Replacing a field coil type speaker.

[ed731pdh]

Radio set here has an 8" Rola field coil type speaker for a Bush AC71 and the inner field coil is totally shot, the copper wire is breaking up with age in multiple places and the paper former is brittle and life expired.  Four wires to the speaker.  Two go directly to the field coil.  Of the other two, one wire goes to one side of the speaker windings, the other side of the speaker winding goes to a short heavy winding that sits over the field coil windings, and then back to the radio itself.

Looking around, difficult to get a replacement so looking to replace with a standard 8" speaker.  The poser is that the HT supply for the valves goes through the Rola speaker first to provide the magnetic field.  This could be replaced with a permanent magnet speaker and a resistor of the right value for the HT supply.  The speaker drive itself is not really much of an issue.

Can anybody see any problems with this?

[Terry]

A smoothing choke would be a better bet if you can find one. The voltage drop across it will be less than through a resistor and, as RC smoothing is not as good as the LC type you would be replacing, you might need to beef up the size of the electrolytics a bit to keep the resistor value reasonably low. Let's face it, RC smoothing has been widely used over the years and is probably related to the capacity and price of electrolytics. As values went up and prices came down, manufacturers found it more economical to fit bigger caps and ditch the expensive choke.

There should be no real problems, though, either way.

Incidentally, the 'short heavy winding that sits over the field coil windings' that you referred to is a hum-bucking coil and adds AC from the field magnet in anti-phase to the audio signal to neutralise induced hum.

[ed731pdh]

Terry, thanks for that. The resistance of the field coil is 2k according to the spec sheet. Looks like a trip on the net for a suitable candidate.

[Crackle]

Energised speakers are often available online auctions and at vintage radio rallies, Mike Lewis normally has a few.
If you do not have Mikes contact details send me a PM and I can let you have his phone number.
Mike

[Refugee]

If the cone is in good condition they are often pretty easy to rewind.

[Crackle]

If the cone is in good condition they are often pretty easy to rewind.

That's true, I have done 2 now, complete strip off all the winding and wind on new enamelled copper.
First one was done with an electric drill in a drill stand laid on its side.
Mike

[Refugee]

An old cordless power screwdriver run from a bench power supply will be even better if you have got one with a duff battery laying about. You get enough torque for winding coils from an amp or two.
I have got an 18 volt power tool that had a duff battery connected up ti a 17AH dryfit for those jobs that just drain the supplied batteries.

[ed731pdh]

Cone is in the same poor condition as the coil former, the paper turning to powder.

[Terry]

I think that clinches the matter!

[Refugee]

Speaker cones are known to turn into mouse nests too.