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03-04-2021, 01:46 AM
Hi Guys
For a given power rating power resistors can vary quite widely in their physical size and materials. For a given power dissipation, the smaller part will get hotter than a larger part. Data sheets for the components show how hot the manufacturer is comfortable in letting the device be operated, and what the life expectancy is operating continuously at these temperatures, and the actual figures can astound you.
All components with leads will initially try to dissipate their internal heat via the leads to the outside world and to whatever electrical conductors the leads are attached to. This is why some diodes - particularly those 3A singles - have such fat leads. The leads should be left as long as is feasible. Power resistors are the same and I've seen solder pads lifted from PCBs because a 5W wire-wound was subject to excess dissipation. The resistor was not discoloured or off-value, but the solder pads had to be cut off the board.
The screen resistor typically carries about one-tenth of the plate current as a maximum, but this is on an instantaneous peak basis. So, in a 50W amp with 4k-aa, Ia might be 300mApk, suggesting Ig2 peak of 30mA. A 1k resistor dissipates 900mW peak in that scenario. Usually the screen resistor is heated more by its proximity to the tube than by its own current / voltage self-heating
Have fun
For a given power rating power resistors can vary quite widely in their physical size and materials. For a given power dissipation, the smaller part will get hotter than a larger part. Data sheets for the components show how hot the manufacturer is comfortable in letting the device be operated, and what the life expectancy is operating continuously at these temperatures, and the actual figures can astound you.
All components with leads will initially try to dissipate their internal heat via the leads to the outside world and to whatever electrical conductors the leads are attached to. This is why some diodes - particularly those 3A singles - have such fat leads. The leads should be left as long as is feasible. Power resistors are the same and I've seen solder pads lifted from PCBs because a 5W wire-wound was subject to excess dissipation. The resistor was not discoloured or off-value, but the solder pads had to be cut off the board.
The screen resistor typically carries about one-tenth of the plate current as a maximum, but this is on an instantaneous peak basis. So, in a 50W amp with 4k-aa, Ia might be 300mApk, suggesting Ig2 peak of 30mA. A 1k resistor dissipates 900mW peak in that scenario. Usually the screen resistor is heated more by its proximity to the tube than by its own current / voltage self-heating
Have fun
