[dave walsh]

Re your question about Mains/Isolation and Auto transformers Ampro [post 12*]. It's good you seem to recognise that an auto Tr isn't protective as it just taps off a different voltage without isolation [common in 60's record players]. This is my explanation re the others but I claim no expertise].

I can see that the mains and Isolation Tr's may look the same to you. The difference, as I understand it is that an Isolation Transformer is primarily a safety device. They are used on Building Sites and when repairing live chassis or doubtful equipment in the workshop, to avoid harm. Standard valved equipment used a "Mains" Tr [yes they are all mains] designed to be incorporated within the amp itself. You would usually get mains in and out [or even more perhaps 350v on a hefty guitar amp] to be rectified for the valves. Yes that is also [by definition] isolated with two windings. There is no direct connection to the chassis and importantly, the metal work can now be earthed but the function/usage is different. You could still get a leak across a mains transformer [unlikely] but it would blow the fuse via earth so that's another safety factor-not just the electrical separation.

An Isolation Tr relies on that factor alone and is built accordingly. It's often bulkier and usually intended for external use only [like some medicines]. There's a lot more to this in some applications but that's beyond my basic understanding, why I'm generally cautious re advising anyone and there are warnings on electrical sites. "Be careful out there" as the Duty Sargeant always said in a certain well known cop show!

Dave W