Practical Hints and Tips
9th, 1932 The Wireless World Article
I finally managed to get an early edition of The Wireless World magazine
for a reasonable price on a eBay auction. Now I will be able to post
a few of those articles from the UK to compliment those from some of
the American magazines. This particular edition is from March 9th, 1932.
My next target is to get a few from the World War II era which although
it began on December 7, 2941 from America's perspective, it officially
began on September 1, 1939 for Europe.
articles are scanned and OCRed from old editions of The Wireless
Warning for the weak
of heart - epochal words like "niggardly" and "parsimonious" are
used herein, and therefore adult supervision should be employed if ignorance
might cause an objection to at least one of the aforementioned.
See all the available
The Wireless World articles.
Practical Hints and Tips
Simplified Aids to Better Reception
When a receiver is fed
with H.T. current from the mains, there is no particular need to be
niggardly in the matter of consumption; a few milliamps. here or there
make practically no difference to the cost of upkeep. But it is a different
matter when dry batteries are employed; in this case, all possible sources
of waste should be rigorously avoided, and it may be helpful to enumerate
some of the more common causes of excessively high anode current.
As often as not, a valve takes more anode current than it should
because an insufficient amount of negative bias is applied to its grid.
Of course, the same thing happens if the grid is totally unbiased, but
then an audible indication that something is wrong is generally given.
Further, it is not enough that merely the bias battery itself should
be in order, and where there arc grounds for suspicion, it is advisable
to test the entire circuit for continuity to make sure that a negative
voltage is actually impressed on the grid.
A "soft" valve will
generally pass a high anode current ; if this defect is not made obvious
by the presence of a blue glow around the electrodes of the valve. it
may generally be detected fairly easily by short circuiting the grid
circuit resistance, and noticing whether this brings about an appreciable
change in anode current. If it does, the valve will definitely be "soft."
It should be remembered that if there is not already a resistance of
sufficiently high value in the grid circuit, this test will not be conclusive,
but a resistance may be temporarily inserted.
Other faults that
may occasionally be responsible for excessive anode current consumption
in battery-operated sets are short-circuited anode feed resistances,
or leakages. or more or less complete "shorts" in the anode circuits;
the bypass condensers may be suspected.
Users of D.C. mains supplies
are always handicapped by the fact that the H.T. voltage available is
inevitably fixed at a considerably lower value than that obtainable
from the majority of A.C. rectifiers. This limitation is particularly
annoying when one is trying to devise means for supplying a power grid
detector with sufficient anode pressure; even if initial difficulties
are overcome the detector decoupling must always be designed on almost
parsimonious lines. Even if actual "motor-boating" is not present,
there is always an uneasy feeling that too much stray L.F. reaction
for really good quality is taking place. The final result is that we
generally arrive at a compromise something like a "semi-power grid"
detector, in which the usual anode circuit limitations may possibly
become evident, due to insufficient H.T. voltage. At the best, we can
hardly hope to use, as a coupling between the detector and succeeding
L.F. valve, an arrangement which will produce anything approaching the
maximum attainable magnification.
The type of diode detector
discussed in The Wireless World of February 3rd suffers from none of
these limitations, and, apart from providing almost perfect detection,
has the additional advantage that it does not impose any serious damping
on the tuned circuit which immediately precedes it. The arrangement,
therefore, is one that should be particularly attractive to D.C. mains
users, who may accordingly be interested in the skeleton circuit diagram
given in Fig. 1. This shows the nucleus of a diode - a L.F. set suitable
for high-quality reproduction of local broadcasting, and in which indirectly
heated D.C. valves are used throughout. Where greater range is necessary,
an H.F. stage may be added in' the usual way.
In the suggested
circuit diagram a pentode output valve is shown, but, of course, there
is. no reason why this should not be replaced by a triode, or where
large outputs are required, by a pair of triodes push-pull.
A set on these lines, with an H.F. amplifier. is definitely capable
of long-distance reception, and there is always the possibility of improved
sensitivity by using the diode anode -which in the simpler form of circuit
is unemployed-for purposes of reaction in the manner suggested in The
Wireless World of June 10th, 1931.
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