What 'ome' to go to?.

[catch29]

My old valve Heathkit S99 has a common and 3ohm & 16ohm speaker outputs, my vintage Kef's are rated 8ohm (nominal?). I've looked on the internet for guidance but just confused myself (it's my age). What would be the correct connection, 3ohm or 16ohm?, I don't want to damage the beast now I've got it working!. Confused of Bletchley.

[ppppenguin]

You won't cause any harm either way. If you use the 3 ohm output you won't get as much volume, but you probably won't notice. Use the 16 ohm output and you might overload the amp if you play a lot of very loud music but you'd have to try very hard to damage the amp.

If the 3 and 16 are taps on a single winding (seems likely) then you could get a 13 ohm nominal output between the 3 and 16 terminals. That's close enough to 8 to give you no worries.

[Kalee20]

If the 3 and 16 are taps on a single winding (seems likely) then you could get a 13 ohm nominal output between the 3 and 16 terminals. That's close enough to 8 to give you no worries.

I can't see that, Jeffrey!

If the 3 ohm tap has n turns, the 16 ohm tap will have √(16/3) x n turns or 2.309n turns.

So from the 3 ohm tap to the 16 ohm tap will be 1.309n turns

and that'll match to a load of 1.309² x 3 ohms = 5.14 ohms.

That's actually a rather better match as it happens!

[ppppenguin]

You're right! I was struggling to remember some 3 phase theory when I replied and my poor brain cells were in overload with square roots of 3. Ordinary square roots of 2 simply lost out.

[Kalee20]

Ah... there you have my sympathy Jeffrey, 3-phase calculations attract √3 errors like wasps around honey. It's a bit like push-pull picks up factors of 2 (or even 2² = 4 if really unlucky), and which resulted me in scrapping 4 hours of winding time last year after the output transformer I'd wound matched not to the intended 8Ω loudspeaker, but to 32Ω...

[ppppenguin]

AlI I was trying to do was calculate the voltage drop on a run of 4 core (3ph+neutral) SWA. Balanced resistive 3 phase load. Ignoring 2nd order effects like temperature. An online volt drop calculator gave me an answer that I wanted to crosscheck. I started with the resistivity of copper and worked from there. After much playing with SQRT(3) I finally remebered that for balanced 3 phase there is no neutral current, so the resistance of the neutral is irrelevant.

Last time I did 3 phase calcs was in the power engineering module that was part of my degree in 1977. Although that course wasn't actually taught by Laithwaite, he was never far away, in his basement toyshop.

[catch29]

I remember Prof Eric Laithwaite on tele demonstrating his anti gravity experiments using gyros. A great British eccentric scientist.