Topband: Signal complaints and auto bias

[Tom W8JI]

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