[Amps] Zener Screen Regulators

[Carl]

----- Original Message ----- 
From: "Ian White GM3SEK" <gm3sek at ifwtech.co.uk>
To: <amps at contesting.com>
Sent: Tuesday, September 04, 2012 5:18 AM
Subject: Re: [Amps] Zener Screen Regulators


> 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

And if you remember I confirmed this in both a Johnson 6N2 T-Bolt and 
National NCL-2000 back in the early 90's.
I saw virtually no IMD improvement with zeners in the T-Bolt since it 
already had VR regulation.

The NCL had a stiff bleeder but there was voltage variation. The zeners 
improved IMD about 2dB (there was a 100 Ohm series "stopper" after the 220 
Ohm current limiter) while those early boards of yours added another 3dB

Carl
KM1H


 _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 10.0.1424 / Virus Database: 2437/5248 - Release Date: 09/04/12
>