AC30 low volume problem

[jim]

I have an British circuit board AC-30 that had blown HT and main fuses.  I replaced the power tubes and
the rectifier.  It works but there is an intermittant low volume condition where when I wack the guitar the amp crackles open and volume is normal until the signal decays.  If I keep playing hard the amp sounds great and when I stop it goes back to low volume.  The cord and guitar are good.  I have chopsticked the preamp section, reseated all the ribbon cables, and swapped the preamp tubes but the condition persists.  Wiggling the controls and jacks do nothing.  Is there a logical way to troubleshoot this?  
Thanks,  Jim

[jjasilli]

1.  Always suspect tubes first.

2.  http://geofex.com/ > -Tube Amp Debug Page > Low power or volume, or volume drops off

3.  Put amp in fault condition.  Systematically measure & record voltages at all tube pins.   

[kagliostro]

Try other final tubes

K

[archaos]

Some things to suspect as well :

    - cold solder joint(s)
    - a defective component (resistor, cap,...) I actually once had a bad S_zo vintage coupling cap (in new condition!) which produced the same symptoms you describe, i.e. intermittent volume drops & crackles. I found the culprit when I gently but firmly chopsticked it. I assume one lead at least had some issue inside the dielectric.
 

[jim]

Another set of power tubes did not fix it.   I does seem like a weak connection rather than a failing part.
The amp works great when I really slam the first grid then it goes back to low volume after the note decays. These circuit boards are brittle and difficult to remove.  I would just to narrow the search a bit before I go looking for bad solder. Jim