Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jim on January 10, 2013, 12:10:19 pm
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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
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1. Always suspect tubes first.
2. http://geofex.com/ (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.
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Try other final tubes
K
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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.
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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