Calculating the wattage of a speaker protection light bulb

I have a very old musician's singing system in my hobby room. Yesterday one of the speakers broke down. In search of the cause, I finally opened the speaker box and found two light bulbs inside. One of them had burned out. They are probably used as overload protection. Unfortunately, the label on the light bulbs is no longer legible. The only thing I could decipher is 24 volts. Does anyone have instructions on how to calculate the wattage of the light bulb depending on the chassis? I couldn't find anything about it on the Internet. This technology is probably pretty outdated.
 
Current speakers (PA and domestic) use 'SK3 type' protection lamps rated at 12.8 V.

Most use RED dot lamps.

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However, a BLUE dot gives more protection as explained in this ebay listing:

https://www.ebay.co.uk/itm/15508218...mvrdCVv71hUgYUuFWK5XkZ5Q==|tkp:Bk9SR6D95_fwZA
 
Just because I can, I'm illustrating the Celestion RTT50 Ring Transmission tweeter, 8 ohm.

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This would appear to be a decades old driver that Celestion rated at 25 watts RMS.

I wonder if its high pass crossover is in good condition?

Perhaps there is an out of spec bipolar electrolytic capacitor that could do with replacing?
 
are there two lamps in parallel?

I was wondering about the arrangement at first when the OP said in post #1, "I finally opened the speaker box and found two light bulbs inside".

I'm now going by the OP's description in post #4, i.e., each of the two speaker boxes contains a single lamp in series with the tweeter.
 
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Now that we know that a single lamp is used for tweeter protection, I'd go with the BLUE dot protection lamp illustrated in post #2.
The blue lamp draws would provide about -8dB attenuation when hot, the red around -5dB, only half the attenuation.

The blue would burn up (fuse) at half the power of the red, somewhere above 14 volts. The red would probably not fuse before a tweeter rated for 25 watts RMS would.

I've had a single burst of feedback burn the fuse out in EV S-40 speakers.
The blue fuse/lamp is cheaper than tweeter diaphragms, though never blew any of the S40 tweeters after we bypassed them.
 
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