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Topband: Signal complaints and auto bias

To: <Topband@contesting.com>
Subject: Topband: Signal complaints and auto bias
From: "Tom W8JI" <w8ji@w8ji.com>
Reply-to: Tom W8JI <w8ji@w8ji.com>
Date: Tue, 29 Jan 2013 10:07:10 -0500
List-post: <topband@contesting.com">mailto:topband@contesting.com>
If your amplifier uses automatic bias, it can cause clicks. There are two things that will prevent or minimize this.

1.) Any auto bias system should not fully cut the tubes off. The tubes should stay very slightly into conduction, very near class B but slightly on the AB side of B. (B is exactly zero idle current, AB is any amount of idle current)

2.) The bias system should be fast on with a hang time on off. It should activate much faster than the envelope rise time, and deactivate slower than the fall time.

3.) The bias system should activate with almost zero power. Even -30 dB background noise on SSB mode should turn on the bias. If it sounds choppy on SSB, it is certainly messing up the CW signals.

A good test is to go to SSB mode with constant room background noise and gradually turn up mic gain while watching the plate current meter. With a 100 watt drive requirement auto bias should fully activate at 10 milliwatts drive (0.7 volts RMS input drive voltage). This is the type of power level we are used to from signal generators, not transmitters.

The AL80B, for example, activates the operating bias with ONE milliwatt of RF power. This prevents the amplifier from changing the rise time of CW envelopes. Some amplifiers activate much higher levels, and this will cause keying artifacts that are as K9YC describes.

Anyone using auto bias and getting complaints should read this:

http://www.w8ji.com/electronic_bias.htm

Having a K3 does not make all signals clean, and not having one does not make all signals dirty. It is the entire system that matters, and there is certainly more than one clean radio in the world.

73 Tom
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