|
|
|
Changing from Battery Bias to Automatic Bias
|
| The following article
first published in 1933, describes the steps needed to convert
a wireless set using battery bias to "automatic
bias". |
|
By. H. J. Barton Chapple, Wh.Sch., B.Sc. (Hons), A.C.G.I,
D.I.C., A.M.I.E.E.
When a valve is used as a low-frequency amplifier or as a
power output valve, and sometimes when employed as a
high-frequency amplifier, it is necessary to apply negative
grid bias. This operation consists of giving to the control
grid of the valve a permanent negative charge, and its object
is to fix the mean or average value of the grid voltage of the
valve at such a value that maximum signals call be handled
without distortion. Incidentally, if the grid bias applied is
maintained at the figure given by the valve makers, listeners
may rest assured that they are also operating their sets under
the most efficient conditions as far as high tension current
consumption is concerned.
|
 |
Advantages of Auto Bias.
There is, however, a rapidly growing tendency, to obtain grid
bias, at any rate from mains receivers, and to a certain
extent in battery-operated sets, by means of what is called
the "automatic" grid bias or "free" grid
bias system. The advantages of automatic grid bias are many.
In the first place, a grid bias battery, like every other dry
chemical battery, has but a limited life, even though it is
not called upon to deliver current.
In time, therefore, its voltage drops considerably and,
because such batteries are small and often tucked away in some
place whore they are easily overlooked, the set may be running
with the low-frequency valve or valves seriously under-biased
without anyone noticing the fact. This not only means bad
quality reproduction but is also likely to injure the valves
because, as grid bias is reduced, the anode current rises.
Serious underbiasing therefore means excessive anode current
which tends to shorten the life of the valve. In automatic
grid bias the bias voltage depends definitely upon the value
of the anode current, so that the bias automatically
increases, should the anode current tend to rise and will thus
bring the anode current back to the normal value.
Again, if for any reason the high-tension voltage drops, as
when the high-tension battery begins to run down, then, with
battery bias, the valves would soon find themselves
over-biased with a corresponding great deterioration in
quality of reproduction. With free or self bias, however, the
bias voltage is again decreased automatically in proportion as
the H.T. voltage drops.
Another reason for the employment of auto bias in battery sets
is the light weight. and small compass of the apparatus
required to provide bias, compared with that of a grid bias
battery. This is of particular value in portable sets where
every cubic inch of space saved is welcome and any reduction
in weight renders the portable set more truly
"portable."
Same Practical Effect.
Although the components for auto bias are somewhat more costly
than a small grid battery, they never require renewal and,
even in first cost they are cheaper than a dry battery if the
output valve is of the big type requiring from 18 to 30, 50,
or even 100 volts bias.
In battery bias, as we have seen, the grid is made definitely
negative with respect to the cathode or filament. In auto bias
the same practical effect is obtained by making the cathode
positive with respect to the grid. That the two systems are
identical in effect will be clear when it is realised that a
valve operates by virtue of differences of potential and not
actual voltages.
Thus, from an operating point of view, it does not matter in
the least whether the cathode is at zero potential and the
grid at 10. volts negative, or whether the grid is at zero
potential and the cathode at 10 volts positive.
In either case the grid is 10 volts negative with respect to
the cathode. |
| Fig.2 - Biasing an indirectly heated L.F. valve |
Fig. 2 shows a typical method of applying
automatic bias to an indirectly heated low-frequency A.C.
valve.
The circuit has been stripped of certain elaborations which
are desirable in practice, but the omission will make the
explanation of the principle clearer. It will be seen that the
secondary of the inter-valve transformer is connected between
the grid and the common negative wire, while a resistance is
connected between
the cathode of the valve and the common negative. It will be
understood that if H.T.- is taken as zero potential, any point
in the anode circuit of the valve will be at a higher
potential. The greatest potential difference will, of course,
be between points A and D (H.T.+ and HT-). C, the anode of the
valve, will be at a lower potential than D but still very
positive with respect to A. B will be at a lower potential
than C but still positive to A. But if B is positive to A, A
must be considered negative with respect to B and, since the
grid of the valve is connected back to A, and is therefore at
the same potential, B is positive with respect to the grid or,
in effect, the grid is negative with respect to the cathode.
In other words, negative grid bias is applied automatically.
For H.F. Valves.
The amount of negative grid bias depends upon the value of the
anode current flowing through the biasing resistance and the
value of the resistance itself; actually the bias is equal to
the voltage drop in the biasing resistance and we shall see, a
little later on, how to calculate the best value. Meanwhile,
however, it is first necessary to give the practical
arrangements by which auto bias can best be applied to valves
of various types. First of all we will deal with indirectly
heated A.C. mains valves, because it is valves in this class
for which the automatic bias principle is chiefly adopted.
|
 |
Most indirectly heated screened grid valves
require a small amount of negative bias in order to prevent
grid current distortion - the usual value of bias being about
1.5 volts. The method indicated in Fig. 2 is quite suitable,
but it is preferable to shunt the bias resistance with a fixed
capacity condenser of .01 mfd. or greater, as shown in Fig. 3.
In the case of an indirectly heated variable-mu screened grid
valve, provision must be made for varying the bias for volume
control purposes, and the grid bias resistance must,
therefore, be a rheostat. The actual value is generally
recommended by the valve maker but is usually of the order of
7,500 to 10,000 ohms for long grid base valves and, of course,
correspondingly less for valves with a restricted grid base.
Many valve makers recommend a special self compensating
circuit for this purpose and, in any case, it is advisable to
include a fixed resistance in series with the adjustable unit,
its value being calculated to give the 1.5 volts or so that is
necessary to avoid grid current. |
| Fig.4 - The method of decoupling the biasing circuit of an
indirectly heated valve. |
On the L.F. Side.
Coming, to low-frequency amplifying valves, we will take first
an early stage valve -not the output valve. Here again the
circuit shown in Fig. 2 is satisfactory, but should be
modified as shown in Fig. 4. Here the grid circuit, instead of
being connected directly to the common negative, is connected
through a high resistance
of about 50,000 ohms, a condenser of from 1 to 2 mfd. being
connected between the secondary of the transformer and the
cathode. Those two components form a decoupling circuit very
similar to that frequently employed in the anode circuits of
valves. Its object is to prevent any extraneous variations,
such as mains ripple, present in the high-tension supply,
being passed on to the grid of the valve when, of course, it
would be amplified by the valve and produce hum. This
decoupling circuit is not always necessary, and listeners who
are converting their sets to automatic bias might try the
effect of the bias resistance by itself, adding the decoupling
resistance if any considerable hum is noticed. |
 |
| A slightly different system of automatic biasing
is recommended for output valves of the three electrode type,
whether indirectly or directly heated. This is shown in Figs.
5A and 5B and is so designed that the biasing resistance is
not included in the output circuit of the valve. In other
words, instead of connecting the grid to common negative and
raising the potential of the cathode, the cathode is connected
to the common negative, which is raised to a positive
potential with respect to H.T. - and the grid is connected to
H.T. --- By thus removing the bias resistance from the output
circuit the loss of audio-frequency energy represented by the
power dissipated in the biasing resistance is avoided. |
| Fig.5b - Biasing a directly heated output valve. |
It will be noted that the two diagrams Figs. 5A
and 5B are identical except that, in the case of the directly
heated valve, the centre tap of the filament winding is
connected to the common negative.
Resistance Circulation.
Indirectly heated pentode valves can, if desired, be biased in
the simple manner shown in Fig. 4, because the impedance of
the load in a pentode output circuit is usually high compared
with the biasing resistance and the losses in the resistance
are therefore not so serious. The arrangements shown in Fig.
5A, however, is preferable. Diagram 5B must, of course, be
employed for, a directly heated pentode.
It must he remembered that the circuits shown in Figs. 5A and
5B are such that the who whole high-tension current of the
seat, and not only the anode current of the output valve
passes through the bias resistance, and in calculating the
value of the resistance this face must be kept in mind.
Provided the calculation is correctly made, and providing the
total high-tension consumption of the set is constant, the
system if perfectly satisfactory. If, however, one or more
variable mu valves are employed An the high-frequency stages,
their anode currents will vary according to the setting of
their own biasing resistance, and this would upset the bias
arrangements for the output valve.
In such a case, therefore, a further variant of the biasing
circuit is necessary. This is given in Fig. 6, and is similar
to that shown in Fig. 4, except that the resistance is again
excluded from the output circuit. |
 |
The method of calculating the bias resistance is
quite simple, Divide the required bias voltage by the anode
current in milliamperes and multiply the answer by 1,000. This
gives at once the value of the bias resistance in ohms.
Example:To find the correct bias resistance for a valve
requiring a bias of 18 volts and taking 30 milliamps.
Resistance = 18 x 1,000/30.
= 600 ohms.
As it it is not always possible to obtain a fixed resistance
of exactly the value required, it is an advantage to use a
semi-adjustable resistor of rather higher maximum value than
the correct figure found by calculation, and this will allow
of a certain margin for final adjustment for best results.
Some designers go farther, and use two resistances for
biasing, one fixed and the other variable, connected in
series. The object is to avoid running the valve. with no bias
or very low bias, if, by error, the adjustable resistance is
moved too far over towards its minimum position. |
|
