David McGown wrote:
Shashi,
Keep it simple. There are lots of 12B4 linestage circuits on the web. Most run the 12b4 around 20mA, with anywhere from 90v to 130V at the plate. Around 12 to 13V at the cathode, so cathode resistor is around 600 to 750 ohms. Bypass it with a ELNA Silmic, perhaps 330uF. Plate resistor between 5K and 10K. Higher value for better linearity. Assuming you use a 7500 ohm plate resistor, the voltage drop at 20mA is 150V, so assuming 120V at the plate, you will need a 270V B+. Work out the power supply from there. Actually, you can use a 115/230V isolation transformer into a bridge rectifier and a cap input supply with a couple of RC stages to filter and drop the voltage down to what you need. Model the PS on PSUD. You can use a choke in place of first R for more filtering.
David
I want to use the power supply parts and chassis from the other preamp which you helped me build this summer. I will try to come up with a fixed bias schematic in the next few days.