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VINTAGE RADIO
By JOHN HILL
An interesting grid bias problem
One of the more interesting aspects of vintage
radio is repairing non-functioning receivers.
As far as I’m concerned, getting them working
again is the most satisfying part of this hobby.
As a person having neither an electrical or electronics trade background,
I have never been properly trained to
do radio repairs. I have developed my
own repair techniques from reading
books, asking others and doing my
own investigations. As a result, there
is a lot I have yet to learn and learning new things helps to maintain my
interest in vintage radio.
Having a limited background makes
one easy prey to any new fault that
comes along and these tricky problems
always remind me of how inexperienced I really am. But perseverance
usually wins and the fault is eventually found and rectified. Solving such
faults is very satisfying.
A recent repair to a mid-1950s
4-valve Radiola produced one of these
mystery faults and it proved to be quite
elusive. Allow me to explain.
A real wreck
This particular receiver was one of
the worst wrecks I have seen for a long
time. It had obviously been stored for
many years in a workshed, judging
by the number of multicoloured paint
splats that were on it. Why people have
to flick their paint brushes at old radios
I will never know!
The little Radiola was filthy and
mouse infested. When the set was
removed from its plastic bag there
remained about a tablespoon full of
The Radiola was a common mid-1950s 4-valve receiver. Considering its filthy
condition when found, it scrubbed up fairly well.
84 Silicon Chip
mouse droppings and other miscellaneous items such as partly eaten
pumpkin seeds, small bones and
other debris. I hate working on sets
like these!
After a thorough clean up, the usual
checkout routine revealed that one of
the oscillator coil windings was open
cir
cuit and the rectifier valve had
almost no emission. As there were
plenty of 6X4s in the valve box, the
weak rectifier wasn’t a problem but
it did arouse a suspicion that there
was something else wrong with the
receiver to reduce the valve’s emission
to such a low level.
Other problems with the old Radiola
were: (1) all the original paper capacitors were still in place; (2) the speaker
grille cloth was a filthy, tattered mess;
and (3) the dial cord was made up of
four different sections of string. As
the knots in the dial cord would not
go around the pulleys, dial movement
was restricted to about one quarter of
its total length of travel. The things
that some people put up with!
An examination of the oscillator
coil revealed that one of its leads had
broken off. If the truth be known, the
wire had most likely been chewed
through by a furry little rodent.
Repairing the oscillator coil was
relatively simple as the broken lead
protruded from the sealing compound
by about two millimetres which
allowed a new lead-out wire to be attached. After reinstalling the repaired
oscillator coil, the set was ready for
a test run, even though the paper
capacitors had not been replaced at
this stage.
The little Radiola fired up straight
away and seemed to be in good tune,
pulling in a number of stations while
using only its built-in aerial.
Regardless of the fact that the set was
performing well, the paper capacitors
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The oscillator coil (centre right) needed repairing because of a broken lead-out
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Replacing the electrolytic and paper capacitors had little effect on performance.
However, leaving them in service is only asking for trouble later on.
were replaced with modern polyester
types, which seemed to make little
difference. Then the rot set in!
Distorted sound
After the receiver had been working
for a few minutes, the sound gradually became more and more distorted.
What’s more, as the sound distorted,
the high tension dropped from around
200V to 175V.
Leaving the set to cool off for a
while produced a similar result. It
worked perfectly for a few minutes,
then the distor
t ion slowly crept
back. So what appeared at first to be
a simple and straightforward repair
had now developed into one of those
mystery faults.
I suspected a faulty output valve
and so a good secondhand 6BV7 was
substituted for the valve that came
with the receiver. I might add, at this
stage, that both of these valves tested
“GOOD” when checked in a valve
tester.
The result was the same – the set
was OK for a few minutes, then went
into a slow downhill slide until the
sound became quite distorted. It was
time to start thinking!
Distortion is usually associated with
the audio end of a receiver (but not
always) and is often caused by a lack
of grid bias. With this thought in mind,
the back bias circuit that provides
the bias voltage to the output valve
was checked. The 100Ω bias resistor
seemed OK and nothing could be seen
that looked remotely suspect.
The grid bias on the 6BV7 output
valve should be somewhere around
-4V, taking into account the plate voltage at which the valve was operating.
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September 1995 85
increase the bias voltage to -4V and
it did! But the problem remained –
the reception was perfect for a few
minutes and then went into a slow
decline just as before.
An interesting discovery
When testing the faulty valves, only one produced a flicker in the shorts
indicator neon. Testing a valve in a valve tester is only half a test. The other half
is to give it a thorough work-out in a working receiver.
It was at this stage that I made an
interesting discovery. I replaced the
original output valve in its socket
while the set was still operating. The
bias voltage immediately went back
to -4V, stayed there for a short period,
and then started dropping again. I
repeated the operation with the other
valve after it had cooled and the same
thing happened.
The fault was in the valve – both
valves, in fact! They worked OK when
cold but not after they had become hot!
Out came all of the 6BV7s I had in
stock. I selected a new valve and fitted
it to the output socket with vastly different results. The bias voltage settled
on -4V and stayed there!
It was too good an opportunity to
miss out on and all the 6BV7s were
checked in the receiver. Out of the
13 valves tested, six of them had the
diminishing grid bias characteristic.
So, in one hit, my 13 “good” 6BV7
valves were reduced to almost half
that number.
The valve tester
These are the faulty 6BV7 valves. Although they all tested “GOOD” in the valve
tester, they had a problem that deprived the control grid of its negative bias.
They worked OK when cold but not when they became hot.
It seemed an appropriate time to check
out the actual bias voltage.
Grid bias voltages are best checked
with a digital voltmet
er as these
instruments have a very high input
impedance, which has little or no
affect on the function of the receiver.
A cheap analog 2,000Ω/V meter can
seriously affect both the voltage reading and the operation of the receiver
under test.
The receiver was set up with two
voltmeters, one to measure the high
tension voltage and the other the bias
voltage. The bias reading was -3V and
86 Silicon Chip
after this reading had peaked, it went
into reverse and dropped slowly until
it almost reached zero.
And as the bias voltage dropped, so
too did the high tension voltage, due to
the output valve passing increasingly
more current. Distortion did not become apparent until the bias voltage
decreased to about -1V.
Perhaps the bias resistor was faulty?
Maybe its value decreased as it warm
ed up?
Not having a 100Ω resistor on hand
I used a 130Ω resistor as a replacement instead. Hopefully it would
It’s time for the valve tester to enter
our story. I set it up to recheck all the
valves that had failed in the receiver,
even though they had previously
checked out OK in the valve tester.
Of the six faulty valves, only one
could manage to produce a flicker in
the tester’s shorts indicator neon.
This short was on pin number eight
which is the control grid. It would
appear that these valves have a problem when they reach full operating
temperature.
It is worth noting that the four new
valves were all OK. Only the second
hand units produced the diminishing
grid bias characteristic, even though
most of them had emission levels
comparable to new valves.
So there’s something new to ponder
over! Is the 6BV7 a more troublesome
valve compared to other output
valves? And what is the mechanism
of the fault anyway?
Initially, the most likely possibility
seemed to be that the valves were gassy. And a fellow enthusiast suggested
that this could be aggravated by a grid
There’s nothing very exciting about a 4-valve Radiola, as it was a very basic
radio receiver. Note the built-in aerial mounted above the chassis. The valve
line up is as follows: 6BE6, 6AU6, 6BV7 (output) and 6X4 (rectifier).
resistor that had “gone high”, as they
frequently did in those days.
However, these ideas had to be
discounted. All resistors are routinely
checked during restoration and any
found to be out of tolerance are replaced. Also, I took the opportunity
to try these valves in another set and
they behaved identically.
That seemed to further confirm that
the fault was in the valves rather than
in any associated circuits. And the gas
theory seemed to be ruled out by the
fact that there was no violet glow in
the valves, which is characteristic of
this condition.
Another idea which was considered
was a fault known as “silver migration”. It occurred in valves with silver
plated pins, the silver “migrating”
across the glass, particularly between
pins with a high voltage between them.
This idea was also thrown out. For
one thing, the warm-up delay didn’t
seem to fit but, more importantly, the
pins were not silver plated.
Finally, the most likely explanation
would seem to be a condition known
as grid emission. Apparently, this can
occur when the grid becomes coated
with cathode material, generally due
to the heater being overrun for long
periods.
So, perhaps that is the answer. But
the question remains as to whether this
type of valve is prone to this problem.
An unusual valve
Incidentally, for those unfamiliar
with the 6BV7, it is a little unusual
in that it is a duo-diode output pentode. As far as I’m concerned, it is
the only one of its kind and I also
suspect that it is a locally designed
and manufactured valve, as it doesn’t
seem to be mentioned in overseas
valve lists.
Actually, a duo-diode output valve
is a logical type to use in a 4-valve
VINTAGE RADIO SWAP MEET
22nd October 1995
Glenroy Tech School Hall
Melbourne
Admission: $3
Enquiries: (054) 49 3207
radio. If a receiver is to have automatic
gain control and diode detection, then
there have to be diodes somewhere,
so why not have them in the output
valve?
Another mid-1950s 4-valve setup
was to use valves of the duo-diode
RF pentode type (6N8, 6AR7) as an
IF amplifier detector and pass the
resulting audio signal to a standard
output valve such as a 6M5 or 6AQ5.
Looking back on the Radiola repair,
perhaps the most annoying aspect was
the fact that the fault was accurately
diagnosed quite early as being a suspect output valve.
It was ironic (read rotten luck) that
the substitute valve used to check out
this theory happened to have exactly
the same fault!
Anyway, learning new things helps
to maintain my interest in vintage
radio. It is unusual faults like the one
just described and their remedies that
make vintage radio repairs both interesting and challenging.
When I finally learn all there is to
know about valve radio servicing, then
all the fun will have gone out of it for
me. However, as that time seems a long
way off, I’m sure that vintage radio will
continue to hold my interest for many
years to come.
SC
September 1995 87
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