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A15 MK2 ++ channel start to whistle

    • A15 MK2 ++ channel start to whistle

      Hi, i have a A 15 mk2 for about 8 years, recently the channel ++ started to whistle. The whistle appears from the volume to two and increases with the increase of the volume, at half gain. I think it isn't a problem of pre microphonic tube, have been tested, and anyway replaced all the pre tubes, included the phase inverter, could be the power tube (never change but they sound good...)?
      Which components can I control?


      Many thanks
    • Hi,
      if it was working properly and you did not change anything to the circuit it's not the circuit itself. It might be
      1) a microphonic valve (preamplifier/PI, so if you already replaced these tubes we can tickle that)
      2) external reason (guitar/speaker), can you reproduce that behaviour w/o your guitar connected to the amp?
      3) wiring => you should check the long (shielded) wires carrying the signal, especially if the shield is still connected properly zo ground. Please check also the switch for boost (+/++) if all connections are OK. You can use a wooden stick to move the signal related wiring a bit with ++ and volume at 3
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