So it's the amplifier in some way that is causing the low volume. The most obvious suspect is the UL84, probably low emission. But only suspect it after checking voltages, in particular the HT+ level. Don't forget to check the AC voltage drop across the UL84 heater, either. With volume set to full, use a 0.1uF or thereabouts capacitor to carry out a 'buzz' test on the UL84 control grid, using your fingers (on the safe end of the cap!) There should be a very healthy buzzy AC hum present. If loud, test at the 'cold' side of the input isolating caps. If this results in a drop in volume, replace the input isolating caps. I'd replace them anyway, with correctly rated types. If the buzz isn't loud at the direct input to the grid, check screen and anode voltages and snip one end of any capacitor across the output transformer to test for leakage. Same with any series capacitor used for tone control purposes. If there is leakage, the cap will probably be warm in use.
From there on you need to consider the output transformer and the loudspeaker.
Hope this helps - sorry if it seems I'm teaching grandad to suck eggs.
Tony
I forgot to mention the screen and cathode electros. If these have dried up, negative feedback effects will drop the valve gain, but whether by enough to cause the severity of problem you describe is debateable.
From there on you need to consider the output transformer and the loudspeaker.
Hope this helps - sorry if it seems I'm teaching grandad to suck eggs.
Tony
I forgot to mention the screen and cathode electros. If these have dried up, negative feedback effects will drop the valve gain, but whether by enough to cause the severity of problem you describe is debateable.
