The Triodino 4 is a stereo single-ended amplifier based on 300B tubes, created to overcome the limitations of the old Triodino 3. After numerous requests for an upgrade from users, I decided to revisit that project, which has been circulating online for years, and bring it to a more complete and mature level.

The Triodino 3 has been built and modified in countless ways, but always remained tied to an extremely simple circuit. This simplicity, while making it accessible, also introduces several practical and sonic limitations. With the Triodino 4, I wanted to preserve the character of the 300B while removing the most evident compromises, improving input sensitivity, power supply, bias management, and speaker control, without distorting the single-ended philosophy.

In essence, the Triodino 3 is just a 300B with its driver stage and little else. It works, of course, but it remains more of a sketch than a truly complete amplifier design. The 300B is an important, expensive, and prestigious tube, and in my opinion it makes little sense to use it in a deliberately minimal circuit where much better results could be achieved.

Main limitations of the Triodino 3:

1. It requires a preamplifier, because the input needs about 6Vpp of swing to be properly driven. Many modern sources, such as DACs, CD players, or phono preamps, have lower output levels, so the power stage may not reach full output without an additional gain stage.
2. The AC filament supply of the 300B, even when carefully balanced, almost always leaves some residual hum at the speaker. With low-sensitivity speakers it may go unnoticed, but with high-efficiency systems it becomes easily audible.
3. The self-bias configuration does not offer the same cleanliness and stability that can be achieved with a well-designed fixed bias. The cathode bypass capacitor becomes a critical component, indirectly influenced by the dynamic behavior of the stage, and the large cathode resistor dissipates unnecessary heat inside the chassis.
4. The damping developed by this zero feedback circuit is, optimistically speaking, around a factor of 2. This means the Triodino 3 can struggle with low-frequency control, especially with certain speakers. The result can be bloated bass, poorly controlled and not always faithful to the musical content.

I therefore took what I consider a draft and completed the work, arriving at a circuit that addresses all the main aspects. I did not want to cut corners on the important parts, because using a 300B in a stripped-down design makes no sense. The Triodino 4 is therefore a project without unnecessary compromises, designed to achieve a mature, quiet, stable, and truly usable single-ended 300B amplifier even with modern sources.

Below, in the premium schematics section, you can find the final stage, power supply, and servobias circuit diagrams.

The first major difference compared to the Triodino 3 is the input stage. In my design I used a modified mu-follower, consisting of a high-quality octal signal pentode loaded by a triode. This solution provides significantly better input sensitivity for real-world use, without forcing the user to add a preamplifier just to properly drive the power stage.

The Triodino 4 can be connected directly to an audio source such as a CD player, a DAC, or a phono preamp, without inserting additional active stages in the signal path. The fewer elements in the signal path, the better, as long as the circuit already provides the required gain. In this sense, the design can be considered a minimal integrated power amplifier, with its own volume control and sensitivity compatible with modern sources.

The 300B operates with fixed bias. Bias adjustment is handled by a servobias circuit that senses the output tube current through a small resistor placed under the cathode and automatically regulates the negative grid voltage. This circuit is not part of the audio signal path, it only works with DC voltages and does not interfere with the amplifier’s sound quality.

The advantage is very practical: the operating point of the 300B remains correct both at cold start and after full warm-up. In addition, when tubes are replaced, the user does not need to deal with a tester, screwdriver, trimmers, and potentiometers to adjust the bias. The circuit automatically brings and keeps the tube in the correct operating condition.

Compared to the self-bias solution used in the Triodino 3, the sonic advantage is significant. There is no reactive cathode capacitor in the output stage, no large resistor dissipating unnecessary heat, and no reliance on the quality of a bypass capacitor for such a delicate part of the circuit behavior. The benefits of a well-implemented fixed bias are well known to those experienced with these circuits.

The filament supply of the 300B is implemented in DC, filtered with a CRC cell, in order to drastically reduce residual hum at the speaker. In a single-ended amplifier with directly heated tubes, this aspect is fundamental, especially when used with high-efficiency speakers. For even better performance, a CLC cell could be used to achieve an even cleaner filament supply. The requested design includes the CRC solution, but small variations remain possible.

Since many people are still skeptical about things I have been saying for years, I included a circuit with switchable negative feedback. This way, those who want to listen with feedback can do so, while those who prefer to try a zero feedback configuration can disable it and judge with their own ears what actually works better.

The feedback has been set at a low level, without making the circuit overly dependent on the feedback loop. Many feedback-based amplifiers become excessively sensitive, noisy, or difficult to control when the feedback is removed. In this case, the circuit remains usable in both conditions: it requires about 3Vpp, that is 1.1Vrms, with feedback engaged, and about 2Vpp, that is 0.7Vrms, without NFB.

The goal is also educational: I want people to directly verify what I have been stating for years. A well-designed circuit does not sound worse simply because it uses a reasonable amount of feedback. On the contrary, it often results in tighter low frequencies, better damping, and more correct behavior with real speakers. Those who do not trust this can disable it and use the amplifier in zero feedback. I am quite confident that after trying both conditions, many will not go back. Also because my transformers are not like others.

But that is not all. The old Triodino 3 used a simple bridge rectifier. Some more dedicated builders modified it by using a tube rectifier, but I wanted to go further. To push audio quality to the maximum, I implemented a tube-based voltage regulator.

The regulator consists of a 6080 or 6AS7 used as a series element, driven by an ECC83. The 6080 is a tube widely used in audio, but originally designed specifically for this purpose: voltage regulation. Historical documentation confirms this: Dual power triode, ruggedized 6AS7G. Intended for use as series voltage regulator.

A practical note for anyone building this project: the ECC83 used in the regulator operates in a nearly static circuit. It does not carry the audio signal, and there is no need to use an expensive NOS ECC83 to achieve good performance. A good modern production ECC83 is more than sufficient. The money saved is better spent on high-quality capacitors, where the benefit is more tangible.

The regulated high voltage brings several advantages. The amplifier operates with a more stable voltage, less dependent on mains fluctuations and dynamic current demands. The audible result is greater clarity, better micro-detail, a more stable soundstage, and improved three-dimensionality. In a single-ended 300B amplifier, these aspects make a real difference, as the circuit is highly revealing and every power supply choice is audible.

Assembly by “R.”

“R” brought the amplifier to me for standard measurements, general verification, and final tuning. This kind of check is important because a project like this cannot be evaluated only by looking at the schematic. Wiring, component layout, grounding paths, transformer placement, and distributed capacitances can significantly influence the final result.

You can admire the FullMusic 300B tubes with mesh plates. For those unfamiliar, and who might think this is a red plate condition, it is not. Mesh 300Bs have a plate made as a metallic grid. The red glow you see is not the plate overheating, but the filament light passing through the mesh structure.

A small technical note: with the FullMusic tubes, the amplifier’s damping factor reached 6.0, while with the setup below, using Electro Harmonix 300B tubes, it settled at 4.4. Most likely, the FullMusic 300Bs are built with the plate closer to the cathode, resulting in lower internal resistance and therefore higher damping factor.

This is interesting because it shows how the output tube can influence not only the tonal character, but also measurable electrical parameters of the amplifier. When discussing 300Bs, people often focus only on brands, trends, or subjective preferences, but the internal geometry of the tube can measurably change the behavior of the output stage.

First build photos by “C.”

I had the opportunity to work on “R”‘s build for final tuning. During testing, I solved a startup stress issue affecting the 6080 by modifying the servobias. Now the 300Bs start in forced cutoff for about 30 seconds, giving time for the indirectly heated tubes to warm up. After this delay, the servobias gradually brings the output tubes into operation, avoiding unnecessary transients and improper startup conditions.

The issue arose because the 300Bs are directly heated tubes and start within seconds, while the other tubes require more time to stabilize. This mismatch could cause noise, hum, and unwanted overcurrent in the 6080 during startup. With the implemented modification, the amplifier starts in a more controlled and orderly way.

I also corrected a few wiring errors in “R”‘s build. In the output stage schematic, I indicated the color coding of the output transformer primary wires to avoid mistakes that could turn negative feedback into positive feedback. I also added a single resistor in the regulator schematic, improving overall circuit behavior.

Here are the measured results:

Power: 8.3Watt RMS per channel
Damping factor: 4.44
THD @ 1Watt: 0.38%
Bandwidth: 10Hz / 20kHz -1dB

I must say that “R”‘s build is not among the best in terms of layout and wiring. It is possible that distributed capacitances and some practical implementation choices slightly penalized the circuit performance, especially bandwidth. Nevertheless, the result is far from poor, in fact considering the type of build the measurements are more than respectable.

A single-ended 300B amplifier should not be judged only by maximum power. Here we have an amplifier delivering 8.3Watt RMS per channel, but with already low distortion at 1Watt and a damping factor higher than typically expected from a zero feedback 300B circuit. This means that, with suitable speakers, the amplifier can provide a much more controlled and credible listening experience than many deliberately simplistic single-ended designs built around a romanticized minimal circuit concept.

Harmonic spectrum

The harmonic spectrum shows an orderly behavior consistent with the nature of the circuit. Distortion is not absent, because a single-ended triode is not designed to chase extreme numbers like a solid-state amplifier, but the harmonic distribution is clean and does not show concerning artifacts. The THD value at 1Watt confirms that the circuit operates linearly in the most commonly used listening range.

Bandwidth on resistive load

The bandwidth on resistive load extends from 10Hz to 20kHz within -1dB. This is a very good result for a single-ended 300B, especially considering that the output transformer is always the most challenging component to design properly in this type of amplifier. The low-frequency response shows that the transformer is not undersized and that the circuit maintains good extension without artificially boosting the bass.

And on reactive load

The behavior on reactive load is even more interesting, because a real speaker is never a pure resistor. Many amplifiers that appear correct on a resistive bench load change character when driving a complex load. Here the circuit maintains a stable and controlled behavior, showing that the design was not created just to look good under ideal measurement conditions.

Square waves at 100Hz, 1kHz and 10kHz

Square waves provide an immediate view of the circuit’s transient response. At 100Hz, low-frequency transformer behavior is evaluated, at 1kHz the general cleanliness of the response is observed, while at 10kHz phase issues, overshoot, instability, or high-frequency limitations become evident. In this case, the behavior is consistent with a well-designed circuit, without anomalies or signs of instability.

Some readers expected to compare measurements with and without NFB, but unfortunately “R”, who now “knows”, chose not to implement the switchable feedback. It is a pity, because it would have been interesting to directly show the instrumental difference between the two conditions. Nevertheless, the circuit has been designed to operate in both modes, leaving the builder the freedom to choose.

So, beyond the measurements, how does this Triodino 4 sound? Below is a comment from Cristian, who named his unit Afrodite.

Hi Stefano

I am listening to the Triodino 4, which I named Afrodite.

I must say, the result is quite different from other 300B single-ended amplifiers I have listened to so far. Vocals remain the strong point of these tubes and the highs have a very fine texture.

What is surprising is the bass, which, with a damping factor only slightly above 4, is particularly well controlled and has impressive punch.

The driver tubes used are 6SJ7 Ken-Rad NOS with metal envelope and a 6SN7 GTA NOS Philips. The output tubes are ordinary E.H.

Great idea the servobias, which works very well, although I think it will not appeal to those who “must” adjust the bias every fifteen minutes 🙂

The result is very good in my opinion.

Cristian

This feedback perfectly confirms the purpose of the project. The Triodino 4 does not aim to erase the character of the 300B, but to allow it to operate under better conditions. Vocals remain the highlight, highs retain their fine and natural texture typical of good triodes, but the bass is no longer left uncontrolled. The higher damping factor, more stable power supply, and servo-assisted fixed bias result in a more authoritative, controlled, and mature sound.

In conclusion, the Triodino 4 is the answer for those who love the 300B but do not want to settle for the usual minimal circuit built around an expensive tube. It is a more complete, more refined, and more modern design, while remaining faithful to the single-ended philosophy. It is not an amplifier for those who want just a couple of components and plenty of forum mythology to feel satisfied, but for those who truly want to hear a 300B used properly, within a circuit worthy of the tube it employs.