[Amps] Screen grid supply

[Angel Vilaseca]

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. 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? 

73


Angel Vilaseca HB9SLV