[Amps] Screen grid supply

[R.Measures]

On Feb 16, 2005, at 3:05 AM, Steve Thompson wrote:

> 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.

If one tube manages to arc from the anode to the screen, the tube is 
already kaput.
>
> 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.

It also reduces RF output a lot in a tetrode amplifier because the 
instantaneous anode V can not dip below the screen potential during the 
conduction half of the cycle.
>
> Steve
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>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org