Many guitarists want to change the sound of their amplifiers in a certain way: more bass, more output or a more vintage-oriented sound. But most of the modifications are expensive and difficult to be done. In this case, to try out a new speaker could be a cheap and easy way to transform the sound of your amp.
The sound of an amplifier can vary if one use a speaker in another size, with another impedance, capacity or made of different materials and by a another manufacturer.
If one put an alnico speaker into a Marshall amp it will sound almost like a Fender Tweed Bassman (these two amplifiers are built-up nearly identical).
Alnico or Ceramic
To me my Silverface Fender Princeton Reverb somehow creates too many haunting mids. While turning up the volume its sound came close to a Fender Tweed amp. To a blues-player this might be a perfect sound but I wanted to use the Silverface Fender Princeton Reverb as a pedal-platform.
After replacing the 10″ alnico by a 10″ ceramic the cleansound became much more balanced.
In a direct comparison the Silverface Fender Princeton Reverb sounds now like a small version of a Deluxe Reverb or a Twin from the Silverface/Blackface era.
Similarly it can be exciting to replace the Marshall-typical Celestion Ceramic by an Alnico. In this way the amplifier sounds quiet more compressed and while overdriving it, one can get close to the sound of the 60s.
The advantage of Vox amps is, that they are offered with both kind of speakers, Alnico and Ceramic. It is worth a comparison. While many guitarists love the overdrive-sound of a Vox amp with an Alnico speaker, the cleansound might be more balanced with a Ceramic speaker. Knowing this can save you from the frequently performed “bright-cut”-mod (by unstitching a certain capacitor one make the signal a bit less shiny).
12″ oder 10″ Speaker
To get more volume it is common to replace a 10″ speaker by a 12″ speaker. But also the other way around can be interesting. Particularly for home-recording it can be quiet interesting to try a bigger amplifier with a 10 ” or even with a 8″ speaker. In this case one has to note the capacity of the speaker. Small speakers will reach their maximum capacity more easily and will -in addition to the overdriven sound of the amp- give a pretty nice compression.
Celestion or Jensen
In the past, Fender amplifiers always used Jensen speakers. But little by little many studio-guitar-players equipped their Delux Reverb amps with Celestion speakers and also the current Fender Princeton Reverb has one. In this case it is also worth it to experiment with different models and manufacturers (like Eminence or Warehouse Speaker).
Watts and dB
With the choice of the speaker you can also influence the volume of your amplifier. While a Celestion Greenback has a sensitivity of 98 db, a Creamback has a sensitivity of 100 db. You can try to produce more pressure and more volume by noting this before you replace the transformer by a new one or modify the circuit. Next to the capacity one should always pay attention to the impedance of the speaker and the amplifier.
If you want to try different speakers you might benefit from a Weber Z-Matcher. This allows you to customize speakers with different impedances for your amp. Additionally you can connect a second box.
In the end two different amps with the same speaker will sound more equal than two identical amplifiers with different speakers.