After almost 10 months of development, I finally brought to life these two single-ended monoblocks using the 211 tube. The project was far from simple — I wanted to use the 211 properly! And by “properly,” I mean that a tube of this caliber cannot be reduced to delivering just 10 watts, as many do simply because they lack the skill to design a proper driver stage. According to the datasheet, the 211 requires a mixed A1+A2 operation (both negative and positive grid bias — and for those who disagree, I recommend using the 845 instead). Therefore, weak driver stages or interstage transformers are out of the question, as they only smear the sound, limit bandwidth, and prevent proper control when the grid goes positive — not to mention that they make the use of NFB impractical, leading to poor damping and weak speaker control.

The design began on paper. I had to think carefully about the schematic because the circuit needed to deliver a large, linear voltage swing into a highly nonlinear load — the G1 grid of the 211. It draws virtually 0 mA throughout the negative swing, then rises up to about 160 mA during the positive half-cycle. The behavior of my circuit is shown in the graph below: in blue the signal (which remains linear), and in green the nonlinear grid current of the 211. My design achieves 25 W RMS with 850 V plate voltage on a 9 k load. Power could be increased to 35 W RMS by raising the supply to 1000–1200 V, but I decided against it, as that would have required series-connected capacitors in the power supply — something I felt could compromise the sound. The current power supply uses a voltage doubler followed by an LC filter with a 200 µF 1000 V polypropylene capacitor.

After fine-tuning the circuit theoretically, I wound the transformers and moved on to CAD design of the amplifiers.

And then, to their physical construction…

Instrumental Measurements
Power: 25 W RMS
Bandwidth @1 W: 15 Hz – 50 kHz (–3 dB)
Bandwidth @25 W: 30 Hz – 22 kHz (–3 dB)
THD @1 W: 0.3%
THD @6 W: 1.7%
THD @15 W: 3.8%
Damping Factor: 5
Rout: 1.6 ohm

Tubes Used (7 total)
1× 12AT7 / ECC81 – Input/pre stage, excluded from NFB loop
2× 6J5 – Voltage gain stages
1× EL34 (triode-connected) – Follower/driver/BIAS control, DC-coupled to output stage
1× 6080 – Voltage regulator for EL34 plate supply
1× PC900 – Error amplifier driving the 6080
1× 211 / VT-4C – Power stage

Spectrum @1 W

Spectrum @6 W

Spectrum @15 W

Square Wave @100 Hz

Square Wave @1 kHz

Square Wave @10 kHz

How does it sound? At first listen, these amplifiers already seemed remarkable — and after extended listening in a controlled environment, they proved even better than expected. They deliver a majestic yet refined midrange and treble, paired with a granite-solid, statuesque bass. The sound is melodic and rich in detail; with certain recordings you can hear the pianist breathe or the guitarist’s hands on the instrument. I have no hesitation in placing them among the finest amplifiers I’ve ever built or listened to in my life. It’s hard to explain, but when you encounter amplification of this caliber, you realize that everything you once thought was “immense” was merely “large.” Every hour spent on such a demanding build was absolutely worth it.