is still a lot of vintage ham radio equipment in use both by the original
owners and by newcomers who buy the equipment at Hamfests and on eBay.
User's manuals are hard to come by, since they often were separated
from the original gear a long time ago. Knowing how to operate, repair,
and align everything properly is still necessary, especially as the
airwaves get ever more crowded and the FCC gets more serious about prosecuting
violators. Old editions of QST are the perfect resource for locating
such information. This particular article covers some of the basics
of oscillators used for CW keying.
May 1941 QST
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged.
See all available
vintage QST articles.
Keying the Crystal Oscillator
And Some Observations on Blocked-Grid Amplifier Keying
By Byron Goodman* WIJPE
Technical Editor, QST.
The subject of crystal oscillator keying is complicated
somewhat by the differences in various crystals, tubes and circuits.
All crystals do not key alike, some circuits are better than others,
and different types of tubes in the same circuit behave differently.
For these reasons, it is wellnigh impossible to set down any hard and
fast rules about crystal keying that will apply in every case. However,
work in the laboratory with the more common tubes and circuits has resulted
in some general principles that can be applied to all crystal oscillators
that are being adjusted for keying.
Fig. 1 - Some oscillograms
of the keying of a grid. plate oscillator (see Fig. 3). The oscillator
is keyed in the negative lead, with no key filter, so that the pure
characteristic can be seen. A, B and C show a 6V6 grid. plate oscillator
tuned for optimum keying, the high-frequency side and the low-frequency
side of optimum, respectively. D shows a 6L6 substituted for the 6V6
and tuned for optimum.
(The oscillograms on these pages all
show the second dot shorter than the first. This is caused by the 'scope
sweep circuit not having a pure saw-tooth form, something that is often
encountered at low frequencies.)
"Adjusting an oscillator for keying" is nothing new to the experienced
amateur who uses several different crystals and has worked with the
problem, but it may come as a shock to those who work on the premise
that a crystal oscillator adjusted for maximum output need only be turned
on and off rapidly with a key to affect good keying. As with self-excited
oscillator keying, the best procedure for adjustment of a crystal oscillator
seems to be first to adjust it so that it follows the key closely at
quite high speeds, and then to introduce some filter to reduce the clicks
to the degree necessary only for good communication at amateur code
speeds. The better crystal oscillator circuits are all capable of keying
speeds up to well over 100 w.p.m., but a keying circuit capable of handling
this speed cleanly results in more key clicks than are necessary for
the more normal speeds of from 20 to 35 w.p.m., and so some lag should
One slight disadvantage of crystal oscillator
keying is that, when several crystals are used (for different parts
of the bands), the total current to the oscillator is not the same in
every case. This means that a key filter adjusted for one particular
voltage-current combination may introduce too little or too much lag
on "make" and too much or too little lag on "break" when a different
crystal (with different total oscillator current) is used. This is likely
to be the case, since all crystals do not key best with the same tuning
adjustment. It is, however, a fine point that is mentioned only to explain
the apparent discrepancies some operators encounter.
As is the
case with self-excited oscillators, cathode keying of a crystal oscillator
seems to be more difficult to filter than power-supply keying (in the
negative or positive lead). The time constant of the oscillator grid
circuit has an effect on the keying, and simply adding a lag circuit
at the key is not as effective as might be thought. The photographs
in Figs. 5B and 5C show a comparison between the effectiveness of key
filters in the cathode and negative leads of a crystal oscillator. Cathode
keying has won popularity because, for the same oscillator, the sparking
at the key and the voltage across the key is less than with power-supply
keying. The obvious answer is, of course, to key a low-power circuit
where these factors become unimportant.
such as the Tri-tet (Fig. 2) and the grid-plate oscillator (Fig. 3)
that use the screen grid as the grounded anode in the oscillating circuit
can, when used with wellscreened tubes, be keyed satisfactorily in
the screen circuit when doubling. If the plate voltage is too high or
if the screening is poor, the crystal will oscillate weakly all of the
time and discourage break-in work on one's own frequency but the circuit
has the advantage that the screen dropping resistor, R2,
and the screen by-pass condenser, C3, serve as a filter that
helps to reduce clicks. When adjusting for minimum clicks, the values
of C3 and another condenser across the key should be varied
until the results are satisfactory. Well-screened tubes like the 6SK7,
6AG7 and the 10-watt pentodes are satisfactory in this application,
but results with the more common beam tubes (6V6, 6L6) will be discouraging,
since the crystal will oscillate continuously.
1 Goodman, "Some
Thoughts on Keying," QST, April, 1941.
Fig. 2 - The Tri-tet oscillator.
The value of C3 will introduce some lag and thus reduce clicks
if a wellscreened tube is used. Negative high-voltage keying is done
at "X", and screen-grid keying at "Y."
sees crystal oscillator circuits with no r.f. choke in series with the
grid leak across the crystal, but the slight saving in expense hardly
justifies the improvement in performance that can be obtained by using
the choke. Several circuits that gave mediocre keying with no choke
showed a marked improvement when the choke was added. The same sort
of improvement is obtained when the value of the grid leak is increased
to 0.25 megohm or so, but this value of grid leak cuts down the output
of the oscillator to a point where it is of little value. The use of
the r.f. choke (and also a large value of grid leak) removes some loading
from the crystal and leaves it freer to start oscillating. As "musts"
for most crystal oscillator circuits that are keyed, it is recommended
that the grid r.f. choke be included and the value of grid leak be made
as high as possible consistent with adequate output to drive the following
stage or, in the case of a single-stage transmitter, to give sufficient
output without a compromise with good keying. The straight tunedplate
triode oscillator is an exception, and it is best operated with cathode
bias only. Frankly, we are at a loss to explain why the cathode-biased
triode works better than one with leak bias while all of the other circuits
are better with leak bias but such seems to be the case, as Figs. 4A
and 4B show.
Simply using an r.f. choke and a high value of
grid leak is not enough to give good keying, of course. A suitable choke
and condenser filter circuit must be used at the key, and the key should
be used in the negative (or positive, if it's hard to get at the negative)
lead, as described in the keying article last month.1 The
same principles apply to adjustment - more choke is used to remove the
click on "make" and more condenser is used to remove the click on "break."
It appears to be slightly more difficult to smooth out the keying of
a crystal oscillator than of a self-excited oscillator, possibly because
one is dealing with a partially mechanical oscillator instead of a purely
electronic one, but in general it will respond to the same treatment.
Fig. 3 - The grid-plate
oscillator. The remarks about F.g.2 also apply to this type of oscillator.
C2 should be 50 μμfd. and C5 will range from 50
to 250 μμfd., depending upon the tube. Some versions of this circuit
use only the input capacity of the tube for C2, but the addition
of the small condenser is worth trying.
The oscillator should be capable of oscillating with only 3
or 4 volts on the plate and an excellent test is to connect several
dry cells in series for the plate supply to check this point. An oscillator
that won't oscillate at a low plate voltage will drop in and out of
oscillation with a "plop" as the voltage is increased from zero and
decreased back again, and hence is not as susceptible to key filtering
as one that will work at a low voltage. Straight pentode oscillators
and some triode oscillators will require additional feedback to make
them oscillate at less than 10 or 15 volts. Under critical adjustment,
the Tri-tet will oscillate with no apparent plate voltage when the circuit
is closed (as is well known), but this is caused by the contact potential
of the tube and the drop through the cathode circuit. The grid-plate
oscillator (Fig. 3) will oscillate at very low voltages with proper
proportioning of the cathode condenser, C5.
month we presented a story pointing out some of the factors influencing
the keying of amplifiers and self-excited oscillators. This follow-up
article treats some of the considerations in crystaloscillator keying
and the blocked-grid keying of amplifiers (and keyer tubes) and, although
it may not offer the cureall for your particular problem, it may start
you in the right direction towards clearing up your keying troubles.
Fig. 4 - Oscillograms
of a keyed 6C5 crystal oscillator. A shows the triode adjusted for optimum
keying with a 10,000-ohm grid leak, B shows the triode with cathode
bias and tuned for optimum keying, and C shows what happens when the
cathode- (or grid-) leak biased triode oscillator is tuned too much
to the lowfrequency side of the optimum keying adjustment. The dots
in C become light and the keying is somewhat erratic.
Fig. 5 - A 6AG7 leak-biased
Tri-tet oscillator with the plate tuned to the fundamental frequency.
A shows the oscillator tuned for optimum keying, and B shows the same
with optimum key filter. Both A and B are keyed in the negative lead;
C is keyed in the cathode with the same filter as B. Adding capacity
to C did not extend the tail on "break" enough to reduce the click to
a good value.
Another important factor in the adjustment of a crystal oscillator
for best keying is that it be keyed while tuned. Electronic bug owners
will find this a simple matter, while the straight key or mechanical
bug owners will have to content themselves with sending a series of
dots while tuning the oscillator. It is relatively easy to hit the best
tuning adjustment by listening to the signal in the receiver, but one
can end up with some rather horrible keying if he just tunes the oscillator
for maximum output and then keys it. This is assuming, of course, that
a proper key filter has already been installed and that the switch to
a different crystal has just been made. The key filter constants can
be determined in the same manner as described last month for self-excited
oscillators. Be sure to listen with the r.f. gain of the receiver well
reduced, else the receiver is likely to give too pessimistic a picture
of the clicks.
If the oscillator circuit is one using a screen
grid tube, the screen circuit should not be overlooked when adjusting
for minimum clicks. The size of the dropping resistor is usually fixed
by the screen operating voltage, but the size of the by-pass condenser
can increase or decrease the clicks, depending upon the type of circuit.
It is suggested that a 0.001 by-pass condenser be used right at the
socket, for a short r.f. path, and then different values of shunting
condensers can be tried at some more accessible point. Here again testing
is most readily done by sending a steady string of dots and listening
to the signal (with the b.f.o. turned off) while different values of
screen by-pass condensers are tried. The screen adjustment is best made
before the key filter is adjusted. The additional capacity should be
added on the tube side of the dropping resistor, of course.
Loading has an effect on the keying of oscillators where the feedback
is obtained from the plate circuit, as in the case of the straight tetrode
or triode oscillators, but it doesn't seem to be very important in circuits
like those shown in Figs. 2 and 3.
A conclusion from the work
described in this article is that the regenerative type of crystal oscillator
(Tri-tet and grid-plate) keys better than the straight triode, tetrode
and pentode oscillators. Not only do they seem to work more uniformly
with different crystals, but their optimum keying is more likely to
occur at the maximum output point. It may very well be possible to make
a triode or multi-element tube oscillator show similar results by adding
additional feedback from plate to grid, but the Tri-tet and gridplate
oscillators are easier to control.
The use of a blocking voltage on the control (or suppressor)
grid of a tube to cut off its output until the blocking voltage is removed
by the shorting of the key, as shown in Fig. 6, is an excellent method
of keying an amplifier. The resistor R1 is the normal grid
leak and R2 is a resistor used to prevent the blocking-voltage
supply from shorting when the key is down. The capacity C1
is the normal r.f, by-pass plus any additional capacity necessary for
a good keying characteristic. A nice feature of grid-block keying is
that it requires no inductance to give a lag on "make," the lag coming
from the time constant of C1 discharging through R1.
On "break," the constant is determined by C1 charging through
R1 plus R2. Since the grid leak, R1,
is determined by the tube .that is being keyed, adjustment of a grid-block
keying system consists of adding enough capacity across C1
until the" make" is as soft as desired and then, if the "break" still
shows some click, raising the value of R2 until desirable
keying is obtained. The same rule as set forth in the previous article
applies - it is preferable to have a harder "make" than "break" for
good copy at high speeds, and this is obtained automatically with grid-block
keying. The same adjustment procedure applies to tube keyers (that use
a blocking voltage) and to suppressor-grid keying.
keying is most convenient in amplifier stages using high-μ tubes that
aren't being driven too hard, since such stages will require a lower
voltage for cut-off.
Unfortunately, grid-block keying does not
work any too well with oscillators. It can be used, of course, but it
isn't possible to get a soft "make" characteristic because the bias
must be brought down to a value that gives a high enough mutual conductance
before the tube will oscillate and it then plunges into oscillation
in the usual manner. Further, a soft "tail" is not added to the oscillator
when grid-block keying is used as is added to an amplifier keyed this
way. The closest approach is suppressor-grid keying of a Tri-tet or
grid-plate oscillator, and these both require that the oscillator run
constantly, prohibiting break-in on one's own frequency without elaborate
shielding and neutralization.
Fig. 6 - Grid-block keying
circuit. R1, is the normal grid leak and C1, is
the r.f. by-pass condenser plus enough capacity to give a good keying
characteristic. R2 is included to prevent a short circuit
of the blocking-voltage supply when the key is closed. Increasing the
size of C1, will make the keying "softer" on both "make"
and "break;" making R2 larger will soften "break."
Fig. 7 - Grid block keying of an amplifier. A shows the characteristic
with only the normal grid leak and r.f. by-pass condenser, B shows the
addition of 0.1 μμfd. across the condenser. The clicks of B were very
slight, with almost none at all on "break." Note that the addition
of capacity in B has made the dots "heavier," requiring a slight readjustment
of the key if a bug is used.
In addition to the keying checks
listed last month, the following applies specifically to keyed crystal
11. Holding the key down and tuning the crystal
oscillator for maximum output does not always give the optimum keying
adjustment. Send a string of dots and tune the oscillator for best keying.
2. A crystal oscillator should be capable of oscillating with
only 3 or 4 volts on the plate if it is to key well.
3. In adjusting
the lag filter at the key, don't overlook the effect of the value of
screen bypass condenser if the oscillator is one that depends upon the
screen for feedback (Tri-tet or grid-plate oscillator doubling or with
4. Use an r.f. choke in series with the
grid leak and as high a value of leak as is consistent with adequate
5. Don't be surprised if some crystals key better than
others in the same circuit.