5 Jun
2015
5 Jun
'15
5:21 a.m.
Len Ovens wrote:
> I don't know about ground switch, but some of them had a "hum" switch
or
> something similar... flipped the ac lines. Sometimes the "neutral" was
> connected to chassis... which with no keying on the ac plug meant the
> chassis might just as easy be hot. Nobody has done that for a long time
> now. The idea was old and outdated 50 years ago. (probably before)
On 06/04/2015 07:28 AM, Glen MacArthur wrote:
Yes that's what it was, however the toggle switch
itself said 'Ground' on
the back the amp chassis.
I used a Fender Bassman Ten amplifier back in the day. Looking at the
amplifier schematic:
http://www.thevintagesound.com/ffg/schem/bassman_10_schem.gif
A single-throw, double-throw, center off (ON-OFF-ON) "Ground" switch is
shown near the incoming (3-prong) AC power cord. The switch connects
the chassis through what appears to be a 0.047 uF 600 V capacitor to one
side of the 120 VAC supply (ON), to nothing (OFF), or to the other side
of the 120 VAC supply (ON). I seem to recall using 3-prong receptacles,
and putting the Ground switch in the center OFF position.
But, things get interesting if such an amplifier is connected to an
ungrounded supply. (And, earlier Fender amplifiers had 2-prong cords
and SPDT Ground switches without the center OFF position.)
Approximating the amplifier power supply transformer as a resistor, the
50 W model would have an equivalent load resistance of:
R = V^2 / P
R = (120 V)^2 / 50 W
R = 288 Ohm
The capacitor and transformer create an R-C low-pass filter, with a 3 db
cutoff frequency of:
fc = 1 / (2 * pi * R * C)
fc = 1 / (2 * pi * 288 Ohm * 0.047E-06 F)
fc = 11.8 kHz
This is well above the electrical supply frequency, so setting the
Ground switch incorrectly (e.g. Hot) will put 120 VAC on the amplifier
chassis, stomp boxes, instruments, speaker cabinets, cables, etc., and
everything their energized parts touch (e.g. musicians and sound
technicians)!
If a person so energized touches a solidly grounded object, a circuit
will be completed from the electric supply energized terminal (Hot),
through capacitor, through the person, through ground, and back to the
electric supply grounded terminal (Neutral).
The impedance of the capacitor at 60 Hz is:
Z = 1 / (j * 2 * pi * f * C)
Z = 1 / (j * 2 * pi * 60 Hz * 0.047E-06 F)
Z = -56.4j kOhm
Neglecting the impedance of the the other items in the circuit, the
magnitude of the ground fault current (electric shock) would be:
I = | V / Z |
I = | 120 V / (-56,4j kOhm) |
I = 2.1 mA
If an energized person instead touches an object connected to another
amplifier with it's Ground switch set correctly (e.g. Neutral), there
would be two capacitors in series and the ground fault current would be
half (~1 mA).
This latter value corresponds to the agreed-upon threshold of sensation
for electric shock:
http://en.wikipedia.org/wiki/Electric_shock
David
