(I really can't believe I'm replying to this... ;-) It's in the
interests of safety.)


rladbury asked:

What I'm still unsure about is:

- What is the exact risk involved of changing the rating of a fuse (in my
case, 1.6a 125v for a 500mA 250v)?


Probably no "risk" as in danger to you, providing you pull the plug first!
A fuse has a voltage rating because that is the maximum voltage that it is
guaranteed to break, without flashing over inside. So you can't put a 125V
fuse in a 250v system, but you can put a 250v fuse in a 125v system. If
you use too low a voltage rating and the right sort of fault occurs then
the fuse will blow, but won't actually break the current because it can
pass through the now silvered layer on the inside - its like not having a
fuse.

The current rating is the maximum that the fuse can carry continuously
without "blowing". So, if you put a 500mA fuse (i.e. 0.5A) fuse in instead
of a 1.6A one it could very well blow immediately as the equipment could
be taking up to 1.6A. Putting a 1.6A fuse in where it should be 500mA is
giving you 1/3 of the protection that you should have. You are increasing
the risk of permanent damage to the transformer primary in the event of a
fault on the secondary (such as a blown rectifier going short-circuit).
Note that the difference in resistance of a 500mA fuse and a 1.6A fuse is
negligable as the rating depends on the composition of the wire as well as
its cross-sectional area. The contact resistance is way higher.


- What is the exact risk involved in using solder wire instead of the
original fuse and its filament wire? (Seems to me the solder would burn at
an even lower temperature,


You may as well stick a nail in - effectively you don't have a fuse. The
solder has a low enough resistance to not heat up enough to melt in most
cases. There is a very real danger of fire in the event of a fault if you
do this. If you try to use solder as a fuse it tends to explode in small
firey globules of hot metal in the event of a fault. Not what you want...


- Would a transformer damaged by the absence of the original fuse be
audibly apparent before leaking currents and creating a risk of shock?


Do you mean that the original fuse blew? If so it would probably blow a
new fuse immediately. After all, that's why the original fuse blew isn't
it? It *could* make the chassis live if there is an earth problem though,
so the transformer should be properly tested before re-use.


- How can I check to be sure my metal case is properly grounded?


Are you using a 3pin mains plug? If so, a resistance test between the
case and the earth pin is as close as you are going to get. It should be
close to 0 ohms.


It is *not safe* to do things like shorting out fuses or wrapping them in
silver paper. There is no danger until a fault occurs, but then there is a
very real danger of fire and even getting someone killed.

Remember that the fuse has the job of breaking the current. It takes a
finite time for the fuse element to heat up and blow. During that time the
current into the fault is climbing higher and higher, up to a limit set by
the resistance of the fault. It is quite possible to draw tens of amps
through a 1A fuse for a small fraction of a second. Without the fuse that
current goes through the load. Normally house wiring will have a bigger
fuse further back, but that is designed to let enough current through to
blow the fuse in the appliance. Consequently, if you short out a 500mA
fast-blow fuse and you get a fault in the transformer primary (or in the
wiring to it) it could put over a hundred amps through, say, a 13A fuse,
down the lead and into the fault. You had better hope that the bigger fuse
blows or the lead could set fire to your carpet or the transformer could
explode (ok, its a worst case scenario, but these things *can* happen).
Even worse, if the amp has a bad earth the fault could "silently" break it
completely and make the case, and everything connected to it, live. That
can easily kill someone but you won't know about it until you touch
something and by then it could be too late.

Just think of one thing to finish off though, suppose the resistance of
the fuse and its connections is 0.5 ohm (*way* too high, even for a bad
contact). The transformer primary impedance could be about 70 ohms for a
200VA transformer so it swamps the fuse's resistance even in that extreme
case. If the maximum primary current is about 1.6A at 120V the fuse will
drop 0.8 of a volt *at full power* - probably less drop than you'll get
from putting the kettle on. You won't hear the difference at all if the
fuse contacts are clean as the series resistance will simply be too low.
If you think you can hear a difference the clean the contacts or clean
your ears! Ignore anyone who tells you that there is a difference - there
isn't.

Don't ignore safety warnings - don't put people's lives at risk for an
imaginary increase in sound quality which can be proved false both by
measurement and by double-blind listening tests.

--
Mick
(no M$ software on here... :-) )
Web: http://www.nascom.info