[Amps] Screen grid supply

[R.Measures]

On Feb 16, 2005, at 12:18 AM, Angel Vilaseca wrote:

> I am building a 3.5-30 MHz amp with 2 grid-driven 4CX250Bs.
>
> I have already built the screen grid supply with resistors from the
> +2000V plate supply, shunted by a string of Zener diodes to ground. The
> regulated 350 V then goes to the two screens through two series
> resistors. There is no shunt resistor to ground. I just duplicated a
> design from an ARRL handbook.
>
> Now I saw in "care and feeding of power grid tubes" from Eimac, and
> elsewhere, that a 30 K resistor from each screen to ground is
> recommended because the screen supply must be able not only to "source"
> current, but also to "sink" current.

The zener string will sink any reverse screen-I.

> Otherwise depending on amp loading,
> screen voltage can soar, and this would increase screen dissipation and
> possibly destroy the tube.
>
> So my question is: Can a grid supply with resistors from the +2000V
> plate supply, shunted by a string of Zener diodes to ground, without a
> shunt resistor to ground, source AND sink current? Or do I need to add 
> a
> 30 K resistor from each screen to ground?

No screen bleeder resistor to ground is needed with a zener string.
>
> 73
>
>
> Angel Vilaseca HB9SLV
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org