Zener Diode in Bias Supply

[Bassman]

Hi Kevin et al.

On the Custom Special schematic in TUT3 there is a zener diode (100v, 1w) straddling Vb and ground.

My amp building experience is limited, I'm used to seeing resistors here.
I believe it's purpose is to provide a stable fixed bias voltage relative to ground.
So if the plate voltage varies for whatever reason, the bias remains the same.

My question is - Why is this better than the bias following the ups and downs of the plate voltage?

The amp I am building will hopefully be gigged.
It has a universal power transformer, wired as Kevin suggested at the back of TUT3.
So different countries, different mains voltages and hopefully lots of festivals with dodgy generators and clubs with terrible mains supplies.

Regards.

[K O'Connor]

Hi Guys

The bias supply will track mains and loading effects of the PT. The zener protects the caps when the bias-set network is not connected, which should only be the case during incremental testing. Normally the zener is 'off' .

Have fun