In the vast panorama of vacuum tubes, the VT-33 – also known simply as 33 or by the military code CV949 – holds a prominent place among tubes designed for battery operation. An American pentode from the 1930s, it was conceived for maximum energy efficiency, making it perfect for portable receivers. Today, however, this small marvel finds new life in high-end audio projects, thanks to its technical appeal and surprisingly refined sound character.
A small masterpiece to rediscover
The VT-33 is a low-power pentode with a filament that requires only 2 V at 260 mA – a dream for any DIYer wishing to avoid the usual hot heat sinks of low-voltage regulators. This filament can be powered directly from a battery or via a simple regulator, ensuring silence and easy integration.
But its true potential emerges when connected as a triode. In this configuration, with the screen grid (G2) connected to the anode through a 220 ohm resistor, the VT-33 turns into a tube with a sweet, natural and incredibly refined sound – perfect for hi-fi applications where tonal quality matters more than power.
Our new transformer: designed specifically for the VT-33
To make the most of this historic tube, we have designed and built a single-ended interstage output transformer specifically optimized for the VT-33 in triode mode.
Main features:
- Primary impedance: 8,500 ohms
- Secondary impedance: 820 ohms
- Configuration: Single-ended
This impedance combination results from a careful analysis of the characteristic curves of the VT-33 connected as a triode (shown at the end of the article) and a load line drawn to maximize linearity, tonal quality, and dynamic coherence. The result is a circuit with a unitary overall gain: the tube provides real amplification while the transformer matches the impedance and brings the signal back to its original level. This allows the creation of preamplifiers with genuine tube tone, perfect for integration into modern audio chains where further amplification is often unnecessary – or even detrimental – given the already high output of today’s sources.
Application schematic and recommended components
Frequency response and distortion: results that speak for themselves
To demonstrate the quality of our interstage transformer, we performed a series of instrument measurements on a sample VT-33 connected as a triode, mounted according to the proposed schematic and without any feedback.
The frequency response graph shows a practically flat bandwidth from 10 Hz (0 dB) up to 65 kHz (?1 dB): an excellent result ensuring the transparency required for realistic and detailed music reproduction, with deep bass and extended highs without harshness.
The harmonic spectrum (THD) graph also confirms the quality of the design: 0.3 % total harmonic distortion without feedback. A value that highlights the musical and natural character of the VT-33 in this configuration, without sacrificing tonal accuracy.
These figures are more than just numbers: they tell of a tube that, though small and originally intended for humble tasks, can offer a refined listening experience when paired with the right transformer.
With this information, even less experienced DIYers can build a small, high-quality audio stage, enjoying the magic of a vintage tube that rivals far more renowned and expensive siblings.
Conclusion
The VT-33, with its minimal filament requirements and elegant voice in triode mode, proves that even the humblest tubes can sound extraordinary when matched with carefully designed components. Our interstage transformer is the perfect complement for those wishing to explore new low-power yet highly captivating sonic horizons. Contact us for orders or further technical information: we’ll be happy to guide you in the discovery (or rediscovery) of this little electronic marvel.