Notes on the Getter
you happen to be Estonian, you might think of something entirely different
than most of us do when we hear the word "getter." In fact, you probably
capitalize the word since it is the name of a pop singer from your country,
If you are a child living in Japan, you would probably think of
Getter Robo, an anime from a popular cartoon series. I, and
I dare say just about everyone else that visits RF Cafe, knows getter
as that silvery deposit
barium) that resides inside vacuum tubes for the purpose of helping
to maintain the vacuum and to absorb pesky random molecules that
otherwise cause electrical noise in the circuit. This article from a
1958 edition of Radio-Electronics discusses the purpose of
getter. BTW, I had never heard of either of the other two Getters due
to OGS (old guy syndrome).
February 1958 Radio-Electronics
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
Notes on the Getter
By Norman V. Becker
Getter inside 12AU7 vacuum tube. (RF Cafe
In high-gain audio circuits, tube noises
such as hiss and frying are some of the most troublesome things encountered.
They can be eliminated only by selecting tubes which are inherently
quieter or by reducing stage gain with negative feedback. In the latter
instance gain might have to be reduced by such a factor as to defeat
the original purpose.
Hiss is created by dc resistance paths
existing between various elements inside the tube. These leakage paths
may be as high as 1,000 megohms and would not upset normal tube operation
if they remained constant. But like a bad carbon resistor, they create
noises of their own through random and erratic changes of resistance.
Leakage paths of this sort are primarily located on the top
mica support wafer, where the support rods are punched through. If the
wafer is contaminated by impurities, it becomes a highly unstable conductor,
connecting tube elements through very-high-resistance paths. Unfortunately,
contamination of the wafer during tube manufacture is almost unavoidable.
Before sealing the tube envelope, as much air as possible is exhausted
by vacuum pumps, but a small percentage of oxygen and other gasses remain
This is where the getter comes into the picture. A small
square loop of wire usually located at the top of the tube, part of
it coated with an explosive substance similar to that used in photoflash
bulbs. High-frequency radio waves penetrate the sealed envelope and
heat the getter to a temperature high enough to fire this coating. This
miniature explosion burns up the remaining atmospheric gasses inside
the tube and, at the same time, splatters a mirror-like silver coating
over a portion of the inner surface (a familiar sight in glass tubes).
Some of this splatter falls on the mica wafer, making it slightly conductive.
To reduce contamination of this sort, certain premium tubes
are manufactured in which the space between getter and wafer is materially
increased. In other types two top wafers are used - the upper one insulated
from the lower - and act as an umbrella to receive most of the splatter.
Another method is to punch oblong slots in the wafer. These openings
effectively lengthen dc leakage paths and thereby reduce noise.
In designing high-gain input stages for microphones and low-output
pick-ups, it is desirable to use premium tubes whenever possible. Special
manuals describing these types are published by tube manufacturers,
giving electrical data, physical dimensions, recommended applications,
etc. In many instances premium tubes are directly interchangeable with
standard types which you might now be using. In addition to reducing
hiss, premium tubes are less microphonic, have lower hum and are generally
more dependable - and are more expensive.
Posted January 20, 2014
Copyright: 1996 - 2024
BSEE - KB3UON
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