12-24-2015, 02:47 AM
Steve, sounds like it may be a moot point, but some tips on replacing a speaker:
There are generally two things to match. One is the impedance driving the audio path, and the other is the resistance of the field coil.
Impedance of the audio path: transformers are used to achieve this match. They transform the impedance of the output tubes to the impedance of the voice coil. Sometimes the transformer was on the chassis, but usually it was mounted on the speaker. Assuming speaker-mounted, then the transformer is already connected to the voice coil and you don't have to worry about that side of it. All you have to do is get a speaker that's the same diameter, has an audio transformer, and came from a radio that used the same configuration of audio tubes. In this case, push-pull 42s or 6F6s. 42s and 6F6s are electrically the same tube (same output impedance), just on different bases (6-pin / octal). So if it's an 8" speaker, then any 8" speaker that a) has an audio transformer and b) came from a radio running push-pull 42s or 6F6s will be a pretty good match on the audio impedance.
Resistance of field coil: the speaker's field coil is an electromagnet that creates a magnetic field for the voice coil to push against. It was also used as part of the power supply, because a coil operates "like a capacitor, but opposite." For that reason, coils make great components for smoothing out the ripple in an AC-to-DC power supply, in conjunction with capacitors. They can do the smoothing and also create a magnetic field at the same time, no problem. When matching a field coil, you're generally trying to match the DC resistance of the field coil, specified in ohms. However, the voltage drop across the coil matters too, because you also care about how heavily wound the coil is. A 1000 ohm coil with 10 volts across it will be wound to carry a lot less current than a 1000 ohm coil with 400 volts across it. The latter would have to carry 40 times more current, and would generate a stronger magnetic field. Those two field coils would likely not be compatible, even though they're both 1000 ohms. If you can match both the resistance of the field coil and the voltage across it within a few percent, you're golden.
That's the gist of it. There are more considerations if you can't find a perfect replacement. Non-perfect replacements may still be usable with adaptations, but there are calculations to be done, regarding how to make up the differences. Not really worth going into unless you're in that situation.
There are generally two things to match. One is the impedance driving the audio path, and the other is the resistance of the field coil.
Impedance of the audio path: transformers are used to achieve this match. They transform the impedance of the output tubes to the impedance of the voice coil. Sometimes the transformer was on the chassis, but usually it was mounted on the speaker. Assuming speaker-mounted, then the transformer is already connected to the voice coil and you don't have to worry about that side of it. All you have to do is get a speaker that's the same diameter, has an audio transformer, and came from a radio that used the same configuration of audio tubes. In this case, push-pull 42s or 6F6s. 42s and 6F6s are electrically the same tube (same output impedance), just on different bases (6-pin / octal). So if it's an 8" speaker, then any 8" speaker that a) has an audio transformer and b) came from a radio running push-pull 42s or 6F6s will be a pretty good match on the audio impedance.
Resistance of field coil: the speaker's field coil is an electromagnet that creates a magnetic field for the voice coil to push against. It was also used as part of the power supply, because a coil operates "like a capacitor, but opposite." For that reason, coils make great components for smoothing out the ripple in an AC-to-DC power supply, in conjunction with capacitors. They can do the smoothing and also create a magnetic field at the same time, no problem. When matching a field coil, you're generally trying to match the DC resistance of the field coil, specified in ohms. However, the voltage drop across the coil matters too, because you also care about how heavily wound the coil is. A 1000 ohm coil with 10 volts across it will be wound to carry a lot less current than a 1000 ohm coil with 400 volts across it. The latter would have to carry 40 times more current, and would generate a stronger magnetic field. Those two field coils would likely not be compatible, even though they're both 1000 ohms. If you can match both the resistance of the field coil and the voltage across it within a few percent, you're golden.
That's the gist of it. There are more considerations if you can't find a perfect replacement. Non-perfect replacements may still be usable with adaptations, but there are calculations to be done, regarding how to make up the differences. Not really worth going into unless you're in that situation.
