I built a Fender 5f1 Champ imitation.
I've taken care to start it up gradually (first with no tubes, then just the 5Y3, then the 12AX7, then the 6V6 with the speaker connected and finally connected a guitar to it.) So far I've seen no shorts or blown fuses. The internal capacitor voltages check out.
After connecting the guitar to either of the inputs, I noticed that the speaker doesn't produce any sound, not even white noise (which is also true, when the guitar isn't connected - this I didn't remember to check before plugging in.)
Instead, the output transformer produces a wheezing, buzzing, cackling sound in response to the volume being manipulated (not just responding to the magnitude, but changing the pitch and sound being produced) and interestingly alters the sound in response to strings being struck on the guitar (most notably, the low E string, with higher frequency notes the effect is not that much noticeable.) Again, all the time, there is nothing coming from the speaker.
I've done a basic check of the output transformer (as detailed in this video) with nearly identical results, but without desoldering the transformer from the circuit.
I didn't check the speaker and I honestly wouldn't know how. However, it is brand new and upon visual inspection, I don't see anything wrong with it.
I've uploaded a short video here.
I've used the standard schematic, with a slight modification (the feedback 22k resistor is split into a potentiometer and a 10k resistor in order to control the feedback):
Where could this behavior come from? What should I check?