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Do not tune your ATU (internal or external) at full output. Always reduce transmitter output to approx. 10W. Power can be increased to the desired level once the best match has been achieved.
Do not operate with a poor match (VSWR > 1.5). Apart from the risk of damage, the complex load presented to the PA will degrade the transmitter's IMD performance. This is not a kindness to your neighbours on the band.
Never cascade ATU's (internal autotuner and external manual or automatic tuner.) Cascading tuners can lead to component failure in the internal tuner, the low-pass filters and/or the PA stage.
Do not "sweep" the VFO whilst transmitting. This is not only in breach of radio regulations*; it can also cause component failure in the transmitter, especially if you sweep across an impedance "bump" in the antenna system.
When using an external amplifier, always connect and configure the ALC line to level the amplifier's output at its design rating. This is especially true of a solid-state amplifier.
Initial adjustments should always be performed with a 50-ohm resistive load at the transmitter output, to avoid radiating unwanted test signals.
When driving a grid-driven (grounded-cathode) tube amplifier with an untuned, swamped input circuit, place a 30 MHz low-pass filter in the coax between the exciter and amplifier. This will protect the exciter from damage in the event of a VHF parasitic oscillation in the amplifier. Solid-state exciters are especially vulnerable in this respect.
* when feeding an antenna
Read my friend Matt KK5DR's article "Tube-type HF Amplifier Tuning".
Copyright © 2007 A. Farson VA7OJ/AB4OJ. All rights reserved. Last updated 18/12/2007
