🚀 go-pugleaf

I chased down a similar sounding problem (no pun intended) some years ago,
and after *lots* of false starts, I found a grid coupling capacitor was
"breaking down" internally, causing the "crackeling" sound.  The function
of this capacitor is to block the dc plate voltage while passing the ac
(audio) component.  You will probably find this type of circuit between
each stage.  It should be fairly easy to trace with a audio signal
amplifier, with no input to the amp.  Poor attempt at simplified schematic
follows:
                              blocking/coupling
                                capacitor
          ________________________| |______             |------------>
  plate  |                     |  | |     |             |  plate
      -------                  |          |          -------
>------ - - -                  \          |---------- - - -     grid
grid   _____          plate    /                     _______
      | /\  |       resistor   \                    |  /\   |  cathode
      | ||                     /                    |  ||
      | ||____                 \                    |  ||
      | ------  filament       |                    |  || filament
cathode                        |----- to B+            ||
 
Hope this helps!
                                     - Frank
*************************************************************************
Frank Tompkins           :   Internet: X1FT@VM1.CC.UAKRON.EDU
University of Akron      :
Akron, Ohio   44325-3501 :
  "I have not failed, I've just found 10,000 ways that won't work"
                                                     - Thomas Edison