Question about Regulated HT for Tube circuit

audiophreak said:

Hi Ian ,
  .. just so I can learn from this , do these resistor values load the regulator better / more efficiently ??

  Thanks again for taking the time,
                                                      Chip

Click to expand...

The TL783 data sheet says it needs to be passing 10mA to regulate properly. In fact if you look at the curves, this is only with the full worst case 125V voltage across the device. As long as you have 75V or less across the device, an 8mA minimum current draw is enough. There is 1.25V between the output and the adjust pins (due to the internal reference). If you connect 150 ohms across this, the current will be 1.25/150 = 8.33mA!! To work out the value of the resistor from the adjust pin to ground you just need to remember that 8.3mA flows though it and it needs to drop your desired output voltage (less 1.25V if you want to be dead accurate). So for 275V you need 275/8.33 = 33K. For 240V you need 240/8.33 = 28.8K.

That's all there is to it.

Cheers

Ian

School'd !!    ;D  Thanks !!   

audiophreak said:

School'd !!    ;D  Thanks !!   

Click to expand...

LOL. I should have mentioned you need to watch the wattage of the 30K or whatever resistor. The power it consumes is just the HT volts times the 8.33mA and the answer is in milliwatts. So for your 240V supply it is 240 * 8.33 = 1999mW or close to 2 watts. It is always sensible to de-rate resistors by about 50% so a 5 watt resistor is a good choice.

Cheers

Ian