This is only a preview of the April 1991 issue of Silicon Chip. You can view 49 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Build The SteamSound Simulator":
Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
|
VINTAGE RADIO
By JOHN HILL
Converting an Airzone to AGC
Although automatic gain control (AGC) became
common in receivers from the early 1930s on, ·
there were a number of sets made at around
that time which lacked this useful refinement.
This month, we look at how a 1937 Airzone
receiver was fitted with AGC.
It was one of those things that I had
been meaning to do for some time. In
my collection of old radios, there is a
rather stately looking Airzone - a 1937
console model which has no AGC.
Just why it lacks AGC is a bit of a
mystery, for nearly all radios of that
era were fitted with this feature .
At one stage, I suspected that an
This is the 5-valve Airzone that was
converted to AGC. It is a 1937 console
model with a 10-inch electrodynamic
loudspeaker. Adding AGC makes it a
far more pleasant set to use.
36
SILICON CHIP
AGC circuit must have been lurking
in there somewhere but had become
inoperative. However, a close inspection of the chassis (as best as one
could without a circuit diagram) soon
confirmed that the set had never had
AGC. There were two reasons for this
.conclusion:
(1). the receiver had no valves which
contained diodes; and
(2). the volume control was a wire-
wound potentiometer in the cathode
circuits of the frequency converter and
IF amplifier valves.
Clearly, the set was never designed
for AGC.
The valve complement (confirmed
by a sticker inside the cabinet) is as
follows: 5Y3, 6A8, 6U7, 6J7 and 6F6 .
This is a fairly standard. layout for ·a
1937 5-valve receiver, with the exception of the 6J7. Most sets of that
vintage would use a 6B6 or a 6B8
instead. Both these valves contain
diodes which would have made AGC
easy to implement.
Valves with diodes
As explained in last month's column, there was a change in superhet
design in the early 1930s, when valves
containing diodes were first intro-
The Airzone chassis. The valve complement is as follows: 5Y3, 6A8, 6U7, 6J7
and 6F6. This is a fairly standard line-up for a receiver of that era with the
exception of the 6J7 (most sets would have used a 6B6 or 6B8).
duced. These diodes could be used
for both an improved form of detection (diode detection) and a more
practical form of AGC than any previous attempts. Along with these
changes came a change in volume
control methods. The volume control
was moved from the cathode circuit
of the IF valve(s) and, in some cases,
the frequency converter as well, to
the control grid of the first audio
valve.
An AGC circuit rectifies the received signal and produces a negative voltage which is directed back to
the grids of the IF and converter
valves, to control their gain. Strong
signals increase the AGC voltage,
which reduces the gain, whereas weak
signals have the opposite effect.
As AGC circuits work with variable mu valves, there was little reason why the old Airzone could not be
converted to ACC. Both the frequency
converter and the IF amplifier valve
were common types, as used in con. junction with AGC in other receivers.
All that was needed was to add a
couple of diodes and a few other bits
and pieces.
Conversion options
There were a number of ways to
approach the diode problem. First, a
different first audio valve such as a
6B6 or a 6B8 could be fitted, which
would have a pair of diodes in the
same envelope. Another possibility
was to fit a separate twin diode valve,
thus retaining the existing 6J7 as the
first audio valve. As a last resort, a
couple of modern solid state signal
diodes would do the job just as well
as any thermionic type - do I sense
some readers throwing up their hands
in horror?
In any case, if the 6J7 is to be used
as a straight audio amplifier, rather
than a detector, its operating conditions should be changed. As a detector - almost certainly an anode bend
detector - it would be biased close to
cut-off, using a high value cathode
resistor and, possibly, a low screen
voltage. This would restict its operation as an amplifier.
In 'the end, I decided to install a
6H6 twin diode valve. One reason for
this decision was the fact that the
chassis already had provision for an
additional valve. Fitting the 6H6 was
as easy as removing a cover plate and
bolting in a new socket. The heater
The 6H6 twin diode was made in both metal & glass envelope versions. At least
the glass version looks like a real valve, even if it is rather small.
pins of the 6H6 socket were wired to
the heater pins of a nearby valve.
An AGC circuit was required and
the one shown in Fig.1 was taken
from an old radio book. There are
some variations in AGC circuits (eg,
simple AGC and delayed AGC - see
last month's story), and some look far
more complex than others. The circuit used requires only one diode and
so the 6H6 anodes and cathodes were
connected in parallel to form a single
unit.
The next step was to fit a new volume control. A 500kQ carbon pot was
installed in place of the old wirewound pot. However, although the
old control was removed from the
chassis, it was left connected in circuit. Mere about that later.
AGC-----,
The new volume control was wired
in accordance with Fig.1, with the
wiper arm going to the 6J7 via an
existing .01µF capacitor. Also added
were the various resistors and capacitors needed for the AGC circuit, plus
the h eater wiring for the 6H6. The
circuit also shows the changes made
to the 6J7 circuit. This . left only the
two AGC outlets to be connected.
These two AGC lines had to be
connected to the control grids of the
first two valves, via the first IF transformer's secondary winding in one
instance, and the aerial coil secondary winding in the other. However,
it was not just a matter of connecting
the AGC lines to existing connections,
because they were both connected to
chassis.
0
-----1---HT
1.5MI
1M
.01
1M'
.,.
Fig.1: the AGC circuit produces a negative voltage and this is applied to
the grids of the first two valves, via the aerial coil secondary winding in
one instance & the first IF transformer secondary winding in the other.
APRIL 1991
37
Only a handful of components was required to convert the old Airzone to AGC.
The 6H6 twin diode was chosen so that the original 6J7 first audio valve could
be retained but other approaches should prove equally viable (see text).
•
•
The 6H6 twin diode valve was easily fitted to an existing valve socket hole in
the chassis of the old Airzone.
These connections need to be carefully traced back from the valve grids
and unsoldered from the chassis, so
that the AGC lines can be connected
to them. After the AGC lines were
connected to the coil terminations,
these points were bypassed to chassis
via O. lµF capacitors. At this stage
everything was ready for a trial run.
Initial results
The initial results were a bit disappointing. The AGC additions had
thrown the front end of the receiver
considerably out of whack. Motor38
SILICON CHIP
boating, whistles and howls indicated
that all was not well and there was a
brief moment when I reflected on the
wisdom of drawing up the original
circuit in case the set had to be returned to "as it was" condition.
At this stage I was glad that I had
left the original volume control in
circuit. It was, in fact, nothing more
than a variable resistor in the cathode
circuits of the first two valves, and
capable of providing variable cathode bias for them.
That was all that was needed to
correct the instability. After finding a
position where everything worked
reasonably well, the potentiometer
resistance was measured and a fixed
resistor fitted in its place.
The set was greatly improved by
the AGC conversion but I was still a
little disappointed regarding its effectiveness. Strong stations were still
fairly strong, while weak stations were
still inclined to be weak.
Out came the single substitute cathode resistor and in went two 5kQ pots;
one in the frequency converter cathode and the other in the IF amplifier
cathode.
With the set tuned to the noise between stations (virtually no signal),
the volume level could be brought up
quite noticeably by fiddling with each
cathode resistor. After replacing these
pots with fixed resistors the AGC was
more effective (Note: the cathode resistance was determined simply by
setting the pots for maximum volume
without instability). These adjustments made a significant difference
to distant stations but seemed to have
had little or no affect on close stations.
One noticeable change with the
AGC conversion was that the receiver
needed retuning. This was particularly evident with the first IF transformer, which had its secondary circuit upset by the addition of the AGC
line. A complete re-alignment of the
receiver was in order and, while it
was not badly out, there were noticeable improvements when the job was
finished .
The conversion turned out to be
quite successful and I love the old
Airzone even more than before.
Other conversions
Flushed with success I decided to
convert another 1937 set, a console
model Playola. Although fitted with
a 6B6 and using diode detection, the
receiver had no AGC, which seemed
a bit cheap and nasty to me. It too
responded well to the conversion and
is now a far more pleasant set to use.
The third and last radio to be converted (a 1934 model Commodore)
was a little different. It uses a 57 as an
autodyne frequency changer, followed
by a 58 IF amplifier, and has no diodes. In this instance, a germanium
signal diode was used, with only one
valve, the 58 IF amplifier, being AGC
controlled. (The old 58 was one of the
first variable mu tubes to be made).
Resurrection
Radio
Vintage Wireless
Specialists
Repairs - Restoration - Sales
Our skilled technicians offer QUALITY repairs
and restoration.
We also have a large stock of Bakelite and
Timber radios fully restored and for SALE.
This temporary test rig shows that the AGC circuit produces a negative voltage
(the larger the signal, the higher the voltage). This negative bias is directed to
the grids of the pre-detector valves.
The frequency changer was not
suitable for AGC control for two reasons: (1) the 57 is not a variable mu
tube; and (2), an autodyn e freque ncy
changer is notoriously cranky (even a
minor change in operating conditions
can cause it to stop oscillating).
The old Commodore responded
fairly well to the treatment, considering that AGC is applied to only one
valve. It certainly is a big improvement on no AGC at all.
Converting a receiver to AGC can
easily introduce other problems because the circuit has been interfered
w ith. Th ere may be other methods of
solving th e problems I encountered,
but th e sets I converted all work well,
an d that's what matters most.
Altogether, these three AGC modifications took quite a bit of time to do,
but they were worth both the time
an d effort. It was an interesting project and the addition of AGC made all
th ese receivers far more pleasant to
operate.
SC
Parts are available for the enthusiasts
including over 900 valve typ es, high voltage
capacitors, transformers, dial glasses,
knobs, grille cloth etc.
Circuit diagrams for most Australian makes
and models.
Send SAE for our catalogue.
WANTED - Valves, Radios , etc.
purchased for CASH
Call in to our Showroom at:
51 Chapel Street, Windsor,Vic 3181
PO Box 111 6
Telephone: (03) 529 5639
UHF HIGH GAIN ANTENNAS
antenn
existing
systems
triangul
powder
coated
receiving
elements ensures excellent UHF reception
compared to other UHF antennas of similar size
and price. Two models are available: the TVA14
for Band Four and the TVA 15 for Band Five UHF
reception. Both models are supplied with back
reflectors to prevent ghosting as well as a
waterproof entry box designed to accept 75 ohm
coaxial cable without the need for addittional
baluns. They also have predrilled holes for
securing the tilt adjustable metal mounting
bracket in either a horizontal or vertical position .
./mported snd dis tributed by:
ARISTA
ELECTRONICS
Avsllsbte through the following retailers:
This simple AGC conversion module was built up on tagstrip & fitted to a 1934
model Commodore receiver. In this case, a germanium signal diode was used,
with only one valve, the 58 IF amplifier, being AGC controlled. The tagstrip
method makes the conversion a bit neater than the more usual point-to-point
wiring method.
B ernys.
Bridgepoint.
Military Road.
Mosman. 2088.
NSW. (02) 969 1966.
Ritronics.
56 Renver Rd.
Clayton.
Victoria. 3168.
(03) 543 2166.
A PRIL 1991
39
|