Where T = RC, or time constant (seconds) = resistance (ohms)*capacitance(farads) for a simple roll off network,
and, Frequency of a simple 2 component high pass network, F= 1/(2pi*R*C),
Your 1500 pf (or mmf) capacitor will pass frequencies roughly three times lower than the original 500 mmf one. The coupling capacitor feeding the stage grid resistor acts as a low pass network in the first order yielding about -20db(volts)/decade or -6dbv/octave of freq. The roll off point (the knee) is where the signal is cut in half, -3dbV. Other stuff affects rolloff such as previous stage's plate load resistor, the tube characteristics, etc. but the effect is dominated by this simple network between stages.
Do the math with 500pf into 1 megohm vs. 1500 pf into 1 megohm, which is the input circuit of the Johnson 12AX7's second triode.
You'll also notice a 200mmf cap. (C53) across the volume control if it hasn't already been cut out. That capacitor reduces the higher audio frequencies into the next stage.
Johnson restricted the audio passband of the Ranger in an attempt to imitate the communication quality of the "serious" ham using a carbon button microphone, very close to what you'd hear on an older telephone instead of any sort of hi-fi.
