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Old 19th July 2013, 09:03 AM   #1
tcqanh is offline tcqanh  Viet Nam
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Default Need information about this Fieldcoil speaker

Hello,

I have these pair of speakers but they have not mark/ label on it and not attachment power supply. So I don't have any information about them.

Could anyone help to tell me what is this speaker (with 30cm diameter) and how to use it ?

Thank you and best regards,

QA.
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Last edited by tcqanh; 19th July 2013 at 09:25 AM.
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Old 19th July 2013, 09:23 AM   #2
VaNarn is offline VaNarn  Australia
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I believe the speakers are a universal model ,known as a G12,that was made in several countries under a licence from the U.S.A. In Australia,Rola were the manufacturers.
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Old 19th July 2013, 10:34 AM   #3
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Lovely old names. Rola became Rola Celestion, the in Batley, Yorkshie, England, became Fane.
Very long throw speech coil and from memory, rated at around 15Watts probably 15R impedance or 7.5R
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Old 19th July 2013, 12:13 PM   #4
IG81 is offline IG81  Canada
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Quote:
Originally Posted by JonSnell Electronic View Post
Lovely old names. Rola became Rola Celestion, the in Batley, Yorkshie, England, became Fane.
Very long throw speech coil and from memory, rated at around 15Watts probably 15R impedance or 7.5R
Wasn't there a link between Richard Allan and Fane or am I mixing things up in my head?

IG
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Old 20th July 2013, 01:17 AM   #5
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by VaNarn View Post
I believe the speakers are a universal model ,known as a G12,that was made in several countries under a licence from the U.S.A. In Australia,Rola were the manufacturers.
Tks VaNarn.

This link shows Rola G-12 has an 8-ohm nominal voice coil, and a 5100-ohm rated field coil impedance. I never use/ build field coil spks so my experience about them is nearly zero.

Pls, tell me how do I get the their specifications ? How is the power supply (voltage ane current) for the coil ?

Vintage Rola G 12 Field Coil 1950s Speaker 8 Ohm 12" Hammond Leslie | eBay
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Old 20th July 2013, 02:01 AM   #6
tcqanh is offline tcqanh  Viet Nam
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I try to calculate PSU for this fieldcoil. Using Ohms' law with P= 12W, Rdc = 5100 ohms. So I get U = 247Vdc and I = 48.5mA.

Is it right ?



This voltage is too much high when compared to currently fieldcoil spks. What is this advantage of high volatage ?
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Old 20th July 2013, 02:13 AM   #7
VaNarn is offline VaNarn  Australia
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Hi tcqanh.It was usual to run the field coil for a dissipation of 12 W and in most cases the electro magnet cum filter choke was used as part of an amplifier/radio power supply( when under class A conditions). As this involved basically current operation,field coil resistance values were supplied to meet the specific needs of each end user.The nominal voice coil impedance for the models manufactured in Australia were 2 to 16 Ohm. Basically to run the speakers that you have,I would measure the DC resistance of the field coils to determine their values.Don't be surprised to find a sizeable difference in the readings as tolerances were quite wide. A 5100 Ohm field coil would need a 250 V DC supply and since this is a high voltage,safety issues arise.Given the age of the wire and doubts about the type of enamelling used for insulation,I would recommend a rewind to a lower resistance say,48 Ohm,and use a regulated 24 V supply to run the field coil.
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Old 20th July 2013, 02:40 AM   #8
tcqanh is offline tcqanh  Viet Nam
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I'm fan of Tube ampli DIY and did build many tube amp, so high voltage like 250V is not problem. You are right. Isolation of field coil here is really issue.

I guess there are some Rola G12 versions. I've just got one specs of them on the internet:

Rola G12 T-S parameters

5100 Ohm Field Coil
12 Watts dissipation
V=260V I=0.048A

Qts 0.38
Qms 15.87
Qes 0.39
Vas 65.49
Sens 100.22 dB
Sd 0.0511
Fo 74.41
Dcr 6ohms
Cms 176.64 mN
MM 25.89g

Is it right information for all of Rola G12 ?
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Old 20th July 2013, 03:19 AM   #9
tcqanh is offline tcqanh  Viet Nam
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VaNarn,

Do you have inform on frequencies response range of theses spks ?

For your opinion, What high freq driver is compatible with these G12 ?
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Old 20th July 2013, 11:12 PM   #10
VaNarn is offline VaNarn  Australia
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If your G 12's are fitted with the original cones,i.e. they have a Fs resonance of 75 Hz or higher,then I would consider having them reconed to achieve an improved performance.A lower Fs, a long throw voice for a larger Xmax and a slightly thicker cone would be all be beneficial as the design concepts for speakers of that vintage aimed at different standards than what is acceptable to-day.Early speakers achieve most of their apparent efficiency via equal gap/voice heights and cone break-up behaviour and have limited power handling capacity.If you wish to use the G12 as is, then the only likely candidate would be a horn loaded compression tweeter,preferably with a wide dispersion and a usefully low xover freq.,say around 1 kHz.
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