February 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 are hereby acknowledged.
2011, designing a frequency converter circuit consists in most cases
of picking out an IC that has the characteristics you need from
a gain and mixer spurious product standpoint, add a couple filters,
and a power supply. In many cases the oscillator is part of the
IC. Of course there are special cases where you have to use a basic
mixer and do everything yourself, but even that is simpler than
designing a tube circuit. It really is amazing what engineers and
hobbyists of yore were able to accomplish using point-to-point wiring
and a slide rule.
Here is a good article form the February
1941 QST magazine that discusses some of the considerations. Maybe
you have an old radio that this knowledge will apply to.
See all available
vintage QST articles
Practical Design of Mixer Converter Circuits
Comparison of Tube Types and Checking Performance
R. Hammond (W9PKW)
THE design of an efficient mixer or converter circuit is often
the one thing that prevents the amateur from building his own communications
receiver. In application the amateur usually is unable to tell whether
or not the stage is giving normal performance and, lacking equipment
for checking gain, no attempt is made to find out if it is doing
the job efficiently. However, there are simple ways of determining
whether or not a mixer or converter is operating efficiently, and
it is the purpose of this discussion to explain these methods and
to give some theory on the operation of converters. The general
characteristics of the several mixers and converters now available
are also given, with a general discussion of the performance characteristics
An elaborate mathematical theory of the operation
of a converter or mixer1
is of no great importance for
our particular problems. Roughly, a converter operates as follows:
Within the tube there is developed a current at oscillator frequency
which is modulated by the incoming signal to produce an intermediate
frequency. The ability of the tube to develop a current at an intermediate
frequency is given by the" conversion conductance," which by definition
is the ratio of an incremental change in intermediate frequency
current to the incremental change in r.f. signal voltage that produces
the current. This conductance in micromhos is published for all
converters, and its use to calculate stage gain is analogous to
the use of mutual conductance with r.f. amplifiers. The gain equation
for a single tuned load is
is the conversion conductance, Rp
is the plate resistance, and RL
is the tuned load resistance.
Published values of plate resistance and conversion conductance
can therefore be used to calculate conversion gain. The tabulation
following gives a comparison of gain for a group of tubes now generally
available. The gain figures were calculated for a tuned load impedance
of 200,000 ohms, which is equivalent to the better transformers
If gain was the only consideration the above would suffice for
the selection of a converter tube. Tube noise is generally not a
consideration when comparing converters simply because the converter
is inherently a noisy device and most converters develop noise voltages
of approximately the same magnitude. The noise output of converters
of the 6A8 and 6SA7 type is approximately 4 times greater than that
of an r.f. amplifier like the 6SK7 or 6K7. Where the ultimate in
signal-to-noise ratio is desired it is necessary to precede converters
of this type with an r.f. stage. Usually the selection of a converter
is based on the characteristics of oscillator stability with regard
to a.v.c. and terminal voltage fluctuation, pull-in characteristics,
oscillator transconductance that determines the ease of oscillation
especially at high frequencies, and other deleterious characteristics
that cause loss in performance at certain frequencies. The chart
on page 41 indicates some of the characteristics of the various
converters. The gain figures and notes on stability and oscillator
transconductance are of particular importance.
the converters perform equally well as mixers or as converters with
the exception of the one characteristic of oscillator stability.
Any of the converter tubes gives good stability if used with a separate
oscillator and the circuits are isolated properly. Of the group
the 6SA7 makes the best mixer because it gives high gain and has
improved internal shielding of the signal and oscillator grids.
The improved shielding is accomplished by using
What's the best mixer tube? How can a mixer circuit be tested
to find out if it's doing the best job it can? Here are the
answers - plus design information of highly practical value.
shielding plates similar to the beam-forming plates used in beam
power tubes. These plates are attached to the side rods of the screen
grid and confine the electron currents to beams which get into the
outer regions of the tube where they are modulated by the signal
grid. The sketch of Fig. 1 shows the construction of the 6SA7. The
side rods of the No.3 or signal grid are mounted so that they split
the' beam and make the electrons travel in radial paths. Electrons
turned back by the signal grid because of a strong r.f. voltage
do not return to the oscillator or No. 1 grid because they are caught
by the collector plates. This reduces coupling between the signal
and oscillator grids and improves stability. Simple structures of
cylindrical grids such as used in the 6L7 and 6A8 do not have this
additional isolation and are therefore not quite as good as the
6SA7. The improvement in stability evidences itself in the form
of greater freedom from "pull-in " - that is, shifting of the oscillator
frequency with signalgrid tuning or with a strong signal on the
signal grid. This effect is usually not as serious as frequency
shift due to terminal voltage fluctuation. The remarks relative
to stability, given in the tabulation on page 41, refer to the stability
with regard to terminal-voltage fluctuation.
for the six converters listed in the tabulation are shown in Figs.
2 to 7 inclusive. The 1A7G, 1R5, 6K8, 6A8, and 6SA7 can be used
with separate oscillators simply by connecting the oscillator grid
of the converter to the oscillator grid of the oscillator tube.
The screen and other positive electrodes should be maintained at
their normal rated d.c. voltages but should be by-passed to ground.
Fig. 2 shows connections for a converter circuit using
the 1A7G and Fig. 3 shows connections for the 1R5. The 1R5 is one
of the new miniature tubes for hearing aids and small portable receivers.
The 1A7G has the conventional 6A8 construction, using an anode for
feedback. The chart above indicates that the gain obtainable with
either tube is approximately 34. The oscillator transconductance
of the 1R5 is slightly higher and the oscillator stability is somewhat
better. These two features are of advantage for high frequencies.
Figs. 4, 5, 6 and 7 show connections for converter
circuits with types 6A8, 6K8, 6J8G and 6SA7 respectively. The high
oscillator transconductances of the 6K8 and 6SA7 make them particularly
suited for all-around usage. They oscillate strongly at high frequencies
where Lie ratios are unfavorable. The 6A8 construction is not satisfactory
for amateur usage because of instability in the oscillator. The
oscillator electrode is a pair of rods located in the tube between
the No.1 grid and the screen. These side rods collect electrons
from the cathode stream and the electrode current is controlled
by the No.1 grid. Unfortunately, changes in signal-grid or screen
voltage also change the anode current. This conductance between
signal grid and oscillator causes instability with variation in
a.v.c, voltage. Fluctuations in screen voltage due to supply regulation
also change the frequency. As a result, the 6A8 is subject to motorboating
or "put-put" at high frequencies. Dial calibrations also drift with
line voltage fluctuations. "Pull-in" is particularly bad with the
The 6J8G construction incorporates a triode oscillator
and a mixer section with a common cathode. This construction results
in good stability insofar as screen and a.v.c. voltages are concerned.
The 6J8G has two serious disadvantages, however, that have limited
its application. The triode section shares a portion of the cathode
area. The area used by the triode is quite small and as a result
the oscillator transconductance cannot be made high. Also, at high
frequencies a peculiar effect is experienced that causes a flow
of current to the signal grid. This current causes a high negative
potential across the resistance in the grid return, and this bias
reduces the gain of the mixer. The effect can be reduced somewhat
by using a high value of screen voltage, but it is then necessary
to increase the bias to hold the cathode current to a safe value.
Fig. 2 - Converter circuit for the 1A7G or 1A7GT.
Fig. 3 - The 1R5 converter circuit.
Fig. 4 - Converter circuit for use with the 6A8. 6A8G or 6A8GT.
Fig. 5 - The 6K8, 6K8G or 6K8GT converter.
Fig. 6 - Converter circuit for the 6J8G.
Fig. 7 - The 6SA 7 converter circuit.
The 6K8 has been used extensively by the amateur and also the commercial
manufacturer principally because it gives fair stability, and design
problems are usually simple. The tuned-grid oscillator shown in
Fig. 5 gives very little trouble and is easy to build. The oscillator
frequency is not independent of screen and .v.c. voltages, but in
most designs the frequency shift caused by one is offset by the
other so that good stability is obtained. The 6K8 has an effect
known as spacecharge coupling which is experienced at high frequencies.
This effect is as follows: The oscillator voltage on the No.1 grid
causes a fluctuation in the number of electrons in the region of
the signal grid. The electron density changes at the oscillator
frequency and as a result a displacement current flows into the
signal grid. At high frequencies where the signal grid and oscillator
frequencies are quite close, the impedance of the signal grid circuit
at the oscillator frequency is quite high and as a result the displacement
current produces an a.c. voltage across the signal grid circuit.
This voltage, when smaller than the bias, reduces the gain of the
tube slightly. Under extreme conditions it overrides the bias and
causes rectification in the signal-grid circuit, causing a serious
loss in gain. The coupling can be neutralized by a small capacitance
- approximately 2 or 3 μμfd - between oscillator and signal grids.
Commercial practice is to use a condenser (known as a "gimmick")
made by wrapping two pieces of wire together to give the desired
capacitance. Neutralizing the space charge increases the gain and
Fig. 1 - Diagram of the 6SA7 structure, showing
The 6SA7 construction has already been described. Using cathode
feedback in the Hartley circuit shown in Fig. 7, excellent stability
is obtained. The gain is quite high and the high oscillator transconductance
makes a good oscillator.
The 6SA7 converter is tricky to
use because the cathode returns through the oscillator coil. This
connection, however, is the secret of the stability resulting with
the 6SA7. The feedback is obtained from the total cathode current.
A.v.c. voltage variations on the signal grid do not change the cathode
current appreciably so that the oscillator frequency is almost independent
of a.v.c. Screenvoltage variation produces a shift in frequency
in the opposite direction and the two effects practically cancel.
The frequency change with either variable is reduced by using the
optimum tap on the oscillator coil. With average oscillator coils
the tap should be adjusted to give a total oscillator voltage of
approximately 10 volts grid-to-ground. Under these conditions the
oscillator grid current measured in the grid leak will be approximately
0.5 milliampere. This current can be measured with a 0 to 1 milliammeter
by connecting it at the bottom of the grid leak.
frequencies it is necessary to keep the leads connecting the cathode
to the coil, and the bottom of the coil to ground, as short as possible.
The cathode lead in particular should be short. The inductance of
this lead is not a part of the oscillator tank and oscillator voltage
developed across it does not contribute to feedback. The voltage
does bias the signal grid, however, and will reduce the gain of
the converter. Under extreme conditions the voltage may be high
enough to cause a flow of current in the signal-grid circuit. This
current results because of high voltage between cathode and ground
and because of phase shift of this voltage with respect to the voltage
between grid and cathode on the coil. The cathode connection to
the coil should also be made so that the lead pulls away from the
coil at right angles. By pulling the wire away parallel to the winding
the cathode-lead inductance may cancel a portion of the tap-to-ground
In band switching arrangements the circuit
of Fig. 8 is recommended. It will be noted that the tap switch on
the oscillator coil is located at the ground end of the coil. This
puts the inductance of the switch and its connecting leads within
the closed tank circuit. Since the tank currents flow through this
inductance it contributes to feedback and gives oscillation with
a minimum of cathode-to-ground voltage. If the switch was between
the cathode and the coil in the position of lead 1 the drop across
the switch inductance would not contribute to oscillation, but would
produce a high cathode-to-ground voltage. As mentioned above, this
voltage is shifted in phase from the voltage in the tapped portion
of the coil and may cause the signal grid to be driven positive
and cause rectification.
The circuit of Fig. 9 shows the
6SA7 as a mixer. It will be noted that the neutralizing condenser
. is used to neutralize the space charge. The 6SA7
as a mixer gives an increase in gain over that realized as a converter.
Space-charge coupling is also experienced with the
6SA7, and a "gimmick" is required for neutralization. This coupling
is characteristic of converter or mixer systems wherein the oscillator
voltage is injected next to the cathode or filament. The 6J8G, although
not having this coupling, has the transit-time effect which is just
as bad and cannot be neutralized. The transit time effect is experienced
with converters or mixers in which the oscillator voltage is mixed
in the cathode stream outside of the signal-grid injection.
* Circuits using both plate and screen current
for feedback can be employed and the effective transconductance
is then 1200 micromhos.
** Transconductance in micromhos at
rated conditions. Note - Gain figures are relative for a tuned load
resistance of 200,000 ohms.
It might be of interest
at this point to give the accepted theory on what causes the transit
time effect. Electrons accelerated through the No.2 screen grid
approach the No. 3 injector grid. At high frequencies, where the
time of transit between cathode and No.3 grid is an appreciable
portion of the period of oscillation, electrons accelerated by the
No.3 grid on its positive swings reach the grid at a time when it
is going negative and are repelled and turned back toward the screen.
On the way back they are accelerated by the positive potential on
the screen and by the increasing negative potential of the No.3
grid. Many of these returning electrons reach the screen and are
drawn off as additional screen current. Some of the electrons, however,
pass very close to the screen and are accelerated toward the No.
1 grid at high velocity; many of them obtain sufficient energy to
overcome the negative potential of the No. 1 grid and flow in the
external No. 1 grid circuit. This flow of current is d.c., and in
a direction such that the drop in the external resistance increases
the bias. If the tube is operated from the a.v.c. string as in the
conventional case, the total return to ground is of the order of
two megohms. A current of several microamperes increases the bias
sufficiently to cause an appreciable loss in gain. The current can
be eliminated for frequencies up to approximately eighteen megacycles
by increasing the bias and the screen voltage.
Fig. 8 - Recommended oscillator switching for the 6SA7.
Fig. 9 - The 6SA 7 mixer, separately excited by a 6J5 or 6J5G
Fig. 10 - Circuit for making performance tests on the 6SA 7
Fig. 11 - Triode mixer with separate oscillator.
The above information
should be useful in determining the converter to be used for a particular
job. Once the converter is built it is comparatively easy to ascertain
whether performance is satisfactory. Of course in the laboratory
the most satisfactory method is to check stage gain with a signal
generator, but few of us have signal generators with which to make
precision measurements. We usually rely on the sound of the set
and whether it pulls in the signals.
The first check
on any converter is to measure the electrode voltages with a high-resistance
meter. The correct voltages are indicated for the various circuits.
Next in order of importance is to check to see if the oscillator
amplitude is high enough. The easiest method of checking this is
to measure the d.c. grid current in the grid leak. This grid current
increases directly with oscillator voltage and is so closely related
to oscillator voltage that manufacturers, instead of rating the
oscillator voltage to be used with a converter, rate the grid current
as measured in a recommended grid leak. On each of the preceding
circuits the rated oscillator grid current is given. In practice
the grid current cannot be held to this value over the band, especially
if a wide tuning range is desired as in commercial broadcast sets.
In communications receivers where the tuning range is small the
variation is not large. A 2-to-1 variation in a set having a wide
tuning range is not bad. If rated grid current is obtained in the
middle of the band the variation over the band is usually not excessive.
The grid current is important because it determines the point of
optimum gain, and other than rated value results in a sacrifice
Converters using the 6A8, 6K8, 6SA7,
1A7G, or IR5 should next be neutralized for space charge coupling.
This is accomplished by connecting a "gimmick" between the oscillator
and signal grids. If a gang condenser is used and the oscillator
and signal grid sections are adjacent, neutralization can be accomplished
by connecting the "gimmick" between the stators of the two sections.
Commercial practice is to solder two small pieces of wire to the
stator lugs and then to twist the ends together. About two turns
is satisfactory. Note: Neutralization is done on the highfrequency
edge of the highest-frequency band. Low-loss wire should be used.
The capacitance should be adjusted to give maximum sensitivity.
There are several phenomena that can take place that
will upset performance after the above considerations have been
observed. Parisitic oscillations take place in the oscillator section
if too much feedback is used or if the values of grid coupling condenser
and grid leak are too high. A 50-μμfd grid condenser is usually
satisfactory for most circuits. Most grid-leak specifications call
for 50,000 ohms. Battery tubes having low oscillator mutual are
specified with as high as 200,000 ohms, and the 6SA7 with its high
oscillator mutual or transconductance is rated with 20,000 ohms.
If the oscillator and signal-grid circuits are not adequately shielded
and isolated, severe coupling between circuits is obtained at some
frequencies. The signal-grid circuit in extreme cases may load the
oscillator enough to cause it to stop oscillating. This effect can
be detected by observing the oscillator grid current as the set
is tuned through the coupling point. A rapid dip in the oscillator
grid current is experienced as the coupling point is passed. Shielding
of coils and isolation of parts and leads eliminates this trouble.
Motorboating on strong signals is the result of oscillator shift
with a.v.c. and other element voltage variation. It was pointed
out that the 6A8 was particularly bad in this respect, that the
6K8 was much better, and that the 6J8G and·6SA7 are very good. Motorboating
can be experienced with the 6J8G and 6SA7 if powersupply regulation
is bad and if the oscillator amplitude is not adequate. Stability
is improved by operating at or somewhat over rated amplitude.
The major troubles experienced with converters produce a
flow of grid current in the signal-grid return. This is true of
the transit time effect with the 6J8G, the space charge effect with
6K8, 6SA7, 6K8, 1A7G and 1R5, and the phase shift of the high cathode
to ground voltage in the 6SA7. The circuit of Fig. 10 shows how
a check for signal-grid current can be made without the use of a
sensitive microammeter. An electron-ray indicator tube such as the
6U5/6G5 will indicate any current flow in the a.v.c. return. Most
returns have about three megohms total and a d.c. current of 1 microampere
will produce 3 volts, which will make a noticeable deflection on
the target. The voltage drop between the bottom end of the coil
and ground should never exceed approximately 1.5 volts. This voltage
can exist because of contact potential in the diode and other grids
connected to the a.v.c. system, and does not indicate trouble.
Signal grid current with the 6A8, 6K8, and 1A7G usually
results from space-charge coupling, as already described. A convenient
test for its presence is to short the signal-grid tuned circuit
with a condenser. This shorts out the voltage and eliminates the
current. 'The "gimmick" when adjusted properly neutralizes space
Signal-grid current because of space-charge
coupling is also obtained with the 6SA7 but in addition current
can flow because of high cathodeto-ground voltage and phase shift
of this voltage with respect to the oscillator grid-to-cathode voltage.
If bypassing the signal grid does not eliminate the current, the
trouble will be found in the oscillator coil and connecting leads.
The cathode lead should be kept short and the circuit of Fig. 8
adhered to. The ratio of length to diameter of the oscillator coil
should not exceed more than about 1.5 to 1. With long coils and
small diameters there is appreciable phase shift with attendant
troubles. As mentioned previously the cathode lead should pull away
from the coil at right angles so that it does not couple to the
Recently, certain manufacturers have used triodes
for mixers. A typical circuit for this type of mixer is shown in
Fig. 11. It will be recognized as similar to many of the circuits
used in the older days. In commenting on this circuit it might be
said that the chief advantage of the triode is that it develops
very little noise. It is thus possible to add extra gain behind
the converter in the i.f. and get high sensitivity with a good signal-tonoise
ratio. The triode in this connection has serious disadvantages,
however. It is necessary to use a special low-impedance primary
i.f. transformer so that the grid-to-plate capacitance of the triode
will not cause loading of the signalgrid circuit. In the practical
case the tuning condenser required to tune the i.f. primary is approximately
2000 μμfd. The high cathode-togrid capacitance causes severe coupling
of the oscillator and signal-grid circuits. This evidences itself
in the form of instability with a.v.c. variation, "pull-in " on
strong signals, and oscillator shift with tuning of the signal grid
circuit. In applications where stability is not of prime importance
a pentode such as the 6SJ7 or 6AB7/ 1853 could be used to give good
signal-to-noise ratio. The low signal-grid-to-plate capacitance
in these types would allow the use of conventional i.f. transformers.
* Ken-Rad Tube & Lamp Corporation,
1 In common terminology, a "converter"
is a tube performing the dual functions of mixer and oscillator;
a "mixer" does not incorporate an oscillator section. Any converter
tube can be used as a plain mixer by providing excitation from a
separate oscillator tube. - ED.
Posted 6/16/ 2011