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1962 Brown Fender Concert Microphonic Problems

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  • 1962 Brown Fender Concert Microphonic Problems

    Hi, I have recently bought a 1962 Brown Concert amp. Everything has been fine for the last month but suddenly I keep getting microphonic problems with the preamp tubes. The first time this happened about a couple of weeks ago I put a nos Mullard in to the v1 position and it was fine again. A couple of days ago I had the same problems with microphonics. I have tried a variety of tubes both NOS and brand new and systematically tried each valve in every position in the amp but the amp is still very microphonic. It has a high pitch squeal even when there is nothing in the input and the whole chassis is microphonic when it is touched. I have tested the voltages in the amp and they are running about 50v higher than written on the schematics. Could the problem be the higher voltages? I knocked down the mains voltages by using a lamp and this stopped the microphonics. Any suggestions or help of how to cure the problem? I'm getting about 240/250v on pin 1 and 6 on V1 which the schematics states should be 190v and even higher results on the PI tube. Is this a voltage issue or are all my 12ax7's bad! Thanks, Mark.

  • #2
    Open the amp and tap on the parts with a chopstick, one at a time.
    Usually it's the coupling capacitors that become microphonic with age.
    The plate resistors and cathode bias resistors can crack with age.
    The cathode bypass capacitors can leak and short...causing too much gain in the preamp.
    BUT don't change them all. Just the "worst" most microphonic ones.
    The old parts in the amp are the reason that the amp sounds "vintage."
    If you gut the old parts, the amp will sound modern, not vintage.
    SAVE and preserve the old parts, as part of your amp archives.

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    • #3
      Thanks for the reply. I tried this a bit earlier but none of the caps seemed microphonic. The only things that were clearly microphonic were the wires from the preamp tube bases. The leads were all very microphonic across the whole length of the preamp. I will go and have another test around the caps.

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      • #4
        It's not the wires,
        It's because the wires are connected to the grids of the preamp tubes.
        I would then just suspect that the cathode bias caps are shorting out, or leaking, causing the preamp tubes to draw too much current.
        Any wire connected to the grid of the preamp tube is going to be very sensitive, like a microphone. It has become "too" sensitive.

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        • #5
          I see the theory about cathode bypass caps increasing the current on preamp tubes by reducing the bias, but the gain increase from that would be pretty limited wouldn't it? It's not a problem I've ever seen, and generally faulty bypass caps cause other more noticable symptoms than a gain increase in my experience.

          Mark, the high pitched squeal you mentioned suggests oscillation to me. This feedback goes electromagnetically from wire to wire, often from plate back to grid and round and round causing squeals and other weirdness as it is often at frequencies you can't hear. It can seem to increase microphonics on grid wires etc, as feedback can multiply gain uncontrollably. Lead dress would be the first thing I would try, as it involves nothing invasive and is easy to do. The plate and grid leads need to be kept separate. Try the plate leads against the chassis and the grid leads in the air, or vice versa. Look out for any power amp wires getting close to preamp wires, specially those that go to the speaker jack as they radiate a lot.
          Last edited by Alex R; 08-06-2012, 06:08 PM.

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          • #6
            Also, if any wires must run parallel & then cross, cross them at a right angle.

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            • #7
              Hi, I have gone over the circuit really carefully and found a couple of dry joints and bad connections. I have made the connections good again and the amp seems to be working fine now. Thanks for the help.

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              • #8
                Originally posted by markammon1979 View Post
                Hi, I have gone over the circuit really carefully and found a couple of dry joints and bad connections. I have made the connections good again and the amp seems to be working fine now. Thanks for the help.
                Nice one, glad you got it sorted. Repairers have a tendency to forget that the first thing we do is to go over the amp with a chopstick looking for little issues like that, it gets to be second nature, but so many problems get sorted out that way without the need for the theorising we all enjoy so much . Join the club!

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                • #9
                  I was gonna say...check for bad solder first. Usually, if you can whack the top of the and cab (or bang on the chassis with a mallet if its out of the case) and it causes the weirdness even after subbing known good tubes, you have solder issues. Not uncommon with older amps like this. Way common on pc board amps....
                  The farmer takes a wife, the barber takes a pole....

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