That's the best way to learn, by experimenting. Cap #21 is on a signal line. The signal goes from plate of 36 to grid of 42 tube. This cap reduces high frequency response of the signal as you discovered. Try different values you might like tone quality of other values.
Norm
:Hello everyone
:I'm not sure I'm good at recognizing what the cap is used for in the circuit I am working on or any circuit for that matter. Just recapped some of my philco 80, and I had to redo it because it sounded like the bass was turned all the way up with no treble(not that this radio has these knobs). I had to replace cap #21 in the schematic,,,...thought it was a bypass cap which I also thought could be as large as 10 times greater than the original value...The value calls for .001 so I tried a .01....all bass no treble!