Vico Vibe: 6v8 Zener Diode vs Voltage Divider

[JJPJ83]

Looking at the power section of a Jordan Vico Vibe schematic, there is a 39k followed by a 6v8 Zener to ground and then a 1M bias pot to the circuit. Here's what I'm thinking...

9v+ runs through the 39k and then the diode takes 6v8 to ground leaving about 2v2 in the hot rail. If I don't have a 6v8 diode, could I just use a 12k resistor in it's place? 39k + 12k voltage divider = about 2v2. Is there a special benefit to using the diode specifically? Or am I misunderstanding how the Zener works?

[thelonious]

9v+ runs through the 39k and then the diode takes 6v8 to ground leaving about 2v2 in the hot rail.

You're thinking of it backward. The zener is designed to regulate to its zener voltage, in this case 6.8v. So it should leave a constant 6.8v at the point where the 39K current limiting resistor and 1M trimmer meet, regardless of input voltage or the load current (within reason).

[JJPJ83]

Ok got it. Thanks for clearing that up.

In that case the voltage divider would be 39k / 120k = 6.72v. Close enough!

[thelonious]

Yep. Assuming your power supply is 9.00V. Which might be why they used a zener to begin with - variations in battery voltage wouldn't affect the bias voltage, at least up until the battery voltage dropped below the zener voltage.