----- Original Message -----
From: "Ian White GM3SEK" <gm3sek@ifwtech.co.uk>
To: <amps@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
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