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Old 11th Feb 2019, 11:58 am   #24
dave walsh
Join Date: Jan 2006
Location: Ramsbottom (Nr Bury) Lancs or Bexhill (Nr Hastings) Sussex.
Posts: 4,614
Default Re: [Restoration AND Light Modification] Murphy 146

More excellent advice there from Trevor and Herald Michael [posts 20/21]. The difference between using an Auto [ie ONE tapped winding] Transformer....a TWO winding conventional Transformer..... or a "live chassis" ie NO transformer at all circuit [only live if the mains wiring is reversed] is explained in the PSU [Power Supply Unit] section of Paul Stennings great notes.

Most valve radios have the conventional type where the two windings are only connected magnetically. An auto Transformer "taps" a voltage off just one so it's less isolated and not as safe. As Trevor said, you can buy an external [Isolating] transformer [with two windings itself] or [effectively] put a "proper" mains transformer [Tr] in place-two windings again! Either way you shouldn't be applying power yet so no rush although starting there and working towards the "front end" [where all the tuning takes place] is standard practise mechanically or when reading up the theory!

I should say that a "conventional" two windings Tr is usually better but it does NOT guarantee safety.. just a separation from the full might of the mains. Often the voltage OUT to the rest of the set is the same as the mains coming in [at 240 AC]. It might even be quite a bit more sometimes, say 300v, or a lot less- eg 6.3v to light up the valves and the tuning scale. The "transform" principle works both ways. No I'm not in competition with Paul's Guide-you can't improve on perfection. I just think that some of this needs emphasising.


Last edited by dave walsh; 11th Feb 2019 at 12:10 pm.
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