Tube Treasures

Output transformers are essential components in vacuum tube amplifiers and other audio circuits. They transfer the audio signal from the power tube to the loudspeaker while providing the impedance matching required for efficient operation. The magnetic core has a major influence on the transformer’s performance. Core geometry, magnetic material, air-gap configuration and winding arrangement affect frequency response, power handling, leakage inductance, efficiency and audible noise. This article examines three commonly used core types: EI cores, C-cores and toroidal cores. It explains how each design works, discusses its main advantages and limitations, and provides practical considerations for selecting a suitable core for an output transformer. Comparing EI, C-Core and Toroidal Designs The following table provides a quick comparison of the three most common core types used in output transformers. Core Type Main Advantages Main Limitations Typical Applications EI core Affordable, eas...

  How to Measure Tube Amplifier Plate Current and Set Bias Safely Introduction Measuring the idle current of a tube amplifier is an important part of checking its operating condition and setting the correct bias. Incorrect bias can cause excessive tube dissipation, premature tube failure, distortion, overheating, or reduced output power. This article explains how to estimate the operating current of power tubes by measuring the voltage across a cathode resistor. This method is particularly useful in cathode-biased amplifiers and in amplifiers fitted with individual bias test resistors. It is important to understand that cathode current is not exactly the same as plate current. In a power pentode or beam tetrode, the cathode current includes both the plate current and the screen-grid current. Therefore, accurate plate-dissipation calculations require the screen current to be taken into account. The procedures described here are intended for experienced electronics enthusiasts and te...