1946 seeburg MA1-L6 amp crackling noise

Discussion in 'General Electronics Chat' started by zsg, Nov 12, 2007.

  1. zsg

    Thread Starter New Member

    Nov 12, 2007
    I just finished up a 1946 seeburg 146s symphonola jukebox restoration. Spent 6 long months worth of free time to complete it. The baby looks like it rolled off the assembly line. My question is in the tube amp. It sounds great and plays strong , but I'm getting a mild level crackling(popping) noise out of the speaker. This is not effected by turning the volume up or down. I can actually drown out the crackling if I play it loud enough. I traced, and retraced all my connections and solder joints. I cannot find what causes this noise. when I play around with any wire in the juke, it changes the crackling noise for better or worse. I thought I had a shorted wire. I checked all wires and found them fine. I unplugged all connections to the amp so its just the amp and speaker. The amp still makes the crackling sound. The speaker uses a field coil. The amp has been recapped, retubed and I replaced the resistors across the 6J7 and 6SN7 tubes. It still makes the crackling sound. I even tried another set of tubes. The amp is powered by the following tubes:
    two 6L6 tubes
    Any thoughts would be greatly appreciated. This final problem is driving me mad. It's all I hear, even in my sleep.

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    That the noise changes when you move wires might suggest soldering problems. I imagine this beast is all point-to-point, so isolating the area of the problem could be a chore. If you can get it down to one wire, that should lead you right to the bad joint. Don't forget the cartridge & tone arm.

    Or, there is the recreational soldering approach, where you just go through the terminal strips and reheat each one with a bit of fresh solder.
  3. Xray

    Well-Known Member

    Nov 21, 2004
    I'm old enough that I worked on tube type radios and TV's in the mid to late 1960's, and I am familiar with the type of crackling sound that you are asking about. I recall in one particular case that a coupling capacitor became defective and was breaking down internally, causing the static crackling sound. Resistors can become erratic also, although less likely than capacitors.

    You might be able to isolate which componant is causing the noise by using an audio signal tracer or "sniffer". That is nothing more than a small audio amplifier that you would use to hunt down a noisy component. Just make sure to use a a small coupling capacitor in the input circuit of your tracer amplifier because of the high voltages that you'll encounter in a tube type amplifier. This is especially important if your tracer is a transistorized amplifier!

    If locating a noisy component with a signal tracer doesn't prove successful, then you may have to just go through the tedious exercise of replacing one component at a time throughout the circuit. You can do this in a quick fashion by attaching a couple of alligator clip leads to a foil capacitor (say, .05uF at 150V). Then one at a time, cut one lead of each capacitor in the noisy amplifier, and connect the test capacitor in its place with the alligator test leads. Reapply power to the amplifier to find out if the crackling noise stops. If it does, then you can replace the offending cap with the correct value. Even electrolytic capacitors can cause noise in audio amplifiers, so you can try substituting them in much the same way. Just make certain that the components that you use for testing have at least the same voltage rating as the components that you temporarily cut out of the circuit.

    Hope this helps!