Self induced oscillation

[John Caswell]

Hi all,
Quite often, when having built something it appears not to perform as the designer specified, with either the voltages not being in the ball park area or the valve getting red even with no signal.
This is often caused by oscillation, usually supersonic and normally only viewable on a 'scope, and is generally triggered off by poor layout/construction techniques and the like.
It can be the very devil to cure, but simple things like keeping control grid wires away from anode wires will effect a cure. In output valves for instance when using them in UL mode a small (100R) resistor wired right on the valve base of the screen grid pin will often stop oscillation dead as will the same remedy on the control grid albeit a slightly larger value. Similarly if strapping a pentode/tetrode to a triode a small 100R resistor between screen and anode will help alleviate any possibility of oscillation
It is also worth noting here that the resistor on the screen grid does not have to be a high wattage rating. Indeed a small wattage rating is to the good, as if the valve goes s/c screen to grid then the resistor will fry protecting the transformer and cathode bias/fixed bias components.
With small signal valves oscillation can often be stopped by fitting a small metal screen across the valve base physically between the grid and anode circuits either earthed or connected to the cathode.

John aka Dr John