Hi Francois.

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In my experience, the 7.5V Zener provides a little bit of bias to limit the quiescent current of a pair of 3-500Zs running at around 3kV to 150mA. This is the requirement for Class AB2 linear operation.

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You could run them with Zero Bias (as was Eimac’s recommendation) to reduce circuit complexity, but the quiescent current would be much higher – for no improvement in linearity – hence the Zener diode.

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Most of us shy away from Zener diodes these days as a few decent size silicon diodes (1N5408 or 6A10) can be placed in series and provide the same bias voltage as the expensive Zener.

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Good luck with it. I do love all of my amps – 2 x 3-500Z on 6m, FU728F tetrode in Emtron DX-2SP for HF, GS35B triode in homebrew 2m amp for EME and YC156 triode for 160m/80m. All the triodes have silicon diodes providing the quiescent current bias voltage.

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Best 73,

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John VK6JX

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