photo #8455311

Willeke Fokkink in 1955 at Philips Heerlen attending to one of the giant valve (vacuum tube) ageing racks. Once the rack was fully loaded, the valve heaters were activated at a 20% higher than nominal rating, so for a 6.3V valve, a Vf of 7.5V would be applied. The control grid would have a positive potential applied and both these effects served to 'boil off' electrons from the emissive cathode which in turn caused a small proportion of the barium and strontium oxides to be reduced to metallic barium and strontium. This was a desirable characteristic as the presence of these transition metals served to increase the cathode's emissive properties. After a prescribed time, the grid potential was reduced to zero, cutting off electron flow and the ageing was completed by running the valve at a low cathode current and with the Vf reduced to it's nominal rating.