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i dunno i'm thinking they just install a cheap isolation trans...i've bought a few such transformers to play with lately...a 50watt rated one from Triad,meets required electrical specs,is wireable for both 120 and 240v input,and is about 2" square max[and that's pushing it really,even smaller are available]...these transformers are brand new,name brand,and cost all of about 10-12 bucks.
i think they're quite popular retrofits in vintage guitar amp circles,they're often used for old and cheaper tube amps using a hot chassis which of course present the same type of safety issues as old radios..only worse!the ISO trans mod is really not unwise in those cases
however these small Xformers have awful regulation and tend to step up the voltage significantly unless loaded to near the max.unless they sized the xformer pretty close to the wattage of the radio,the tube filiments are gonna get pretty roasted in a AC/DC set,and if they did size it right the Xformer is gonna get warm and be close to it's rated limits at all times...it ain't an elegant solution,but i bet that's what they're doing
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