[Phlogiston]

OK All of the above is good advice. So, the next thing I would check for NO audio output is continuity across the primary of the output transformer. Check the secondary and the speaker voice coil too but these are far less likely to fail.

Replace all of the paper and electrolytic caps.

You could try next to inject a signal on the grid cap of the 75. You can just touch it IF YOU KNOW WHAT YOU ARE TOUCHING. Listen for hum. Don't get electrocuted.