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A system recently delivered by
a customer for refurbishment comprised a pair of Quad II amplifiers
of roughly the same sort of vintage, a control unit and a mono
tuner. All are valve equipments, totalling quite a serious number.
I didn't count them but between 20 and 30 in all. One of the
two amplifiers had probably suffered a mishap in the past as
the pair now had a single large "glass" KT66 and a
smaller "glass-tubular" version in each, probably so
they'd look similar. One of the rectifiers was a G version and
the other a GT. An EF86 had a telltale white top, indicating
it's glass had been cracked, and was replaced with a new one
before going further.
The Quad II system is nicely engineered mechanically. It's also
nicely designed electronically but electrically speaking it's
bad.
I'm not one to advocate over zealous safety legislation, in fact
I believe that once we started on the road with the new, so-called
"Safety at Work" rules, and environmental protection,
we were set on a path of stupidity. I'd rather have "good
engineering practice" as a basis for regulations, but we
haven't any more so there it is.
The Quad designers used two-pin Bulgin mains connectors and being
un-polarised can be fitted either way up and, being only two-pin,
of course there can be no earth connection carried through that
connector.
A first scenario for potential mishap may be a pair of amplifiers
standing side-by-side powered from the mains through separate
mains cables. What if there's faulty mains wiring? One chassis
may be at mains live potential and the other at mains neutral
waiting for the unsuspecting user to bridge the two (or one to
ground) with part of his anatomy.
Of course this scenario may sound far fetched but what if the
mains wiring is in perfect condition? At what potential is the
chassis of an amplifier with respect to ground? I suggest you
try this and use an AC voltmeter to test the hypothesis. Don't
involve ones body parts for the test, just a voltmeter.
When the system is complete, with all the units of the correct
vintage (by which I mean originally designed to be interconnected)
and connected together with undamaged and unmodified cables,
all will be tolerably well. I say "tolerably" as there
still will be no connection to ground for AC to leak away. Alternating
currents will (want to) flow to ground from transformer inter-winding
capacitance, mains decoupling and stray capacitance generally.
What can go wrong? The mains connections to either amplifier
are carried from the control unit via the audio cable or should
that be… "The audio connections are carried from the
control unit via the mains cable". Either way, I think it's
not such a good idea safety wise. When new it was passable but
as these things age it becomes more than likely that things can
go wrong. We're looking at items made between 30 and 50 years
ago after all.
For example, what about the example in front of me? Both two-pin
mains connectors have fallen off and are long since gone and
the controller connection to mains is via a two-wire plastic
cable poked through the chassis and soldered to the rear of the
two-pin connector on the rear of the chassis. A length of plastic
cable sheath has been pushed over one of the exposed pins of
the mains connector. The other pin is unprotected, as presumably
the protective part has fallen off. An accident waiting to happen.
If not from the exposed pin but from the bare wires that should
be terminated in mating plugs. Maybe the original owner knew
the hazards but I bet the current owner is a little hazy on the
dangers?
I decided on a straightforward solution. Remove the three two-pin
mains connectors and fit three-pin IEC connectors (the type that
are fitted to computer power supplies). The modification involved
only a little filing of the apertures so that the new parts fitted,
and will readily allow the old parts to be refitted, if so desired,
by an enthusiast so inclined.
The pair of amplifiers was so modified. The mating cables could
have been made up by using the original wiring but I decided
to add a pair of separate mains cables fed through new grommetted
holes in the rear of the controller and soldered to the original
feed points under the chassis, suitably changed by the addition
of a third IEC connector, a 3-pin plug which allows use of a
new IEC mains lead fitted with legal moulded 13-amp plug.
The mains wiring within the controller was basically left unchanged.
The wires integral to the audio cable were cut off and the new
integral mains cables soldered to points going to the new mains
socket and the on/off switch. Incidentally, one mains cable is
only active when the STEREO button on the controller is pressed
and, because of this, the corresponding mains cable was marked
with white paint so that it corresponds to the white (or cream)
sheathed audio cable.
In theory the modifications may degrade the system. This is because
each 3-pin mains connector has its earth pin connected to the
corresponding chassis and when everything is coupled up there
are paths outside the signal connections for currents to flow
between the units. This may only be apparent if a record deck,
with its correspondingly very low-level audio signals, is plugged
into the controller, and then only under certain conditions,
which can be overcome.
Any other safety matters? One of the amplifiers was missing its
lower cover. Not good. If one lifted a working amplifier and
one's fingers strayed underneath the chassis it could be very
uncomfortable, if not the last thing one did.
I made a new cover from a piece of steel cut from a piece that
originally screened the innards of a computer monitor. Four rubber
feet finished the job.
What of the components inside the system?
I checked all the resistors and tested the critical capacitors
and found very few to be in need of replacement, just a few coupling
capacitors.
With everything connected together the sound was really very
good. Even with a cheap workshop loudspeaker one could have imagined
that the Radio 4 people were actually in the workshop.
A little work was necessary to the tuner. Apart from fitting
a new dial lamp, the tuning mechanism needed some work as it
had stiffened. I applied a little oil to the moving parts (including
the surface of the dust cores used in the permeability tuning
arrangement). A tweak on the settings of the cores realigned
the low-end BBC stations to conform roughly with the dial markings,
although setting the tuning still feels like there's an elastic
band somewhere in the mechanism. Thankfully a pair of indicators
assures one that tuning is OK.
If you have a Quad II system or something
similar, I should stress that, unless you are an electrical engineer,
do not attempt to modify mains wiring of an equipment and don't
remove protective panels as old valve equipment is not like its
modern transistor-based equivalents. The HT supply in the former
can rise to 500-volts compared with the 30 or so encountered
within transistor amplifiers etc and only ten milliamps or so
flowing through a human body can prove lethal. |
