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
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
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
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.
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
"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 be added.1
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.
Combination oscillators 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."
sometimes 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
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.
Last 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
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
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.
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
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 oscillators.
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 wellscreened
4. Use an r.f. choke in series with the grid leak
and as high a value of leak as is consistent with adequate output.
5. Don't be surprised if some crystals key better than others
in the same circuit.