Calculating power dissipation
Posted: Tue Sep 20, 2011 7:51 pm
Looked through the archives but could not find what I was looking for. I'd like to know the proper way to figure power dissipation for EL84 plates in the Stereo 35. I've noted the instructions for biasing the tubes in the diytube manual, as follows:
"With an insulated adjustment tool and a DMM (at low voltage setting), adjust bias pots clockwise (up) or counter-clockwise (down) to get the voltage at each TP (i.e. the test point at R19,R22,R25,R28) to be 350mV. This means the cathode current is 35mA. Plate current is about 30mA and the screen is 5mA."
So, given that P=EI, exactly which figures for voltage and current should be used to calculate power dissipation at the plate? Should we:
- Subtract the cathode voltage from the plate voltage and consider the result to be the voltage "across" the plate alone, or is there also a voltage drop across the screen to be subtracted, as well?
- Use 35mA or 30mA as the current through the plate?
Thanks for any help with this.
Peter
"With an insulated adjustment tool and a DMM (at low voltage setting), adjust bias pots clockwise (up) or counter-clockwise (down) to get the voltage at each TP (i.e. the test point at R19,R22,R25,R28) to be 350mV. This means the cathode current is 35mA. Plate current is about 30mA and the screen is 5mA."
So, given that P=EI, exactly which figures for voltage and current should be used to calculate power dissipation at the plate? Should we:
- Subtract the cathode voltage from the plate voltage and consider the result to be the voltage "across" the plate alone, or is there also a voltage drop across the screen to be subtracted, as well?
- Use 35mA or 30mA as the current through the plate?
Thanks for any help with this.
Peter