Thorola Model 4 Speaker

Hello to all. I am a novice trying to restore a Thorola Model 4 horn speaker that goes with my Thorola Model 55 radio and have two questions.

I need to rewind the wire on the coil that sits in the magnets in the base of the speaker. I have checked info in the museum and it seems these types of speakers are 1k, 2k, or even 4k ohms. What should the resistance be on this speaker?

Secondly, if I rewind the coil and the resistance is "low" can I add a resister to bring it up to the correct value? I'm planning on rewiring it with as much #28 wire as I can get on there. Or should I use #32 or #40?

It appeares that someone had taken this apart before because the inside of the base is a mess. The original wire on the coil had come unwound because the spool was broken so there was no way to measure the original resistance. The original wire is extremely fine.

Thanks.

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You need similar thickness to wire to get enough turns as the sensitivity is related to number of turns. A series resistor would only make the DC bias voltage and current correct, it would reduce already poor sensitivity.

There are two options:

1) Use similarly fine enamel copper wire

OR

2) Use a 2000 Ohm moving iron earpiece, Some horn drivers are really just an earphone driver.

Usually 2,000 Ohms is fine, 1000 is a bit low. 4K might be rarer.

A less authentic option is a 2,000 Ohm to 8 Ohm transformer and a suitable size 8 Ohm loudspeaker to drive the horn. You often see transformer and 2 Ohm to 8 Ohm 8" to 10" speaker fitted in place of 2000 Ohm moving iron (reed) paper cone speakers as 1940s to 1950s repairs of 1929 to 1932 radio models.

Note that the voltage ratio = turns ratio for a transformer, but AC impedance is square of turns or voltage ratio, so 250:1 of 2,000 Ohms to 8 ohms is 15:1 ratio, A small 110V to 7.5V transformer might drive an 8 Ohm speaker. The Grid Bias on the output tube is adjust to give perhaps 4mA. One trick to avoid core saturation using a mains transformer for Audio single ended is a cancelling current. This happens automatically with push pull as the DC currents in primary cancel. For single ended you can put 15 x 4 = 60mA in a second secondary winding, or 4mA on the other primary of a 2 x 110 transformer for USA or Europe,

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Thank you very much Michael. Now I see that the resistance comes from the wire which has some resistance value per unit of length. I put a micrometer on the wire and it measures .006 which is #34. I'll try and carefully rewind it and let you know the outcome.

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