WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I can see that it's an OPAMP classic current source but I can't figure out what is zener D3 for.
Picture doesn't show but it's a classic triode preamp B+ regulator.
Looks like some kind of clamp to me, perhaps to prevent latchup under certain conditions and/or simply to protect the opamp inputs being pulled above or below the opamp supply voltage.
you are both right, thank you.
You put me on the right path. So, I found some explanations about this opamp's behaviour. But the protection recommended is by schottky diodes.
So, why a zener is used here ?
A zener allows the voltage across the diode to reach the value of the zener. Using a standard type diode would clamp the voltage across it at a permanent 0.6volts and not allow any voltage swing. The zener allow swing up to the zener voltage.
