[Boatanchors] SPEAKER SWITCHING

[brianclarke01 at optusnet.com.au]

Hi Norman,

When a valve amplifier is operating at full volume, the Voltage swing at the plates of the output tubes should range from 0 V to twice the supply Voltage - when the secondary is loaded. When there is no secondary load, the load reflected to the plate[s] is very high and the upper excursion will be even higher than twice the supply Voltage. Such Voltage excursions are likely to result in flashover and permanent carbonisation inside the transformer.

When the secondary is shorted, the load reflected to the plate[s] is very low. The Voltage excursion will be much lower and no damage can occur - it is almost impossible to draw enough current to fuse the primary winding.

73 de Brian, VK2GCE.