Hello from Jimb.
I have a question that for some may seem stupid however I want to understand , even if it is a stupid question,
The electrodynamic speaker from my STC 55 is beyond my expertise to repair . I have salvaged the choke and the speaker transformer . I am going to use a permanent magnet speaker in conjunction with these units. I have two questions ,firstly what speaker impedance best suits the old speaker transformer it measures only 1.5 ohms . The second is the silly question I do not have any idea about. The choke unit has a secondary set of windings that fed into the speaker cone coil what did this do ?
Is it ok to do what I intend I see no logical reason not to.
Regards.
Jimb

A speaker coil with a DC resistance of 1.5 ohms would have a nominal AC impedance rating of about 3 or 4 ohms.

An electrodynamic speaker has two coils: field and voice. The field coil provides an electromagnet against which the voice call operates. Often the field coil did double duty as a filter choke for the B+. In such arrangements it was also common to find that if the speaker is unplugged, the B+ is cut off via a bridging strap in the plug.

I'm not familiar with the setup that you describe with a choke having a secondary. That sounds more like a transformer.

The usual issue when replacing electrodynamic with permanent is the loss of the choke function, in which case a high wattage resistor is substituted.

(BTW: I have never seen anyone on a radio forum reply "what a stupid question")

There are actually three windings in an electrodynamic speaker. They have a "Humbuck" winding to counteract Hum from the AC.

Some do separate the field coil & mount it elsewhere, but in order to be effective it should be mounted with a ferric plate on its face.

Current squared by resistance applies when selecting the wattage of a resistor to replace the field. I used a panel mount type in the last one where I subbed (field was actually missing???? dumb hacker).

Perhaps he should have asked questions? Then I would not had to sort out his mess & damage. One should take solace & note the Wisdom in a Chinese proverb that basically says in respect of asking "Better a fool for five minutes, than a fool forever" : So by asking you find out, by not asking you will never know.

Being to scared to ask the question does not fix things.

http://au.rs-online.com/web/c/passive-components/fixed-resistors/panel-mount-fixed-resistors/

http://au.rs-online.com/web/p/panel-mount-fixed-resistors/0158474/ 158-474 1K 50W $5.10 Is an example of what I used & is not likely to cook if its got the voltage rating. Easily mounted on a side plate as a further heatsink.

Marc

Thank you GTC and Marc.
I have mounted the speaker field coil back with the ferric plate that was sandwiched between the field coil and the speaker it is about 5mm thick. The centre pole has been retained and has strong magnetism.
The extra winding must be the humbuck winding. I have mounted the speaker transformer back on top as it was before.
I will still need to obtain a suitable 5 inch speaker from somewhere.
Regards.
Jimb.

There are actually three windings in an electrodynamic speaker. They have a "Humbuck" winding to counteract Hum from the AC

Ordinarily internally connected, I think.

The humbuck coil is in series with the VC so there is on a good drawing with one, the extra coil shown.

You have one end of the secondary on a node going directly to VC terminal; The other to Humbuck, and the other end of the humbuck to the other speaker VC terminal. So there are three terminals involved & one must be careful not to wire the humbuck coil out when rewiring one.

Marc