Bad preamp tube socket cause microphonics?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by shooto, Aug 18, 2006.

  1. shooto

    shooto Supporting Member

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    would it?- I'm getting noise so I do the tap test- V1's tube is looser than the rest (which are all pretty stable) and it "pings" when tapped- Tried another tube in that position and it "pings" also when tapped...so, I think it's the socket- is that it and are they easy to replace on your own?

    btw, Laney AOR 50-

    thanks
     
  2. shooto

    shooto Supporting Member

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  3. conundrum

    conundrum Member

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    If it just makes a "ping" sound and not a long squeal then it's not a problem. Any tube you put into V1, unless it's terribly dull, will do that.
     
  4. shooto

    shooto Supporting Member

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    well, it seems like that's the only thing that could give me noise- the connection on the pins are much looser than the others so I think that looseness is causing vibration of the tube and the noise-

    am I on the right track?
     
  5. rockon1

    rockon1 Member

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    Perhaps you could describe the noise and under what conditions it happens. (all the time,clean or dirty channel if applicable,only when played loudly.etc) If youve eliminated other possible causes(cables, guitars etc)
     
  6. shooto

    shooto Supporting Member

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    ok, I did some more investigating- It happens when I plug into high gain input (which I believe is v1)- when I tried low gain input (which by-passes v1) it is quiet as a mouse at any preamp and master volume-

    I plugged back into high gain input (AOR channel) and switched channels to the regular channel (channel 1, which is really the same as just plugging into low gain, you by-pass AOR gain or v1) it was again totally quiet- click to AOR channel (that kicks in V1) and crazy noisy again-

    it's gotta be V1's socket...like I've said, I switched tubes same thing...switched cables, guitars...same thing-

    go to low gain (by-passing V1) and it's totally quiet-

    I think it's gotta be the socket-

    easy to replace or do I need to take it to my amp guy?
     
  7. slider313

    slider313 Silver Supporting Member

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    You can try to retension the pins with a large needle or safety pin. This may help.I don't think the socket would give you microphonics,so you may have to go through a few preamp tubes to find one that is quiet. Some amps have more gain than others. I find the Sovtek 12ax7wa is a good low noise tube for V1.
     
  8. shooto

    shooto Supporting Member

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    retension the pins?...like stretch them out?...why would the looseness occur with other tubes and the original tube works fine (not loose) in other positions (sockets)?
     
  9. WaltC

    WaltC Member

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    What I'm sure Slider meant to say is to retension the socket pin holes. This may help BUT be careful 'cause even preamp tubes can have 2 or 3 HUNDRED Volts DC on the plates fed by a capacitor that can zap you even if the amp isn't plugged in.

    If you're not knowledgeable and comfortable with working around tube voltages, then have your local tech take a look.
     
  10. shooto

    shooto Supporting Member

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    ohhhhh...will the charge dissipate if left un-plugged for a day or two so I can work on it?
     
  11. WaltC

    WaltC Member

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    It may, depending on how the filter caps are wired (if there are bleeder/balancing resistors, etc). What I do is make a cap discharge wire by taking a Radio Shack alligator clip wire (with a clip on each end), cutting it in half and soldering in a 10K 1 watt resistor (any value over 100 ohms will work to keep the discharge from being too rapid <G>) in the middle. Then you can clip one end to ground and the other to any plate load resistor, leave the standby switch on and all the filter caps will discharge over a couple of minutes (may be much faster, but a couple of minutes will be safe). Leave the discharge wire connected while you work to prevent the caps from recovering (re-charging themselves, they *can* do this!) and you should be good to go. Of course, be very careful, lots of Volts here and you can get hurt and/or killed (disclaimer). You can get by with just the wire without the resistor in the middle if you know what you're doing and use the plate load resistors to slow the discharge, but the new modified wire can be reused lots and will come in handy in future work.
     

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