This is only a preview of the February 1998 issue of Silicon Chip. You can view 29 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 "Multi-Purpose Fast Battery Charger; Pt.1":
Items relevant to "Command Control For Model Railways; Pt.2":
Items relevant to "Demonstration Board For Liquid Crystal Displays":
Items relevant to "Build Your Own 4-Channel Lightshow; Pt.2":
Articles in this series:
Articles in this series:
Purchase a printed copy of this issue for $10.00. |
VINTAGE RADIO
By JOHN HILL
Clean audio for old Henry
It has now been 12 years since I first
became interested in collecting and restoring
vintage radio receivers. Over that period, I
have found it necessary to rework some of
my earlier restorations, for the simple reason
that they were not done correctly in the first
place.
Experience is not something that
is acquired overnight. As one slowly
advances in the art of valve radio
repairs, there is a gradual realisation
that some past restorations may not
be as good as they could have been.
This was the case with old “Henry”,
a massive 7-valve con
sole receiver
of early 1930s vintage. Henry was
so named because of his masculine
appearance. With square corners,
chunky proportions and shear bulk,
Henry looks quite imposing and takes
up plenty of space.
Henry is one of those numerous
receivers that bear no manufacturer’s
name. These sets were made by wellknown companies for various retail
outlets which often (but not always)
put their own trading name on them.
In Henry’s case, the chassis could
have been built by almost anyone and
has no recognisable parentage. However, it certainly looks impressive,
being built from large early 1930s
components.
While the original restoration was
broadly successful, there was slight
audio distortion. Although unnoticed
at the time, I have become increasingly sensitive to vintage radio receivers
with less than perfect audio.
Many cases of audio distortion in
old receivers are due to the anode
bend detection method that was in
common use during the early 1930s.
However, in Henry’s case, diodes
in the 2A6 first audio valve handle
Henry is an early 1930s 7-valve
superhet receiver of rather large
proportions and is typical of the era.
The challenge was to cure his audio
distortion problems, which have been
present since restoration many years
ago.
76 Silicon Chip
the detection and AGC functions.
Therefore, any distortion must be
due to causes other than the detection
circuit.
So, like several of my early restorations, Henry required a reworking
session.
Common problems
When this old receiver was originally restored, I found that there were
three common vintage radio problems
in need of attention - leaky paper
capacitors, dead electrolytics and an
open circuit output transformer.
The replacement transformer was
selected mainly for its size (so that it
would fit the existing mounting holes)
rather than for its impedance specifications. But unknown to me at the
time, the correct output transformer
for this particular receiver has quite
different specifications from most.
Henry has an unusual output stage
which consists of two type 59 output
pentodes in parallel, rather than pushpull. The two valves are wired grid to
grid, plate to plate, etc. This arrangement provides twice the output power
but is not as good as push-pull which
has a number of advantages, including
lower distortion.
A parallel output stage requires
an output transformer with half the
primary impedance of that used for
a single output valve. In the case of
parallel 59s, an output transformer
with a 3kΩ primary is required but
that is not what was installed when
the set was restored. The transformer
used would have been more in keeping with a battery receiver, as it had
a 10kΩ primary.
So a bad impedance mismatch
needed to be corrected for a start.
On top of that, one of 59s had an
open heater. Readers unfamiliar with
this output pentode may be surprised
to learn that the valve has two heaters
and will still work reasonably well
when one is open. However, as there
was a distortion problem to correct, a
replacement valve was required.
Next was the problem of resistance
values. The set used a particular brand
of resistor that is notorious for going
high, so it was not surprising that
some were up to 100% out of tolerance. All the cathode bias resistors
were wirewound types and the bias
voltages were OK.
It was obvious that the partly
defunct valve, the out-of-tolerance
The high tension choke (left) and high tension power transformer (right) are
mounted on the top of the chassis.
resistors and the output transformer
would all have to be replaced. The
replacement transformer was a 2.5kΩ
“Isocore” type in a pressed steel can.
While it looks a few years too modern
for the set’s age, it was the only transformer that came near the required
3kΩ primary. Being an Isocore type,
with floating, “hot” laminations, it
should be relatively troublefree.
The modifications had the desired
effect and the sound from the old receiver was greatly improved. It is quite
amazing how well some ancient loudspeakers perform. While they may
not be equal to modern equipment,
they are OK with the limited range
of frequencies covered by AM radio.
A 2.5kΩ “Isocore” output transformer
was used to replace the earlier 10kΩ
unit. This was necessary because the
primary impedance required for
parallel connected output valves is
half that of a single stage.
This close-up view of the chassis shows the two 59 output pentodes which are
wired in parallel. The old 59 was in production for only a short time and was
superseded by the 2A5.
February 1998 77
Old Henry was built using very large 1930s-style components and boasts no less
than seven valves, including two 59 output pentodes wired in parallel.
So old Henry is working noticeably
better than before and I was pleased
with the outcome. While there really
wasn’t much to do regarding this
The receiver is impressive
because of its size alone.
Inexperienced collectors should
note that there are a number of
unprotected high voltage
connections on the top of the
chassis.
78 Silicon Chip
particular repair, it does demonstrate
how attention to details can make a
difference.
Hernry’s attributes
As Henry is an unusual receiver, I
will finish off this month’s column by
describing some of his more interesting attributes.
Perhaps the most striking aspect
about the chassis is its size and the
choice of components used. In many
ways, it appears to be over-engineered, the power transformers being
one such example. There are two power transformers, one mounted above
the chassis, the other below.
The high tension transformer is on
top. It has two secondary windings: (1)
a centre-tapped high voltage winding
for the type 80 rectifier plates; and (2)
a low voltage winding for the rectifier
filaments.
A separate filament transformer is
mounted underneath and supplies the
remaining six valves. It delivers 2.5V
and is rated at many amps. As the
valve heaters collectively draw 8A,
this transformer is quite large and of
robust construction.
There is also a huge 30H, 85mA
high tension choke mounted above the
chassis next to the HT power transformer. These two units weigh quite
a few kilograms and concentrate a lot
of weight at one end of the chassis.
Both power transformers and the
high tension choke operate at barely
warm temperatures even after several
hours operation. Even then, the temperature increase is mostly due to the
close proximity of the rectifier and
output valves.
The front end valve line-up is: 58
RF amplifier, 57 auto
dyne oscillator/mixer and 58 IF amplifier. The
gain of the two 58s is controlled by
AGC. There is no AGC applied to the
frequency converter as the autodyne
circuit was unsuitable for AGC. The
IF is 175kHz.
The 59 pentode
Mention has already been made
regarding the 59’s twin heaters and
cathodes. The old 59 has other peculiarities that are also worth mentioning.
The 59 has a large 7-pin base which
is marginally bigger than the standard
7-pin base of other valves such as the
6A7 and 6B7. As a result, 59s will not
fit some valve tester sockets without
the aid of an adaptor.
Another oddity of this valve is that
its suppressor grid has its own base
pin connection (hence the 7-pin base).
Other pentodes have six pins, with
the suppressor internally connected
to the cathode.
Some servicemen of yesteryear do
not speak very highly of the old 59
valve, claiming that it was weakly
constructed, trou
b lesome and inclined to go gassy. Personally, I’ve
encountered none of these problems.
(A staff member who was familiar
with these valves recalls that they
were prone to what was virtually
instant destruction in the event that
the plate voltage was lost – as with
an open speaker transformer which
caused a red hot screen. No-one ever
reached the switch in time!)
The 59 was in production for only
a short time and was replaced by the
2A5. The 2.5V series of valves was,
in fact, short lived, being superseded
by 6.3V types at about the time Henry
was built.
Dangerous voltages
Perhaps the worst aspect of Henry is
the unprotected – and potentially lethal – high voltage connections above
the chassis, where one would least
expect to find them. Inexperienced
collectors/repairers should take note
of the following.
Most manufacturers of the era
endeavoured to keep high voltage
nasties confined underneath the
The front end or RF part of the receiver. The valves inside the shield cans are:
58 RF amplifier, 57 autodyne mixer and 58 IF amplifier.
This large 3-gang tuning capacitor is typical of many early 1930s receivers.
Miniaturisation had not been thought of then.
chassis. However, this was not always
the case, as an examination of Henry
clearly reveals.
The rectifier socket is mounted
above the chassis. Its bare external
connections are within easy reach of
any careless fingers that may venture
close enough to touch those high plate
voltages.
Similar bare connections (in the
form of terminals) are to be found on
the high tension choke. These connections are easily reached (even when
the chassis is in its cabinet) and they
have a DC potential in excess of 300V.
Finally, another dangerous and
potentially fatal nasty is on the high
tension power transformer. A bare
unused 240V primary connection
protrudes from the transformer cover.
It seemed prudent to tape over this
hazard. Mains voltages, by reason of
their low source impedance are by far
the most dangerous.
So that’s about all there is to report
on old Henry. He has always been one
of my favourites and now he’s better
than ever.
In fact, Henry is one of those nice
old receivers that makes collecting
vintage radios such an enjoyable
SC
hobby.
February 1998 79
|