Infernal_Death
Well-known member
Hi all
I am working right now on a bass preamp (which has a small EL84 5W power amp section) and i am still a bit clueless about the psu. I need bipolar 15V supply for the ic's as well as a B+ voltage and the heater voltage. I am thinking about going the route Jakob already did in his projects (using 2 toroids, one backwards).
Here is the schematic for the preamp:
http://mitglied.lycos.de/InfernalDeath/swr_interstellar_overdrive1.gif
So now i try to understand the psu in the schematic. Let's see:
We got 220V incoming, after the rectifier we should have about 310V. So i looked at the datasheet of the 12AX7 and there is says that a 12AX7 draws 8mA. Looking the same way a EL84 seems to draw 48mA. That makes 104mA together. So i guess that's the current pushing through R65 which is a 270r resistor. By ohms law this makes a voltage drop of 28V through the resistor. Now first question. How do they come to 250V from 310V through this resistor ?
So even if we have 250V behind the resistor, through the resistor R66 (12K) we have again voltage drop of 96V (assuming that only the 8mA gets consumed through R66). So the 12AX7 seems around 154V ?
I am quite sure i have quite some errors in my thinking. Could anybody please be so kind to explain to me how this psu is working ? I need to adept it for my project and i want to make sure to not blow something up before connecting :wink:
As always thanks for reading.
Flo
I am working right now on a bass preamp (which has a small EL84 5W power amp section) and i am still a bit clueless about the psu. I need bipolar 15V supply for the ic's as well as a B+ voltage and the heater voltage. I am thinking about going the route Jakob already did in his projects (using 2 toroids, one backwards).
Here is the schematic for the preamp:
http://mitglied.lycos.de/InfernalDeath/swr_interstellar_overdrive1.gif
So now i try to understand the psu in the schematic. Let's see:
We got 220V incoming, after the rectifier we should have about 310V. So i looked at the datasheet of the 12AX7 and there is says that a 12AX7 draws 8mA. Looking the same way a EL84 seems to draw 48mA. That makes 104mA together. So i guess that's the current pushing through R65 which is a 270r resistor. By ohms law this makes a voltage drop of 28V through the resistor. Now first question. How do they come to 250V from 310V through this resistor ?
So even if we have 250V behind the resistor, through the resistor R66 (12K) we have again voltage drop of 96V (assuming that only the 8mA gets consumed through R66). So the 12AX7 seems around 154V ?
I am quite sure i have quite some errors in my thinking. Could anybody please be so kind to explain to me how this psu is working ? I need to adept it for my project and i want to make sure to not blow something up before connecting :wink:
As always thanks for reading.
Flo