On Wednesday 16 February 2005 08:18, 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.
Resistance between the zeners and the screen grids is bad.
If the anode arcs to the screen, the zeners must be able to handle the pulse
of current without being destroyed otherwise you will probably kill the tubes
and bases.
I recommend reading the article on screen supplies at www.ifwtech.co.uk/g3sek.
Zener screen supplies can work ok, but you need to think about what can
happen when things go wrong. Running at lower anode voltage reduces the
likelyhood of problems a lot.
Steve
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