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 (if any) 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.
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)
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 of each.
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 now available.
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.
In general 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. Converter
Typical circuits 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 6A8.
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 image ratio.
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.
At high 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 inductance.
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 Cn
. 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
** Transconductance in micromhos at rated conditions.
Note - Gain figures are relative for a tuned load resistance of 200,000
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 oscillator.
Fig. 10 - Circuit for making performance tests on the 6SA 7 converter.
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
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 in performance.
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 coil.
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, Owensboro, Kentucky.
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