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- A Most Dangerous Amp
- Phil Allison
October 3, 2006, 8:32 am
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Hi to all,
As some of you already know, my business involves servicing professional
audio equipment including valve instrument amplifiers. Some of these amps
are quite new but others are decades old.
Last week, one of the oldest examples I have ever seen arrived in the
workshop carried by its proud owner. A "classic" early 1960s built Gibson
GA 40 aka - the "Les Paul". It has a single chassis is fitted at the top
of an open backed cabinet with a 12 inch speaker - see pic.
The amp was made in the USA for their local market and hence the 115 volt AC
power system. As you can see from the schematic it has no AC supply safety
earth - standard US practice at the time.
Instead, there is a "polarity switch " that connects a 0.02uF, 400volt DC
capacitor to the chassis from one or other of the AC supply wires. Users
are supposed to pick the one that causes the least humming noise !!
Now, this particular GA40 amp had been long ago converted to operate from a
240 volt AC supply. A 230/115 volt AUTO-TRANSFORMER ( also made in the
USA ) had been installed in the bottom of the speaker cabinet. It was
fitted with a 2 pin US type outlet and a short, black rubber " figure 8 "
cable trailing out of a hole in cover plate. A dubious looking in-line join
extended this cable via 2 metres of round black rubber cable finally ending
in an Aussie 3 pin plug.
1. The short figure 8 cable (carrying 240 volts AC ) was covered in open
cracks allowing the tinned copper conductors to be seen and touched.
2. The 230 /115 volt auto-transformer was wired with its common connection
going to the 240 volt AC active wire.
3. Because of #2, the "polarity switch" connected the amplifier's *
FLOATING * chassis to either 120 volts AC or 240 volts AC via the now 45
year old, 0.02 uF, film cap.
4. Such film caps are not rated to withstand 240 volts AC and commonly fail
short when made to do so.
When I enquired, the owner admitted he sometimes receiving " shocks " from
the amp and had no idea what the "polarity switch " was for. He also
revealed that his 6 year old daughter liked to plug mic into the amp and
sing through it.
Given that mics and electric guitars connect people directly to the
metalwork of any amplifiers they are being used with - the whole situation
was highly lethal.
Suffice to say, the old amp now has a modern 3 core lead supplying the
auto-transformer, the transformer's frame is wired to the AC supply ground
pin and the chassis of the amp is permanently wired to the frame of the
transformer. The oversized and very dangerous 0.02 uF film cap ( referred to
as a "death cap" in the USA) has simply been removed.
Re: A Most Dangerous Amp
** Perhaps I need to explain what a "polarity switch" does on a guitar amp
in the USA.
Up until the early 1970s, US domestic power outlets were of the two pin
variety with an active and a neutral pin with 110 to 125 volts AC @ 60 Hz
across them - with * NO * provision for earthing the appliance. The
neutral conductor was held close to ground potential by connecting it to an
earth stake at the street pole or elsewhere. This worked well enough for
domestic appliances like lamps, heaters, electric jugs, vacuum cleaners and
However, a guitar amp is another matter. Because of its high voltage gain
and high input impedance at audio frequencies, the chassis needs to be
earthed in order to minimise capacitive injection of AC line noise and 60 Hz
hum picked up by a less than perfectly shielded electric guitar or mic.
When it is not earthed, an amplifier's metal chassis "floats" and will
typically take up an AC voltage around half that of the AC supply due to
stray capacitance in the AC supply transformer and internal AC wiring -
about 50 to 80 volts AC in the USA. This AC voltage can easily be measured
with a high impedance AC meter, ie a VTVM back then or a DMM nowadays.
The "solution" ( read Yankee dodge) invented by guitar amp makers was to add
a paper or plastic film capacitor of about 0.02 uF to .05 uF from chassis
to neutral - swamping the effect of stray capacitances and bringing the
chassis voltage very close to ground potential.
The particular capacitor value was chosen to be large enough to do the trick
but no so large that when linked to the AC active line the maximum AC
current that flowed would cause a serious electric shock to any user who
happened to be in contact with the ground ( ie bare feet) or any earthed
metal. A 0.05 uF cap has an impedance at 60 Hz of 53 kohms - allowing 2.3
mA to flow.
However, since USA style two pin AC supply plugs can be inserted into the
wall outlet * either way around * you never know which wire is active and
which neutral inside an appliance - so makers fitted a SPDT changeover
switch to allow the user could find out by trial and error. An electric
guitar or mic would hum or buzz considerably less in the right ( ie neutral
side) position. This switch was christened the " POLARITY " switch and was
always placed right next to the AC supply on/off switch.
Such guitar amplifiers were designed to be local US models only and NEVER
meant to be sold or used in ANY country with 230/240 volt AC power.
However, these amps CAN be safely converted for use with a 240 volt AC
supply if connected to an ISOLATION step-down transformer. The polarity
cap inside the amp cannot conduct any AC current to earth when supplied by
an 240 /120 step down isolation transformer - nor can it possibly be
subjected to 240 volts AC and hence fail short.
Re: A Most Dangerous Amp
** Not class "Y" nor even class "X" - not even AC voltage rated at all.
Just ordinary paper, oil filed paper or mylar film caps of 400 or 600 volts
Some US amp makers ( ie Mesa Boogie) even left 0.047uF polarity caps in
place on their 240 volt AC export models sold here in the 1980s and 1990s.
After some time subjected to 240 volts AC, they simply exploded with an
Scared the wits out of the amp's owners !!
Re: A Most Dangerous Amp
I played a gig on the roof of a bus by the beach years back and got one
serious shock from my guitar and mic. Had to kick the lead out of the
guitar. Stopped playing and found out later the whole thing was running on a
generator. The bass player never had a problem though.
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