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White noise through a tube amp?

Discussion in 'Amps and Cabs [BG]' started by Balog, Feb 28, 2013.

  1. Balog


    Mar 19, 2009
    Mukilteo, WA
    I recently traded into a US made Fender Pro Jr combo for practicing while the kiddo is in bed and I need super low volume. I really like it so far, but whenever I turn it on it makes a low level of the sort of white noise you hear with a radio between stations. Is this the sort of thing I can fix, or should I take it to a tech?
  2. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    Could be bad tubes or dirty pots/jacks. Or it could be bad components. Try simple things first, I suppose. What amp? And how old?
  3. That's a moderately high gain amp, and that hiss is typical of many higher-gain tube amps.
  4. toomanyslurpees


    Jan 21, 2009
    Those amps are fixed biased and that fixed bias is really hot, there are mods online if you google it to put an adjustable bias in (basically changes a fixed resistor value to a smaller resistor in series with a pot, I rebuilt a projr point to point style for a learning experience and it's much quieter with a cool bias on the power tubes.
  5. JHAz


    Jun 29, 2011
    Dude, it's called tube rush. Sit back and enjoy it. :

    couldn't help myself.

    Or try different preamp tubes and if it really seems wrong take it to a tech. Too lazy right now to pull out my PJ and see how much noise it has. Not enough to bother me in actual use, though.
  6. Ukiah Bass

    Ukiah Bass Supporting Member

    May 10, 2006
    Might also be caused by a ground loop in your gear/home.

    P.S. I occasionally get a quiet hissing sound in my home studio but not at other locations. The amp's builder thought it might be a ground loop.
  7. RickenBoogie


    Jul 22, 2007
    Dallas, TX
    OK, I'll say it. It's a high gain guitar amp, and not the best choice for practicing bass with. Awesome for guitar though.
  8. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Is it a totally consistent noise, or is there some texture to it, like frying eggs sort of sound?
  9. mystic38


    Dec 4, 2012
    Mystic CT
    this is highly unlikely.. sounds like its a hiss, not a buzz or hum.

  10. Balog


    Mar 19, 2009
    Mukilteo, WA
    Seems totally consistent, but I will double check when I get home from work.

    Thanks for the advice everyone, no experience with high gain tube amps to date so this is a learning experience.
  11. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    No doubt it has the Fender tone stack, too, so depending on your settings it could be boosting the treble to a point where it makes some noise, as when you boost frequencies, you also boost their corresponding noise floors.

    Or it could be broken ;) I'm betting on it being normal tube amp noises.
  12. Steve Dallman

    Steve Dallman Supporting Member

    Two things make this noise, bad tube, or noisy plate resistors. Since it's a new(er) amp, it is likely a preamp tube, and probably the first one.
  13. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Get a good 12AX7 and substitute the first tube, if it doesn't change replace the original and substitute the second tube. Won't cost you much to try that. Last stop is the EL84's. So far, you've invested in tubes that you'll need in the future. Next stop is a tech. It is a fairly simple amp so tech time should be minimized.
  14. I still say it's just normal for a high gain amp...

    A new preamp tube in V1 or in the PI may help, but even big name costly amps like Mesa, have what I call a lot of hiss.

    Too, the OP is hearing the hiss in a very quiet environment (home practice). The hiss won't be noticed in a music store or on stage--that's one reason I think most amps manufacturers aren't upset with the hiss. Also, some people are more sensitive to the hiss than other people.
  15. Hi.

    Without hearing the hiss, I'd vote for a normal hi-gain amp hiss as well.

    I have yet to hear a single tube amp like that without the "background hiss", unless there's a noise gate of course.
    Some are better and some are worse, and tube rolling may help, but IMLE dead-quiet hi-gain operation is very difficult with 12ax family of tubes. Especially CP.

  16. WingKL


    May 12, 2007
    Actually, since this is a tube amp where you can pull out the different amplification stages, you can get a sense of which stage or how much all the stages are contributing to the hiss by removing the tubes one by one starting from the first tube. You can do this without first buying any spare tubes. Of course, you do the tube pulling while the amp is off. Then to see if it's a particular preamp tube that is bad, you can swap it with another one in a different stage. If the noise level changes, then it would likely be the tube as the gain staging tends to change noise levels depending on the signal-to-noise ratio of the device and where it is in the amplification chain. To be clear, you cannot swap between the el84 and 12ax7 tube positions.
  17. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Probably normal high gain amp hiss. With that said, the noise figure of a triode is related to the transconductance (similar to a JFET), so a bad or inferior tube could be more hissy.
  18. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The Pro Jr. isn't what I would call a high gain amp. It is a 15W amp consisting of a 12AX7 preamp with a tone stage between the two triodes, a 12AX7 phase inverter, and a fixed bias push pull pair of EL84 power tubes. The pre-amp is fairly standard and what you often see in a low power Fender amp like a Champ. EL84 amps can get hissy but don't have to be.

    The Pro Jr. is an amp that people like to mod. Some people mod it for bass. You'll never know if anything has been done to it unless you look inside the chassis. As with any new used amp, taking it to a tech to have it checked out is a good idea.

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