Yeah trying to drive a speaker with a 12ax7 a fraction of a watt is all you'll get . There is however a few small amps that use a 12au7 in a push pull output stage to get a watt or two ,one of these is made by blackstar .
To do it properly you'd probably need a 12 ax7 at the front end ,one stage for voltage gain maybe followed by tone controls , the second triode set up as a phase invertor (split load) ,then a 12au7 in push pull coupled through a transformer . A 12ax7 only draws a milliamp or two of anode current max and it expects a very high load impedence compared to what normal transformers are designed for, the 12au7 is much lower impedence ,so maybe a small edcor push pull something like 12-15kohm : 4,8 or 16 ohms depending on the speaker you have will work ok .
Have a look at what this guy did with a couple of ax7's and an au7 on the output, even though the volume isnt massive ,its still an impressive sound .
https://www.youtube.com/watch?v=bgue2NYCVgY
The 48 volt mains transformer you have wont be much good either ,you'll be wanting around 220-250 volts ht .Try flea markets and garage sales ,something like an old taperecorder chassis might have everything you need,mains ,output transformers etc.