UX-250 Single-Ended Transformer-Driven Power Amplifier Part 1 【DIY Amp Lab】
Revised Operating Conditions for a Maximum Output of 5.3W, Designed and Built by Iwamura Yasuo
A single-ended amplifier using the UX-250, a popular directly heated triode (DHT), was built. Since the UX-250 is prone to grid current, it is operated with self-bias and driven by a transformer with a Finemet core. Unlike conventional designs, the load impedance is set to 7kΩ and the self-bias resistor to 1.5kΩ. Despite the relatively light operating conditions, the maximum output reaches 5.3W.
For the input and driver stages, the dual triode 12R-LL3 was selected, considering its plate resistance. The amplifier achieved favorable performance results: a frequency response of 26Hz to 27kHz (-3dB), a distortion rate of 0.07% (1kHz, 0.01W), and a damping factor of 2.4 in the midrange.
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