Lower IMD from Tetrode Power Amplifiers

John Nelson, GW4FRX has been a constant campaigner for cleaner signals, and has been responsible for many key developments in tetrode power supplies (see Chapters 6 and 11 of The VHF/UHF DX Book). This bar-chart shows his measurements of two-tone IMD performance of a pair of 4CX250Rs in class AB1 at 500W PEP output, with three different types of screen supply.

IMD bar-chart

Test conditions

  • Two Eimac 4CX250Rs, operating (heater) time 4500hrs
  • Va 2000V
  • Vg2 350V
  • Zero-signal anode current 100mA per tube
  • Two-tone test signal, 1kHz spacing, class-AB1 drive conditions (no g1 current).
  1. Amplifier tuned and loaded initially for -30dB 3rd-order IMD at 500W output, using GW4FRX stabilized screen supply (350V). IMD measured.
  2. 150 ohms series resistance added to screen supply to simulate poorer regulation. IMD re-measured.
  3. Screen supply replaced by 350V zeners with 100uF in parallel to improve audio-frequency regulation. IMD re-measured.

See comments below



  1. A screen voltage regulator with low dynamic impedance at all audio frequencies from 3kHz down to syllabic frequencies of a few hertz will produce significantly better IMD performance, especially for the higher-order products that make your signal 'wide'.
  2. Improved screen stabilization can give IMD performance that is notably better than stated in the Eimac data sheets.
  3. Zener stabilizers are no longer state-of-the-art.

On the air, these conclusions have been verified by many British and European stations. Tight screen voltage regulation really does make a difference to your reputation!

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Updated 9 April, 2013
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