[Amps] Zener Screen Regulators

[Ian White GM3SEK]

peter chadwick wrote:
>The down side to a zener regulator is that the regulation is dependent 
>on dynamic resistance of the zener. GW4FRX found many years ago that 
>zeners were better than VR tubes, but changing to a proper shunt 
>regulator gave an appreciable improvement in IMD with 4CX250Bs on 2m.

Details can be seen at:
<http://www.ifwtech.co.uk/g3sek/cleansig/imd1.htm>

This information is what originally set me on the path towards designing 
the Tetrode Boards, first for my own use and then as boards and kits for 
other hams.

Other tetrodes may be more tolerant of screen voltage variations than 
the 4CX250-350 family, but a tightly regulated screen voltage will 
always improve the DC stability and dynamic IMD.

The dynamic impedance of a passive zener regulated supply in the 
300-400V region is about 150 ohms, though most are much worse than that 
because people then connect further impedances in series with the output 
- the screen current meter, for example, and assorted "stopper 
resistors". Anything connected in series with the output will add to 
that dynamic impedance.

That is why the Tetrode Boards include an active (feedback loop) shunt 
regulator for the screen supply, with the metering and screen-grid 
protection circuits located *inside* the loop so they do not affect the 
voltage regulation.

Another BIG part in choosing a screen supply is damage limitation - 
especially if there is a major arc from anode to screen. My design aim 
has been that everything must survive without damage: the screen supply, 
the tube, the socket (with its vulnerable screen bypass capacitor) and 
also the HV supply. This adds complexity to the circuit design, but the 
reward is that in most cases the amp can come straight back on-air after 
an arc fault, with just a push of the Reset button.

All of this is discussed at greater length in my QEX article from 1997:

<http://www.ifwtech.co.uk/g3sek/boards/tetrode/qex-article.pdf>


(Bad timing in joining this debate, as I am just about to hit the road 
for the funeral of G4DEZ down in England.)


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek