Following the article dedicated to the modification of the Nobsound 6p3p, today I present a new project commissioned by a client, the complete from-scratch realization of a Single Ended amplifier using 6L6G and 6L6GC tubes, including schematic and a set of custom-designed transformers.
The goal was simple but ambitious, to replicate the sonic quality of the circuit used in the Nobsound, improving every aspect, removing the compromises imposed by low-cost components, and delivering a fully Hi-Fi result.
The heart of the project, SB-LAB transformers
For this new amplifier, which I decided to call Lupino 6L6, I used SB-LAB SE4K2-6L6 output transformers, specifically designed for 6L6 tubes in Single Ended configuration. These transformers, unlike the small and undersized cores of the Chinese model, are built with suitable materials and dimensions to guarantee an extended and linear response across the entire audio band, including the low-frequency range. Click here to learn about the whole 6L6 tube family and its variants.
I also designed a dedicated power transformer, the 25S112 model, which provides the correct voltages and currents without being stressed to its limits, as often happens with low-cost units. The filtering is handled by an SB-LAB 16S64 choke, sized to ensure silent, stable, and hum-free operation, even with high-consumption tubes.
Updated and optimized circuit
From a schematic point of view, the circuit follows the one used in the modified Nobsound project, but with several improvements. In particular, I reduced the amount of negative feedback, making it compatible with the use of higher-quality transformers.
In fact, when well designed transformers with a large core and adequate primary inductance are used, it is possible to obtain excellent low-frequency extension without resorting to excessive negative feedback. The result is a more open, natural, and airy sound, while still maintaining good control over real speaker loads.
I tested the circuit on an open bench, temporarily mounted on a wooden board for preliminary instrument measurements. The results confirm the effectiveness of the design, frequency response flat from 10 Hz to 40 kHz within minus 1 dB, total harmonic distortion (THD) limited to 0.8 percent, and a damping factor (DF) of 5.1. These are more than respectable figures for a Single Ended class A amplifier, ensuring controlled yet natural and dynamic listening.
Alex’s build
The Lupino 6L6 project was also successfully built by the client Alex, who commissioned the complete set of SB-LAB transformers. With great care and attention to detail, Alex assembled his amplifier following the proposed schematic, achieving a result that is both clean in appearance and technically impeccable. Below are some photos of his work, showing the construction simplicity and the level of quality that can be achieved with well designed components.
Frequency response with the SE4K2-6L6 transformer
Harmonic distortion
Square waves at 100 Hz, 1 khz, 10 khz
Lupino 6L6 is a project designed for those who want to build a 6L6 Single Ended amplifier from scratch without compromises. Thanks to purpose designed SB-LAB transformers and a properly dimensioned power supply, it delivers higher performance and a refined tonal character, while keeping the simplicity and immediacy of a proven schematic. Anyone wishing to learn more or order the transformer set for this project can contact me directly through the contact form.