The single biggest factor is the voltage added to the tube plate (anode). You cannot control the electron emission from the cathode (this is determined by the tube cathode material and tube age), so if the plate voltage is too high, the heat generated by electrons hitting the plate cannot be dissipated fast enough, which will cause a ‘red plate’. This usually happens on power tubes, not on preamp tubes.

To observe if a tube has ‘red plate’, turn off the room light at night and compare the color of the plates to other same kind of tubes. If one is obviously brighter than others, you should check the bias voltage (plate voltage). Adjust it until the tube is not showing red plate and within the amp manufacturer’s suggested bias range.

Do not bias more than 15% below the suggested bias voltage provided by the amp manufacturer – this will cause the tube not operating at optimal condition and will in fact shorten tube life.

Some amplifiers are designed with the tubes running with plate voltage higher than others – we suggest you follow the amp manufacturer’s recommendation.