Some useful and precautionary field coil data obtained 

while testing a Jensen F12N


This data was acquired on a Jensen F12N with a field coil of 992.8 ohms and may be considered as 1000 ohms.  Before applying power to any field coil,  its DCR should be measured.  The voltage to be applied can be calculated by Ohm's law, P=E2/R.  It is suggested that the power be set at about 14 watts at ambient temp as most field coils probably operated with less than 18 watts.  Considering their age today, 14 watts is considered prudent.

The voltage applied should be measured under load of the field coil.  While AC will work, it will produce a 60hz ripple and may result in an audible hum.  These field coils were originally used as power supply filters for the vacuum tube plates.  The very early field coils of the dirty thirties and earlier had much higher DCR due to the 250 volts or so needed for the tubes. This was AC of course and with a 120hz ripple from the full wave rectifier, hence the demodulator coil (hum bucker) wired in series with the voice coil and placed at the end of the field coil close to the voice coil..  This equates to 19 watts for a field coil of 3300 ohms, one such coil I have on a 1923 Western Electric field coil speaker.


The table below shows the increasing temperature of the field coil over time.  When the temp. was stabilizing, the power was increased slightly.  This was stopped when the temp stabilized at 135 degrees, showing an increase of only 2 degrees over the next 19 minutes.  This is also due to the effect of the increase in DCR of the field coil.

It is deemed prudent to keep the power to the field coil at about 14 watts.  Over long periods, the temp will rise and so will the DCR of the coil, thus dropping the power transfer to slightly under 14 watts.

In this case, if the initial power was held at 14.26 watts, after a few hours when the DCR rose to 1148 ohms, the actual power transfer would be 12.54 watts.  It is felt that this would be satisfactory considering the age of the field coil.  The effect on the acoustic output of the speaker is considered to be negligible.

Over the course of this test, the approximate rise in DCR is about 2.8 ohms per degree rise in temperature.  This can also be expressed as 0.72 ohms per minute in an ambient temperature of 80 degrees and over a 3 hour period with `14 watts applied at the start.

In short, such a speaker under the above conditions can be operated all day without overheating the field coil.

Data run Oct. 18, 2012.  (It's hard to believe that 6 years have passed since then.)



Applied DC voltage Applied Power


Coil Temperature


Time in minutes power applied NOTES
120 14.26 105 25 ambient temp 80 degrees
112 33
118 40
125 15.5 121 45 temp rise stabilizing so power was increased
126 52
127 59
130 74
130 16.73 135 92 actual power = 14.72W as coil DCR is now 1148 ohms
137 111



Three views of the F12N which is all original.

For a larger view, left click on the appropriate pic.

For more info on this unit, see     and


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