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03-04-2021, 02:54 PM
Hi liquidair
The observations regarding MV position versus screen resistor heat reflect the fact that the amp is clipping and the plate cannot go any closer to ground than it already is. meanwhile, if the grid drive is still able to increase, the internal resistance of the tube may in fact decrease but any "excess" current from the cathode has only two places to go: to the screen and to the control grid.So, in a hard clip scenario screen resistor stress is maximised.
Note that there is no "usual" or "optimal" plate resistance per tube type. You may see commonly used values, typically initiated by a manufacturer's data sheet showing a possible application. The example is meant to provide a reference of performance not to denote a limitation or optimal condition of operating the tube.
Using the higher Raa at a lower Va and Vs does not in itself lead to hotter-running screen-stops. Rather, the use of a lower Raa might simply because the current can be higher at peak clean output.
Alsso note that Human skin temperature is around 24C. PTs are rated to be 40C or less at full power, although some smaller units stretch this to 60C. Power resistors can have temps up to 150C and tubes up to 250C. You can see that where the component may be quite happy with a 50C rise there is no way you can touch it even for an instant without it leaving a mark
Our discomfort with the hot part is more aesthetic and that it is beyond the Human Scale. Were you to measure Marshall's screen-stop temps at clipping you would be as alarmed as you are with those in your build.
The observations regarding MV position versus screen resistor heat reflect the fact that the amp is clipping and the plate cannot go any closer to ground than it already is. meanwhile, if the grid drive is still able to increase, the internal resistance of the tube may in fact decrease but any "excess" current from the cathode has only two places to go: to the screen and to the control grid.So, in a hard clip scenario screen resistor stress is maximised.
Note that there is no "usual" or "optimal" plate resistance per tube type. You may see commonly used values, typically initiated by a manufacturer's data sheet showing a possible application. The example is meant to provide a reference of performance not to denote a limitation or optimal condition of operating the tube.
Using the higher Raa at a lower Va and Vs does not in itself lead to hotter-running screen-stops. Rather, the use of a lower Raa might simply because the current can be higher at peak clean output.
Alsso note that Human skin temperature is around 24C. PTs are rated to be 40C or less at full power, although some smaller units stretch this to 60C. Power resistors can have temps up to 150C and tubes up to 250C. You can see that where the component may be quite happy with a 50C rise there is no way you can touch it even for an instant without it leaving a mark
Our discomfort with the hot part is more aesthetic and that it is beyond the Human Scale. Were you to measure Marshall's screen-stop temps at clipping you would be as alarmed as you are with those in your build.
