The project I present today is yet another amplifier based on the famous 300B tube, but with a different approach than usual. This is not the classic “showcase story”, it is a complete rebuild aimed at transforming a mediocre piece of equipment into a serious, measurable single-ended power amplifier that is genuinely pleasant to listen to. This article is useful for those who want to understand why many commercial 300B amplifiers sound worse than the myth would suggest, and above all what is actually required to make a 300B perform as it should, correct power supplies, a driver capable of real voltage swing, appropriate components, and output transformers worthy of the task.

The roots of the project go back to 2011, when I was still a hobbyist. At the time I bought a used Chinese amplifier, the “Music Angel XD-850MKIII”. I was attracted by the low price, the look with the tubes on display, and the curiosity to see whether, with a few modifications, it could become something worthwhile. The initial enthusiasm faded quickly though, it was not a starting point to be “refined”, it was a starting point to be rebuilt.

From that experience YA300B was born, an acronym for “Yet Another 300B”. It is a project that starts from a real-world case, shows its flaws without any discounts, and then arrives at a complete technical solution, with high-level instrumental and sonic results. In 2024 the project became relevant again because a customer wanted to build it, allowing me to revisit the original schematic, improve it with the experience gained over 13 years, and measure the amplifier more rigorously than I had done at the time. If you already own a Music Angel, or a similar “candelabra-style” amplifier, this transformation can really be worth it. Alternatively, as we will see later on, it is also possible to build everything from scratch.

Music Angel XD-850MKIII, a disappointing experience

The Music Angel XD-850MKIII, which initially seemed promising, turned out to be a disappointment. As I wrote in the old 2011 article, sometimes curiosity pushes you to buy one of these Chinese amplifiers, they are cheap and look appealing, but how do they really perform? For beginners, the listening impression may seem positive, because it is relative to previous experiences. With the experience gained over time, however, I found this amplifier decidedly unsatisfactory. In short, it was a device I was happy to turn off, proving that relying solely on the “charm of the 300B” is often an illusion.

The amplifier sounded harsh, to the point of giving goosebumps from discomfort during listening. Instead of selling it and passing the problem on to someone else, I decided to have fun modifying it. What I proposed at the time was not a simple update, but a total transformation, from a “candelabra” amplifier to a serious, good-sounding amplifier.

The transformation, from candelabra to amplifier

When I talk about “candelabra” amplifiers, I mean those models designed mainly to visually showcase the tubes, rather than to deliver truly convincing audio performance. The Music Angel XD-850MKIII is a perfect example, a single-ended power amplifier with 300B tubes, where the 300B are driven by another single-ended stage using 2A3 tubes. These, in turn, are driven by an input stage based on a directly heated RF double triode, the DCC90, known for its microphonics, but made “scenic” by three 18/10 stainless steel rings, complete with a free set of knives and a non-stick pan.

That said, having a ready-made chassis is always a great convenience, much better than building something from scratch in an improvised wooden box, maybe with bathroom drawer handles from IKEA or lasagna trays used as chassis. This project was therefore born under the banner of fun and DIY. It must also be said that the modification ended up costing more than the amplifier itself, but I did it to recover the best from an ill-considered purchase, to learn, and to gain experience, experience that later proved very useful. Even though very little of the original remained, reusing the chassis saved time and money compared to building a completely new device. The final result is high-level, so if you already own a similar amplifier, this transformation can really be worth it. Alternatively, it is possible to build it entirely from scratch, as we will see later in this article.

Inside the original Music Angel

For those wondering whether it is possible to obtain a real improvement without going as far as such a deep modification, the answer is unfortunately no. Partial or “cosmetic” interventions do not lead to concrete results, without rethinking the device from the ground up, all the original limitations remain.

The original circuit concept was severely lacking. Analyzing the schematic, it immediately emerged that the filaments of the 300B and 2A3 were powered by a sort of “pseudo DC”, obtained with a full-wave rectifier followed by a single electrolytic capacitor. The situation was even worse for the DCC90 filament, powered by a half-wave rectifier. Such an approach not only seriously compromised sonic performance, but also made listening fatiguing and unpleasant. As if that were not enough, the unit failed shortly after purchase. At that point I decided to intervene radically, completely removing the original circuit and starting over from scratch.

I then moved on to measuring transformers and inductors, hoping to save at least part of the original components. The reality, however, was very different, and the image conveys the situation far better than any description. I was faced with transformers that were completely rusted, as if they had been sitting in a damp basement for decades. They were small, clearly undersized, crudely assembled, with deformed laminations that were poorly closed. There was no impregnation at all, and the connection wires were extremely thin, more like hair than conductors worthy of the name.

The laminations looked as if they had been cut with scissors, irregular, damaged, wrinkled, and covered in rust. The winding structure was equally questionable, with a section of secondary at the beginning, another at the end, and the primary crushed in between. Overall, the result was bleak, even worse than what can be found in the audio transformers of some old tube radios.

Wire tension was practically nonexistent, the winding was loose, with more air than insulation material between layers, and completely without impregnation. Measurements immediately confirmed the suspicions, the bandwidth extended from about 200 Hz up to 86 kHz at -3 dB. With such a small number of sections, no impregnation, and large air gaps, the lack of bass was inevitable. The loosely wound structure, even soft to the touch, combined with the large amount of internal air, further emphasized the aggressive and harsh character of the amplifier.

In the image below you can see the inductor, just 1 Henry, of such small size that it was almost certainly in saturation during normal operation. It was fixed with a hard yellowish waxy substance of unclear nature, a detail shared with the power transformer.

YA300B Project, “Yet Another 300B”

The circuit is configured as follows. The input stage uses the two triodes of a 6SL7GT in cathode-coupled configuration, providing a modest preamplification of 9.5 dB. The next stage uses a 6SN7 triode, half a tube per channel, loaded with a transistor CCS. This approach proved necessary to obtain sufficient voltage swing to drive the output stage using the same supply. Using a resistor as a load would have required a much higher voltage, because the 300B, in this configuration, requires a 240 V peak-to-peak swing to be fully driven. Otherwise the driver distorts before the output stage, which is what happens in many other 300B designs, including commercial units costing 5000 euros and more. This is behavior to be avoided.

A fully tube-based solution would have been to use a higher voltage for the driver and adopt circuits such as the mu-follower, as in the later projects Triodino 3.5 and Triodino 4. However, this choice would have required an additional tube and an extra power supply stage, making everything more complex. The CCS, on the other hand, is an important approach for those who appreciate detailed listening and do not dismiss it out of prejudice without having tried it. It is worth remembering that CCS in general, and especially those based on MJE340/350, are used as anode loads or current sources in many highly regarded preamplifiers.

To obtain the Premium schematic read here and contact me here.

I spent a great deal of time optimizing the driver and managed to achieve harmonic compensation between the 6SN7 and the 300B, significantly reducing output stage distortion right up to the onset of clipping. Global negative feedback is very moderate, about 4 dB, and is applied only to the driver, optionally switchable off, while the 6SL7 stages are excluded from the feedback loop from the start. The following photos illustrate the reconstruction process of the Music Angel.

I removed the protective turret of the DCC90 and enlarged the hole to accommodate an octal tube.

I wired the new circuit.

Following a logic of intelligent reuse, for the wiring I reused part of the original components together with elements taken from other dismantled devices, pairing them with new components where necessary. The final result is the unit shown below.

Finally I installed a pair of Full Music 300B tubes, in a circuit matched with a suitable power supply chain and quality transformers.

Here is the rectifier used, a NOS 5U4GB.

The return of the project in 2024

In 2024 a customer became interested in building this very old project of mine. I therefore revisited the original schematic and slightly improved it in light of the knowledge gained over the past 13 years. The changes were targeted and not revolutionary, but they still helped to further refine the result.

The customer, being at his first builds, encountered some technical difficulties. He therefore brought the unit to my lab to solve a couple of small issues. Having the amplifier on the bench also allowed me to carry out more accurate measurements compared to those done many years ago. In the following photos you can see his build.

Thermal view of the two MJE350 transistors

Instrumental performance of the amplifier

During instrumental testing, excellent results emerged. Undistorted RMS power is 8.2 watts per channel, with a maximum power of 9.8 watts under deep clipping conditions. The damping factor is 7.7. Total harmonic distortion at 1 watt is 0.52 percent, while bandwidth extends from 10 Hz at -0.2 dB to 20 kHz at -1.2 dB. These values confirm the quality of the project. Below are the usual reference plots.

THD

Bandwidth

Square wave at 100 Hz

Square wave at 1 kHz

Square wave at 10 kHz

How it sounds: absolutely exceptional. The soundstage is surprisingly open, with deep, clean, and well-controlled bass, and a high-frequency extension capable of sending shivers down your spine for its cleanliness and naturalness. Every microdetail is rendered with great precision, with a wide and credible soundstage. The 300B, when inserted into a coherent design and driven correctly, can deliver truly extraordinary results, with tonal richness and depth that leave you speechless.

Thank you for exploring YA300B, a complete evolution based on the Music Angel XD-850MKIII chassis. With a radical redesign and high-level performance, YA300B demonstrates the potential of the 300B when it is supported by correct design choices. If you are interested in building this project, either by modifying an existing Music Angel or building from scratch, consult the premium schematic and the available transformer set. Contact us for further information and to purchase the kit.