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many do not) you can either speak a little
more softiy, or connect an att enuat o r pad
(a sort of resis tive voltage divi de d be tween
the microphone and the amp lifier.
What about the ou tput co nd itions? In
o ur class A amplifier, which is normally
used to provide voltage gain, th is isn' t
usually adjusta ble. If it were, taking too
littlc power out wo uld tend to produce the
same symptoms as putting in to o much
input signa l, wh ile attempting to get out
to o m uch power wo uld show u p as heating
of all the amplifier co mp o nen ts in ad d it io n
to distortio n.
Now let' s see what happens when we
misu se J class C rf power amplifier. For
o peners this t ime , we'll assume that it's the
fin al ou tput stage of a CW t ransmitter.
In many class C am plifiers using "fixed"
bias, the bias level ca n be adjusted by the
operator. To o Iow a bias level will result in
low power o utput. and too high a bias level
will generate excessive ha rmonic output.
Most class C circu its these days, however,
usc a "self bias" arra ngement in whic h the
o perat ing grid bias is deve loped by the flow
of grid current through a "grid leak"
res istor; with these , bot h th e bias level and
the in put signal arc set by the same
control, which may be called the "drive"
adjustment, or the "driver t uni ng."
In p ut signa l level is no t as critical with a
class C amplifier as it was with class A,
be ca use we are no t so concerned with
signal distortion. Th e ma in t hing is to have
en oug h, withou t ovcrdr iving the circuit,
an d the designer usually specifies a broad
range of operating condi t ions.
In circuits using self-b ias, loss of drive
causes loss of bias as well, with resulting
high curre nt in t he amplifie r stage and
damage to all components in the pla te
circ uit. For this reason it's goo d prac tice to
adjust the drive level (input signa l) with the
amplifier plate and screen voltages turned
off. If input signa l is too small (too lit tle
grid current), bias will be too low and
power output will be lo w. If it's too large
(e xcessive grid current) harmonics will he
excessive an d the final-stage grid may be
damaged .
With a CW transmitter, it's bes t to start
with the am ount of drive signal the de-
signer rec o mme nds. After dri ve is dcveloped,
plate voltage may be ap plied an d the
output adj ust ments rnade , and the n drive
may be red uced until power outpu t drops
by a barely perceptible amoun t, and advanced
by some 15 to 20 1 Yr, from that
poin t. T his assures minimum driv e wh ile
still providing rated ou tput fro m the amplifier,
an d will probab ly result in an input
signal level with in the designer's intentio ns.
For an A I"! tr ansmit ter, this adjustme n t
of drive and load ing may not be adeq uate .
A mod ulat ed amplifier req uires enough
dr ive to supply th e mo du lation pea ks as
we ll as the carrier level, h ut we'll go into
th at later when we examine th e mo du latio
n process.
Ou tp ut signal level adjustment in a class
C a mp lifier is th e most critical adjustment
o f them all. It should not be attempte d
until the input signa l level has bee n
brought into t he corre ct range, because the
tu be or transistor may be destro yed in t he
absence of in put sign al.
Most tra nsmitters have tw o adjustme n ts
for o ut p ut tuning, one marked " tuning"
and the ot her ma rked " loading." The
normal practice is t o set the load ing con ­
tro l for min imum output, th en rapid ly
adjust the tu ning cont rol un til plate curren
t d ips sharply. This di p ind icates tha t
the outp ut tank circuit is tuned to resonance,
and is ac ting as a hig h impedance.
While this is all 'right fo r tu neu p, wh ich
nor mall y is don e rap idly, it can damage the
eq uipmen t if ex te nde d op era tion is attempt
ed with too-little power be ing taken
ou t. Whether you usc it or no t, a 5 00W
amplifier is deve loping its 500W wort h of
curre nt and vo lta ge swi ng, and if you take
only 50 of t hem ouf th e rest are going to
be looking for mischief insi de the amplifier.
T uning coils may overheat an d me lt
their plastic su ppor ts, or capacito rs ma y
arc over.
To take more powe r o ut , the loading
con trol is adjusted to increase plate current,
meanwhile readjusting t he tu nin g
cont rol to keep the dip at its mini mu m
value. The rea djustment is necessary because
the two controls interact ; increasin g
the loading reduces the " Q" of the tank
FEBRUARY 19 71
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