Sound system for old village hall

In article , Dave Plowman (News) scribeth thus

Actually if you can see the cone or put a very thin bit of wire or something similar thru the grill when you connect a battery say 1.5 odd volts the cone will displace in one direction if its directly connected.

If theres a transformer in there then unless thats an autotransformer there will be no DC connection to the speaker cone so it won't move either one way or the other....

Yes right!...

Most of them long gone now....

Odd tho I never remembered Vortexion or Grampian as making PA systems good tape machines and mics ...

via a 4 ohm resistor, or you'll likely kill it.

NT

if its an odds & ends setup in a village hall, I'd assume nothing about the knowledge of whoever wired it up.

If theyre low R speakers, its not hard to convert the setup to 100v line if needed, or vice versa.

Some cheap 100V line systems did use autotransformers. Some of them were even tapped to sort of control the volume at a particular speaker. And even with a proper transformer, you'll hear a click as you connect/ disconect the DC voltage due to the change of current inducing a voltage in the secondary winding.

If you're in a mining area, it may even be "surplus" 25 volt line as used down t'pit. Very unikely, though.

The best way to check is, as has been suggested, to put an ohmmeter across each speaker. Anything below 30ohms will be a plain speaker, anything over a hundred will be 100 Volt line. A click as you make the connection is good, as it's proof that the speakers do actually work, and survived the party that blew the amp up.

Grampian made quite a lot of PA and guitar amps and their valve stuff is sought-after; =A393 and 11 bids for this one:

In message , tony sayer writes

Vortexion was one.

My favourite was CTH, who I believe may have been in Huntingdon

You might well still get a click even via a transformer as the field collapses.

Vortexion certainly made PA amps as did Grampian. Don't think either made speakers - although they might well have sold ones with their name on them.

their most seen product.

If the speakers are wired with more or less bell wire of any significant length it's unlikely that they would have ever worked properly. Wiring of low-impedance speakers requires really heavy 'cable' of the order of 4 sq mm and is still limited to a few yards for satisfactory working.

Open up the speaker cabinets, partly to clean out fluff, cobwebs and dead mice, and to check whether there's a small transformer to convert from 100 (or perhaps 70) volt line to the correct impedance for the drive unit, typically 4, 8 or 16 ohm.

Transformers are available (I think 70 volt was only ever used by Tandy, BICBW) to fit at the amplifier (if it doesn't have a 100 volt output already), and at the individual speaker(s).

Cobblers.

it will, but not for long...

In a previous life I worked at Vortexion. The PA amps and mixers were the bread and butter of the firm. Export mainly, especially the 'little' portable with a 12v input and built in vibrator invertor. Happy days.

-- Jim White Wimbledon London England I will not hide behind the fifth amendment

indeed. The resistance of copper is such that long runs can be done with bellwire. There is some resulting power loss, but not a great deal.

I gather 70v line was popular in the US, but not here.

NT

It all depends on what sort of power in involved.

that would explain why someone thought it was a "tandy special".

Did you know Les Hill who later worked for Thames TV?

Hmm. It depends on the impedance of the speaker. But 4mm is overkill for most anyway. You must have been reading the Russ Andrews site. ;-)

Peter Walker gave the formulae in Quad handbooks. From memory:-

'The loop resistance of the cable should not exceed 20% of the nominal speaker impedance'.

For most domestic runs, 2.5mm is more than adequate.

of course

only Tandy sold 70v line gear over here afaik

NT