[DaleHCook]

What you are looking for is a transformer that will match 6K to 1 1/4 ohms.

We don't know what the voice coil impedance is - just its DC resistance. The approximate voice coil impedance is fairly easy to measure with the speaker mounted in the cabinet, but I prefer a more sophisticated measurement using instrumentation. From experience I would estimate a 3 to 4 ohm range for the voice coils of most antique radio speakers. The optimal plate-to-plate load impedance for a push-pull pair of triode-connected 42s in class AB2 with fixed bias is indeed 6,000 ohms (see the characteristics for the 6F6, which has the same characteristics as the 42 and the 2A5). Among universal output transformers something like a Stancor A-2855, A-3880, or A-3890 would work. I do not know offhand if their physical sizes are any smaller than the Hammond 125-D suggested by Ron (but can check if needed), but they are all rated at 15 watts so, like the Hammond 125-E, they may all be larger than the original.