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Fog horn type of sound from my amp?

Discussion in 'Amps and Cabs [BG]' started by Dave44, May 2, 2003.

  1. Can anyone here give me a rough idea of what the heck it was I heard last night on my practise amp? It's a Fender Bassman 10 from 1972. Acually I had the guitar in my stand and was getting a cup of joe and all of a sudden this loud sound like a fog horn started to wail on me. Turned the amp off then on again and it worked fine but I'm nervouse. I'm almost positve it wasn't feed back the volume was only on at about 2.5 and the guitar strap was laying across the strings so they weren't vibrating. I have it scheduled for repair next week but I'd like to give the tech as much info as I can before leaving it with him.
  2. It was feedback, but not from your guitar. It was probably from a microphonic tube. The speaker vibrates the internal elements of the tube, which is in phase and the whole amp goes into oscillation. Next time just whack the amp on the top with your fist and it'll shut up. To fix it permanently, you'll need to replace probably the first or second 12AX7 with a new one.

    As a test, turn the volume up slightly and then thump on each tube and listen for a loud ringing sound from the speakers. They'll all do it a little bit, but there's probably one or 2 tubes that will do it really bad. No biggie, just one of the joys of tubes. :D


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