[Harp-L] Re: Identifying hazardous amps

[fjm <2738540@xxxxxxxxxxx>]

Isn't the reason the amplifiers tube filaments are wired in series not 
that the amp lacks an isolation transformer but that the amp doesn't 
have a filament transformer?  I've seen plenty of old radios with 
isolation transformers but no filament transformer. Why can't you ground 
the chasis of an amp lacking an isolation transformer?  Power tools 
don't have isolation transformers and they're grounded.  Isn't the 
purpose of an isolation transformer to isolate the amplifier from 
electrical noise?  I don't have a Harmony 400 schematic in front of me 
but it seems likely that simply plugging one into a gfci protected 
circuit would solve the problem.  Unlike the situations with many early 
amplifiers where the chassis is connected to the incoming line voltage 
via a capacitor or pair of capacitors where a gfci won't work because 
the voltage and current involved ends up below the trip threshold of the 
gfci a straight short to ground would quickly trip the gfci.  Personally 
I'd rather be depending on a gfci than a ground and ultimately a panel 
breaker set to trip at 15 amps.  If there's a good reason besides noise 
that you can't ground an unisolated circuit I'd really like to have it 
explained to me.  Thanks, fjm