel laminations with grain-oriented (GO) laminations. I therefore built transformers with an impedance of 4500 ohm, ideal for the 6L6GB in single-ended configuration, with another specific goal in mind.
In several previous articles I have already pointed out that many zero-feedback amplifiers, including some from prestigious brands, use output transformers with insufficient primary inductance or cores close to saturation. This approach is used to mask the most obvious defects caused by the absence of negative feedback, especially those famous “boomy” and annoying bass frequencies. It is clear that, if you choose the zero-feedback route and categorically refuse negative feedback, those problematic bass issues must be eliminated somehow. The most common solutions are filtering the amplifier input, using intentionally bandwidth-limited output transformers, or pairing the amplifier with loudspeakers that are naturally weak in the low frequencies (for example single-driver speakers or open-baffle speakers).
As you can see, I included a switch that allows the negative feedback (NFB) signal to be disabled at will. I replaced the 6N1 tube with an ECC83, a necessary choice to achieve an adequate damping factor while keeping the output stage an easy load to drive. The original rectifier, a 5Z4, was also replaced with a GZ34, which is essential to recover usable voltage: the stock Chinese transformer, in fact, tended to have more voltage sag than expected.
Note: this schematic is intended for tubes of the 6L6 / 6L6G / 6L6GA / 6L6GB / 5881 type, and also the 6p3p. The 6L6GC and 6N3C-e are excluded, as they require a higher screen-grid voltage. However, it is possible to adapt the amplifier to those tubes by changing the value of the 12k resistor in series with the screen grid to a lower value, until the desired operating point is achieved. Always verify plate and dissipation values under operating conditions, because screen current can vary significantly from tube to tube. I am not indicating a precise value because the idle current of the screen grids is often unpredictable and may deviate from datasheet values. Let’s now move on to the assembly steps:
For anyone curious about how I managed to solder tin onto stainless steel, I recommend watching this YouTube video: https://youtu.be/SHxo5tcNNMg. The procedure shown is identical to the one I used, with the only difference being that I used ready-made ferric chloride instead of preparing the solution from scratch. Without this galvanic treatment (which I carried out with the help of my modified power supply), soldering on stainless would be virtually impossible. Of course, a flat-tip soldering iron (screwdriver type) of at least 150 watt is also required.
Let’s now move on to the usual instrumental measurements. I want to clarify a point that is often a source of confusion: some people claim that, in a circuit without negative feedback (zero feedback), total harmonic distortion (THD) increases progressively from minimum power up to maximum power. Conversely, according to the same theory, in a feedback circuit THD would be higher at very low volume, then decrease at mid volumes, and rise again as clipping is approached.
This behavior, more similar to what you would expect from a class B output stage than from a single-ended amp, is sometimes used as an argument against using negative feedback in systems intended for low-volume listening. The idea is that, at low power levels, a feedback circuit would distort more than one without feedback.
To verify this claim in a concrete way, let’s look at the THD results measured in the two configurations (zero feedback and negative feedback enabled), keeping the rest of the circuit unchanged and comparing several power levels, from 0.1 watt up to just below clipping. We will compare the values to understand what actually happens.
With negative feedback enabled
Zero Feedback
0.1 watt RMS – (0.18%)
0.1 watt RMS – 0.85%
0.5 watt RMS – 0.47%
0.5 watt RMS – 2.45%
1 watt RMS – 0.73%
1 watt RMS – 3.6%
3 watt RMS – 1.31%
3 watt RMS – 6.2%
Just below clipping (5 watt) – 2.35%
Just below clipping (4 watt) – 8.4%
As you can see, distortion increases progressively from a minimum value up to the clipping threshold, both in the zero-feedback configuration and in the feedback configuration. However, distortion is always higher in the version without negative feedback, even at the minimum power level measured.
For transparency, I left the schematic and the necessary references to replicate the test. Anyone can build the circuit and repeat the measurements, using transformers made by me or by any supplier of their choice. In this case it is a classic 22×30 core with a 4500 ohm primary and 4 and 8 ohm secondaries, without any particular ambitions.
The maximum output power of the circuit with negative feedback enabled is about 6 watt, with a noticeable distortion threshold starting around 5 watt. The damping factor is 0.3 in zero-feedback mode and reaches 9 with negative feedback enabled. The bandwidth, in the zero-feedback configuration, is 40 Hz – 40 kHz at -1 dB. It should be noted that the transformer was specifically designed with a low-frequency roll-off, a mandatory choice given the small core size. With negative feedback enabled, the frequency response extends from 10 Hz (at -0.6 dB) up to 40 kHz (-1 dB), offering a noticeably wider overall response. Below I show the comparative plots… and some extra content.
Bandwidth with negative feedback (resistive load)
Bandwidth with zero feedback (resistive load)
Bandwidth with negative feedback (reactive load) DF = 9
Bandwidth with zero feedback (reactive load) DF = 0.3
In general, there is little awareness of what a low damping factor actually implies. Beyond the usual descriptions, such as the speaker cone that, once pushed, keeps moving by inertia instead of faithfully following the signal, there is a lack of concrete understanding of the effects on the amplifier’s real-world behavior during use.
Everyone seems to focus exclusively on harmonic distortion and its components, while few people bother to observe what happens when a low-damping amplifier actually drives a loudspeaker. For this reason, I made two short clips showing the oscilloscope screen, with the amplifier connected to a speaker and driven by a function generator.
Warning! The following videos contain steady sine tones. It is strongly recommended to turn the volume down to minimum before playback, especially if you are using headphones. These sounds may be annoying or even harmful to your hearing, as well as to headphones, computer speakers, or smartphone speakers. Audio is nevertheless essential to correctly understand the phenomenon being analyzed. I assume no responsibility for any damage to audio devices or hearing resulting from watching these videos. Proceed with caution.
Let’s start by analyzing the situation without negative feedback (Zero Feedback). On the oscilloscope screen two traces are visible: the lower trace represents the signal generated by the function generator, while the upper one shows the signal actually present at the speaker terminals.
As you can clearly see, while the amplitude of the lower trace (the generator signal) remains constant, the upper one, that is the amplifier output signal, varies significantly as the frequency changes. It is behavior that resembles a tone control built into the amplifier, with the difference that in this case the points of maximum and minimum response are determined by the loudspeaker’s characteristics.
What you see in the plots for the reactive load is exactly this phenomenon: a frequency response altered by low damping. To those who only talk about harmonic distortion, I ask a question: this is not harmonic distortion, but it is still a distortion. How should we classify it? Now let’s see what happens in the same situation, but with negative feedback enabled and a damping factor of 9.
In this configuration, with negative feedback enabled, you can see that the output signal amplitude shows only minimal, almost negligible variations. As I have stated in other contexts, I believe a damping factor of 5 is optimal for small-power amplifiers such as single-ended designs. In higher-power equipment you can safely go up to 10. Exceeding these values, in my opinion, rarely brings benefits in this kind of application: too much negative feedback can reduce subjective listening pleasure and make the result feel less natural.
Now let’s move on to another extra: a direct comparison between the sound of the feedback circuit and that of the zero-feedback configuration. How did I do this? I connected an attenuator directly to the speaker terminals and sent the attenuated signal to the line input of an old laptop, which I normally use to drive a laser engraver.
I apologize for the recording quality: the laptop is almost 10 years old and I paid 40 €, so some background noise from its sound card is inevitable. Despite that, the sonic differences between the two configurations are still audible.
It should be said that a more accurate demonstration would have required a binaural microphone, capable of capturing the distortion physically introduced by the loudspeaker, and not only the electrical component. Finally, I add that, since I used transformers with a small core and a primary calculated with modest inductance, the amplifier does not show the typical boomy and uncontrolled bass even in zero-feedback mode.
To begin, I chose a royalty-free audio track titled “At First Sight” by FiftySounds, downloaded from EpidemicSound.com. You can listen to the original file at the provided link.
Please note: to fully catch the differences and sonic details, it is essential to listen with headphones.
I played the track from my media center, connected directly to the Nobsound, and I recorded the signal taken from the speaker terminals in both configurations (zero feedback and negative feedback enabled), trying to keep the level as comparable as possible between the two tests.
Then, using an audio editing program (Audacity), I overlaid a segment of the original recording with the same passage filtered through the amplifier. Thanks to the software’s “solo” function, during playback I can quickly switch from one version to the other, allowing you to clearly hear the differences in real time. Let’s start with the comparison between the original track and the amplifier output in zero-feedback mode.
In the audio view, the upper track is the original file, while the lower one is the recording of the amplifier in zero-feedback configuration. By ear, you can clearly notice a loss of body in the low frequencies and a certain dullness in the highs, along with an emphasis in the midrange. Also, even if the electrical recording already gives you an idea, it is in the real room that you can perceive how the zero-feedback playback sounds “dirtier”.
Now let’s move on to the recording with negative feedback enabled. At this point, all the skeptics who believe the usual urban legends will expect an unpleasant sound: metallic, harsh, like broken glass, nails on a chalkboard, or that infamous fork-on-a-plate screech. Let’s see (and hear) whether that is really the case.
As in the previous test, the upper track is the original audio, while the lower one is the recording of the signal after passing through the amplifier, this time with negative feedback enabled.
And here is a concrete demonstration: in this case, with proper use of negative feedback, the signal is not degraded in any appreciable way. On the contrary, the zero-feedback version sounds worse. Those who claim to prefer zero feedback, in reality, are chasing a certain type of distortion, and they do not always admit it.
The recording with negative feedback is so faithful to the original that it is almost indistinguishable, if it were not for a slight background noise and a small loss of brilliance, which is also present in the zero-feedback version. These limitations are due exclusively to the built-in, far-from-professional sound card of the old laptop used for recording. To wrap up, I am providing a download of the two full recordings. In the future, if I have the chance to repeat similar experiments with more serious equipment (see binaural microphone), you can be sure I will do it. Click here to download the zip with the two files in flac format
Hi, a pentode distorts more than a triode without feedback. Also, 5 watts is a small amount of power, distortion in single ended amplifiers increases as output power rises, so if you want low distortion at low levels you also need plenty of headroom. 3.6% at 1 watt from a 5 watt pentode on cheap transformers with a small core, without feedback, is quite normal.
As for what they say about the capacitors, that is not true. I used 220 uF and 470 uF, not 10 uF. The theory on that website is wrong on many points.
Let us start from the fact that a class A stage has a constant average current draw, so there is no bias drift. If the capacitor is too small, at low frequencies it will not be able to hold the charge, you will get partial local degeneration under the cathode, with loss of tube gain only at low frequencies. This means bass roll off, but also a phase shift introduced by the reactance of the capacitor, which always lags the signal and introduces distortion, and that is not good. Saying that using too large a capacitor creates problems is simply nonsense. The larger the capacitor, the more the voltage movement under the cathode is reduced, until it becomes completely negligible. It works like a high pass filter.
Of course you cannot use an excessively large capacitor under a cathode because at power on the tube has to charge it, and for a certain time it is forced to deliver a large current that could stress it. You can also simulate the circuits with LTspice to observe how the low frequency response shifts when you change the values of the cathode bypass capacitors.
I also read the article. 10 uF is far too small to hold the cathode steady in a hi-fi power tube stage, but that article was written with a guitar amplifier in mind. Guitar players do not want to hear deep bass and they actually like various forms of distortion, so his evaluations of “how it sounds” are probably from a guitarist’s point of view. In fact, I have built guitar amplifiers too, and that is probably something I would also do to better color the sound, but for a guitar amp, not for hi-fi.
That was interesting… thank you, I like your switchable recording idea, but I have to ask, why does a 5 watt amplifier with no feedback have 3.6% distortion at only 1 watt, that seems a lot isn’t it, or is that normal for pentode ?
” ….. However, in order to maximize gain and keep the phase shift associated with the cathode bypass capacitor to a minimum, the capacitor value should be increased to around twice this value, or 10uF. If the capacitor is made too large, however, the stage will suffer from poor transient response, as the cathode voltage will slowly move up and down, causing the plate current, and thus, the plate voltage, to also shift up and down. This can result in an audible distortion and nonlinearities in response to a large transient input. For this reason, the capacitor should be made no larger than necessary.”
Hi, a pentode distorts more than a triode without feedback. Also, 5 watts is a small amount of power, distortion in single ended amplifiers increases as output power rises, so if you want low distortion at low levels you also need plenty of headroom. 3.6% at 1 watt from a 5 watt pentode on cheap transformers with a small core, without feedback, is quite normal.
As for what they say about the capacitors, that is not true. I used 220 uF and 470 uF, not 10 uF. The theory on that website is wrong on many points.
Let us start from the fact that a class A stage has a constant average current draw, so there is no bias drift. If the capacitor is too small, at low frequencies it will not be able to hold the charge, you will get partial local degeneration under the cathode, with loss of tube gain only at low frequencies. This means bass roll off, but also a phase shift introduced by the reactance of the capacitor, which always lags the signal and introduces distortion, and that is not good. Saying that using too large a capacitor creates problems is simply nonsense. The larger the capacitor, the more the voltage movement under the cathode is reduced, until it becomes completely negligible. It works like a high pass filter.
Of course you cannot use an excessively large capacitor under a cathode because at power on the tube has to charge it, and for a certain time it is forced to deliver a large current that could stress it. You can also simulate the circuits with LTspice to observe how the low frequency response shifts when you change the values of the cathode bypass capacitors.
I also read the article. 10 uF is far too small to hold the cathode steady in a hi-fi power tube stage, but that article was written with a guitar amplifier in mind. Guitar players do not want to hear deep bass and they actually like various forms of distortion, so his evaluations of “how it sounds” are probably from a guitarist’s point of view. In fact, I have built guitar amplifiers too, and that is probably something I would also do to better color the sound, but for a guitar amp, not for hi-fi.
That was interesting… thank you, I like your switchable recording idea, but I have to ask, why does a 5 watt amplifier with no feedback have 3.6% distortion at only 1 watt, that seems a lot isn’t it, or is that normal for pentode ?
Also I read about cathode bypass capacitors, on https://www.aikenamps.com/index.php/designing-single-stage-inverting-feedback-amplifiers
” ….. However, in order to maximize gain and keep the phase shift associated with the cathode bypass capacitor to a minimum, the capacitor value should be increased to around twice this value, or 10uF. If the capacitor is made too large, however, the stage will suffer from poor transient response, as the cathode voltage will slowly move up and down, causing the plate current, and thus, the plate voltage, to also shift up and down. This can result in an audible distortion and nonlinearities in response to a large transient input. For this reason, the capacitor should be made no larger than necessary.”