[Radio Wrangler]

Followers, whether cathode followers or emitter followers have an instability gotcha. it gets worse if it's driving a capacitive load (Scope deflection plates!) and it gets worse if the anode/collector is well decoupled at RF.

If you look at the stray capacitances between electrodes and the inductances of the connections, you can rearrange things and find you have a Colpitts oscillator circuit made out of the strays, the decouplers and the capacitive load. This is where spurious oscillations at VHF come from.

The resistor in the anode spoils the gain of the oscillator and tames it. It's a more powerful technique for a valve than sticking a resistor in the grid connection but it can waste some signal power and some DC power. In this case it won't be significant.

An alternative way of looking at the impromptu oscillator is that the inductance of a decoupling connection is rotated (active filter gyrator style) into something which looks like a negative resistance, and that can cancel the losses of any stray resonance and turn it into an oscillator.

This vulnerability is used. Many UHF oscillators use the "unstable emitter follower" circuit and it foxes people because there are no visible feedback paths.

You can look at a circuit and from the measures to prevent spurious oscillations, you can form an opinion of the experience level of the designer (unless a good designer got overruled by the accountants).

David

David