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Old 23rd Oct 2021, 9:30 pm   #1
nomisimon
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Default Celestion PPM Soundex Speaker Impedances

Hi everyone

I recently acquired a quite nice Celestion PPM Soundex speaker from around 1932 I think. It still sounds remarkably good. There is a tapped transformer inside which connects to three terminals to provide low, medium and high impedances for the output stages of receivers of the day. I've measured the DC resistances but really need to know what expected 'impedances' were that the tappings provide. I've found adverts that describe the speaker unit and that it has the three choices, but none tell me the actual values.

I wonder if anyone can help, it would be much appreciated?

I've attached some images of the speaker model I have.

Thanks

Simon from Northumberland
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Old 23rd Oct 2021, 10:07 pm   #2
mark_in_manc
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Default Re: Celestion PPM Soundex Speaker Impedances

I suppose you could measure it? Loudspeaker impedances were (more recently) sometimes quoted as the modulus of the impedance at 500Hz - some way above the mechanical resonant peak, and below the rise due to coil inductance. So if you put a resistor of a similar kind of value to that which you are expecting - say 5-20 Ohms - in series with the cabinet, and poke a 500Hz sine wave into it, and measure the voltage across the cabinet and the voltage across the resistor, you can work back to the modulus of the cabinet ip impedance.
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Old 24th Oct 2021, 9:17 am   #3
Mr 1936
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Default Re: Celestion PPM Soundex Speaker Impedances

Hi Simon

That's a fine looking speaker, with a shape not unlike the Philips "Ovaltiney" discussed recently.

I can't help with the impedances other than to second the suggestion of measurement above.

I did turn up a couple of scraps from the August 1932 issue of Wireless World, (see attached pics) which suggest it was introduced that year.
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Old 24th Oct 2021, 10:14 am   #4
nomisimon
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Default Re: Celestion PPM Soundex Speaker Impedances

Hi mark_in_manc, yes that is an option...but I thought someone might say 'oh yes, I can answer that easily'. The impedance is not that of a speaker, it's the impedance needed in the anode of a receiver output stage. I think I may have found the answer on a Marconiphone 144 speaker. Thanks to both you and Mr 1936 for your input, Simon.
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Old 24th Oct 2021, 11:19 am   #5
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Default Re: Celestion PPM Soundex Speaker Impedances

hi Simon

...don't know if this helps, the following blurb found in Aug '32 Wireless World:

>>"The same general form of construction and all the attributes of the PPM type are fołnd in the new series of energised models. The D.C. Soundex is an inexpensive chassis, for its price is 3os. only, which includes the input transformer. It is obtainable as stock type fitted with field windings of 2,500 ohms, 4,700 ohm's, or 6,600 ohms, and an adequate flux density can be obtained with a 4 -watt dissipation, the normal working range being between 4 and 8 watts"
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Old 24th Oct 2021, 11:43 am   #6
nomisimon
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Default Re: Celestion PPM Soundex Speaker Impedances

Thanks Sandy_r...I think that is precisely what I need.
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Old 24th Oct 2021, 11:46 am   #7
ms660
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Default Re: Celestion PPM Soundex Speaker Impedances

That's not the transformers primary impedances.

Lawrence.
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Old 24th Oct 2021, 12:05 pm   #8
sandy_r
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Default Re: Celestion PPM Soundex Speaker Impedances

if you know (or measure) the expected quiescent current of the o/p valve and subtract an average current for field coil you can work back to the DC load presented by the transformer primary, given its drop from supply voltage

Last edited by sandy_r; 24th Oct 2021 at 12:11 pm. Reason: clarification of calc
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Old 24th Oct 2021, 2:15 pm   #9
nomisimon
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Default Re: Celestion PPM Soundex Speaker Impedances

I actually believe those numbers are the typical anode impedances of the valves the transformer, and hence the speaker unit, are designed to work with. Similar speakers have similar impedances specified.

Simon
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Old 24th Oct 2021, 2:34 pm   #10
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Default Re: Celestion PPM Soundex Speaker Impedances

Quote:
Originally Posted by sandy_r View Post
It is obtainable as stock type fitted with field windings of 2,500 ohms, 4,700 ohm's, or 6,600 ohms, and an adequate flux density can be obtained with a 4 -watt dissipation, the normal working range being between 4 and 8 watts[/I]"
British Rola Co.:

https://worldradiohistory.com/hd2/ID...=%22soundex%22

Lawrence.

Last edited by ms660; 24th Oct 2021 at 2:47 pm.
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Old 24th Oct 2021, 3:16 pm   #11
Mr 1936
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Default Re: Celestion PPM Soundex Speaker Impedances

"if you know (or measure) the expected quiescent current of the o/p valve and subtract an average current for field coil you can work back to the DC load presented by the transformer primary, given its drop from supply voltage"

You've lost me there, Sandy

I read that advert for the energised speaker and took the values to be the DC resistance options for the (magnetising) field winding, i.e. not the AC impedance presented by the voice coil/tapped transformer combination.
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Old 24th Oct 2021, 3:29 pm   #12
mark_in_manc
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Default Re: Celestion PPM Soundex Speaker Impedances

Quote:
Originally Posted by mark_in_manc View Post
I suppose you could measure it?... So if you put a resistor of a similar kind of value to that which you are expecting - say 5-20 Ohms ...
Oops - I forgot about the transformer, which is the whole point of the measurement! So I'd perhaps start at a 1k resistor, and measure for each of the tappings available.
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Old 24th Oct 2021, 4:37 pm   #13
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Default Re: Celestion PPM Soundex Speaker Impedances

Is the Low direct to the speaker?
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Old 25th Oct 2021, 8:13 pm   #14
sandy_r
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Default Re: Celestion PPM Soundex Speaker Impedances

Quote:
Originally Posted by Mr 1936 View Post
You've lost me there, Sandy
.
Apologies for the confusion '36
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Old 25th Oct 2021, 11:08 pm   #15
sandy_r
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Default Re: Celestion PPM Soundex Speaker Impedances

Quote:
Originally Posted by PJL View Post
Is the Low direct to the speaker?
the bakelite panel holding the 3 screw terminal-posts appears to show raised 'links' for the connection variants:

High: use the 2 outer connections
Low: use left and mid connections
Medium: use mid and right connections

the Celestion PPM 19 transformer allows matching to 4 levels of o/p valve and it does this using 3 screw terminal-posts and a wander-plug with 2 sockets (Red and Blue) to provide the necssary combinations (see photo)

the 3 terminals are identified as L, H and C - presumably for Low, High and Common, given that C appears in each connection pair (the column with the wander plug info can just be seen on the far right as Red, Blue, Red, Blue)

this may give Simon some clue as to the intended functionality of his soundex speaker - it might be worth delving inside the case and following the wiring. Interestingly, Simon hasn't mentioned having to provide separate field energisation to his extension speaker (which he reports as sounding "remarkably good")

some extension speakers also include a switch in series with one of the speaker connections, presumably to allow the extension speaker to be switched out of circuit with the radio's AF stage without disturbing the connection to the radio
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Last edited by sandy_r; 25th Oct 2021 at 11:24 pm. Reason: addendum
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Old 26th Oct 2021, 6:50 pm   #16
nomisimon
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Default Re: Celestion PPM Soundex Speaker Impedances

Hi Sandy_r.

I did rewire the transformer to terminal wiring as it was missing...someone had started to rewire the speaker as a regular low impedance extension speaker. There is no energising supply needed. It's just a regular speaker with an output transformer on it. As I mentioned, the switch is wrong and disconnected.

Thanks for the help and information, I think I have what I need now to use it as designed. Someone will have a copy of the actual instruction leaflet somewhere.

I have some plans for vintage TRFs and the like.

Regards

Simon
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Old 27th Oct 2021, 9:30 am   #17
sandy_r
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Default Re: Celestion PPM Soundex Speaker Impedances

thanks for the update, Simon; happy days enjoying the soundex with the warm glow of a vintage TRF!

all the best
sandy
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Old 27th Oct 2021, 4:20 pm   #18
nomisimon
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Default Re: Celestion PPM Soundex Speaker Impedances

Sandy

This speaker could save me providing an output transformer for each one, definitely a result.

Cheers

Simon
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