Lower IMD from Tetrode Power
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
- 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
- Amplifier tuned and loaded initially for -30dB 3rd-order IMD at 500W
output, using GW4FRX stabilized screen supply (350V). IMD measured.
- 150 ohms series resistance added to screen supply to simulate poorer
regulation. IMD re-measured.
- Screen supply replaced by 350V zeners with 100uF in parallel to improve
audio-frequency regulation. IMD re-measured.
See comments below
- The best IMD performance (white bars) came from GW4FRX's own extremely well
This has excellent DC stability and a very low dynamic
impedance at all audio frequencies, from 3kHz down to syllabic frequencies of a
- Second best (checkered bars) was the highly stabilized supply with a
150-ohm series resistor added to artificially increase the output impedance.
- A poor third (black bars) - especially for higher-order IMD - is
the traditional 'chain of zeners' stabilizer with a 100uF parallel reservoir
- 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'.
- Improved screen stabilization can give IMD performance that is notably
better than stated in the Eimac data sheets.
- 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!
Back to GM3SEK's Amateur Radio
9 April, 2013
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