[Amps] Direct rectification of AC mains to drive the amp,

[Manfred Mornhinweg]

Bill,

you are missing the main point here!

> Another way to float the scope is with a sine wave type inverter running
> from a 12 volt battery. Mount the inverter and battery in a box and you have
> a portable power supply to run your scope or anything else. Add a trickle
> charger for the battery and you're all set. 

Yes, sure!  But if you then connect the scope to a direct-line-powered 
circuit, the scope chassis and knobs STILL will have a dangerous voltage 
on it!  That's exactly the same as using an isolation transformer to 
power the scope, or to simply use the scope's internal transformer for 
insulation, and lift its ground. In all three cases the scope's chassis 
will end up connected to the line through the circuit being measured!

The Fluke Scopemeter is a good solution, because that scope is fully 
insulated. Its enclosure and controls are all plastic, and its probes 
too use externally insulated BNC connectors. That's why it's quite safe 
to use on line-connected circuits. But most normal/older scopes have the 
probe ground connected to their metal enclosures, and so they are NOT 
safe to use them this way, and if someone does it anyway, he needs to be 
careful!

Hmm... I'm starting to see why some people here recommend not to incite 
anyone to build directly line-connected electronics... ;-)

As Peter says, running the amplifier, and NOT the scope, through an 
insulation transformer, is a safe thing to do during the test phase. The 
same applies to Bill's battery/inverter solution - it's safe to power 
the amp from teh battery/inverter, but NOT the scope! The downside is 
just that legal limit amps draw at least 2.5 kilowatts, possibly up to 
3, and if poor power factor is involved, that could be 4kVA. Isolation 
transformers for that power level are expensive, and inverters and 
batteries even more so!

Despite all this, I will STILL build my electronics directly powered 
from the line, when it is convenient. And sometimes I will even float my 
scope to CAREFULLY take some measurements. But please, all of you out 
there, don't do this if you don't understand CLEARLY and COMPLETELY what 
sort of safety issues are involved! We don't have too many hams left 
these days, let alone any willing to even use a scope...

Manfred

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