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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 8th Nov 2019, 1:06 pm   #21
dave walsh
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Default Re: 1930s field speaker?

I like the thoughtful way in which you mounted this "ex-cabinet" LS Chris! Unless I'm seriously mistaken, the consensus seems to be that it's actually a permanenat magnet type-in which case what happens when you wave a screw driver at it?
I've always known that early speakers often had their own power source. I only came across the designation "Mains Energised Loudspeaker" though, not "Field" but [of course] the electro-magnetic creates one! It's certainly a fine example either way but given the vintage and "ad hock" production regime at the time, you may never locate it's original home. Based on past experience though, just me saying that, in the way of things, may result in a positive ID from a member

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Old 8th Nov 2019, 8:26 pm   #22
cdm1christopher
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Default Re: 1930s field speaker?

I have another a Houston type e 1930 loudspeaker pic attached that reads 1k on winding and the pye around 600ohm. If your saying there should only be a few ohms then something must be wrong then although I thought these 1930 and pre needed a higher output because of the design of radios back then. Just stabbing in the dark.

But if your saying should only be a few ohmes then obviously something wrong?

Any info on those readings most welcome.
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Old 9th Nov 2019, 1:18 am   #23
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Default Re: 1930s field speaker?

"Mains energised" has always struck me as rather a misleading term- the field coil has to be energised with dc, direct mains would be useless! You can get away with a fair amount of ripple voltage since the high inductance of the field coil means that the actual ripple current in the winding is quite low.
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Old 11th Nov 2019, 1:45 am   #24
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Default Re: 1930s field speaker?

It's one of those hangovers from earlier times. As David mentioned earlier on, the only way to get the field strength early on was an electromagnet. Originally they had a dedicated mains powered supply, but domestic versions usually used the HT current, and for economy it doubled as the HT choke. Alternatively, some were wired across the HT instead. Either way, they're energised. The mains bit is a lingering memory, nothing more.

Academic really, as this thread is apparently about an early PM speaker.
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Old 27th Feb 2020, 4:03 pm   #25
cdm1christopher
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Default Re: 1930s field speaker?

Been a while since being on the thread but I have discovered that the colour underneath was white as per David's I have also found out that the bolts for the magnet are brass as is the bracket which holds the speaker onto the timber box which is solid mahogany.
I've also noticed that the 4 studs near where the speaker terminals are located must have had another transformer attached.
So I've decided to strip it right back clean up the brass re paint and use a general foam surround as the leather is too heavy. So someone decided to just spray the whole thing brown! Was pain to get of.
It works fine off my quad mono block and the centre screw adjusts whether you want to listen to just the high notes which are beautifully reproduced or loosen off and the bass also comes into play.
Not finished yet but will post pic.
The transformer is quite large if it's intended for output only. Again bit rusty so been dismantled and will be rust free and painted. Also interesting the end part of the large magnet has a copper looks like thin slivers similar to a radio air capacitor fins which are also centred on pins which is held in place by the 4 bolts. That was also sprayed over and greased not sure how the paint adhered to be honest.
Anyhow I'll post some pics in due course.
So fair bit of work but worthwhile.
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Old 27th Feb 2020, 4:13 pm   #26
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Default Re: 1930s field speaker?

Could not resist heres some pics.
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Old 27th Feb 2020, 4:14 pm   #27
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Default Re: 1930s field speaker?

one more
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Old 29th Feb 2020, 9:52 pm   #28
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Default Re: 1930s field speaker?

Quote:
Originally Posted by cdm1christopher View Post
the centre screw adjusts whether you want to listen to just the high notes which are beautifully reproduced or loosen off and the bass also comes into play.
I'm not too sure about that TBH. I think you may be loosening the centre pole piece and letting it trap the voice coil, so I wouldn't keep doing that or you may damage both the coil and its former. The cone should move in and out absolutely freely when carefully gripped on both sides between finger and thumb with no sign or sound of rubbing from the centre.
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Old 1st Mar 2020, 12:10 am   #29
cdm1christopher
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Default Re: 1930s field speaker?

Can the mods close this thread please.
Many thanks Chris
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