The 1626 tube, originally designed as an RF power oscillator, is an intriguing component in the world of thermionic valves. First introduced in 1941, the 1626 was intended for high frequency stability, operating in class C with strong negative grid bias. This setup allowed the tube to generate short bursts of power in the tuned anode circuit, which then oscillated to produce sinusoidal waves.
The 1626 is also known by the U.S. government code VT-137. Although originally developed for RF applications, it has recently been successfully used in audio amplifier projects.
When employed in audio, the 1626 typically runs at about 250 V with a maximum bias current of 25 mA. One of its key features for audio use is its low internal resistance. This not only makes it an excellent driver for output tubes but also facilitates the design of high-performance interstage transformers—essential for top-quality sound.
In short, although the 1626 started life as a class C RF transmitter tube, its ability to perform well in audio applications makes it a versatile and valuable choice for tube-amp enthusiasts.
Continuing our look at 1626 applications, we present the i5k-1626 interstage transformer—a key component to fully exploit this tube’s audio potential. The i5k-1626 is specifically designed to work with the 1626, offering a nominal primary impedance of 5 k?.
A major strength of this transformer is its flexibility with reflected load. The secondary load can vary from 4.7 k? to 6.8 k? without significantly affecting system performance, making the i5k-1626 highly adaptable to different amplifier configurations.
The transformer features a 1:1 unbalanced ratio, meaning input and output voltages remain equal while preserving signal integrity. Thanks to this ratio, the i5k-1626 can deliver an output swing exceeding 120 V peak-to-peak. This translates to about 0.4 W RMS of clean power—ideal for driving subsequent amplifier stages.
In conclusion, the i5k-1626 interstage transformer is essential for anyone wishing to use the 1626 tube in audio. Its 5 k? nominal impedance, tolerance for varying loads, and ability to provide high-voltage output without distortion make it a versatile, efficient component. These qualities ensure optimal drive for output tubes, helping create high-quality tube amplifiers. Below is the frequency-response graph:
Note: Following the i5k-1626 design, customers can request versions with different secondary impedances. This flexibility means the transformer can be used not only for audio power amplifiers but also for transformer-coupled preamps and headphone amplifiers. Custom builds ensure optimal performance and high sound quality tailored to specific needs.
When purchasing output transformers together with other components such as chokes or power transformers, significant quantity discounts are available compared to single-unit pricing. See the discount scale below.
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Dimension diagram
