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Thread: Amp hiss/drone

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    Question Amp hiss/drone

    Hello,

    I'm having problems with amp hiss. It seems to get better and worse at different times. It increases as I turn the amp volume up, but it's still loud enough to be annoying when I have the amp volume low and the volume on the guitar up all the way. The amp is a Fender Blues Jr. It has the stock 12ax7 tubes (the amp is about a year and a half old, bought new) and new JJ el84's. I'm more concerned with this now because I've just started recording from the amp.

    A recording of the hiss is attached below.

    Another problem is a constant low drone (that you can also hear in the mp3 file). It's usually much quieter than the hissing, and it keeps at a constant volume.

    One thing I know is that the cables I'm using can't be the problem. The amp gives off the same hiss (and hum) even when there are no cables plugged into it. I also tried plugging it in to a couple other outlets in other parts of the house with no change in sound either (I don't know if grounding issues would be localized to a specific outlet though). I also asked this question on another forum and they suggested to peek at the capacitors inside for any noticeable damage but there is none.
    Attached Files Attached Files

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    Last edited by tboy; 11-18-2008 at 11:38 PM. Reason: attached file

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    bump

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  3. #3
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    Try a different 12ax7 in v1, or swap them around (V2 & V1) see if that helps. Good tube vendors can screen 12AX7 for excessive noise, look for tubes that have been graded as 7025 equivalent/specified for V2 in BF Fenders etc.

    Can't hear the drone thru the PC, pull the 2x 12AX7 and see if it goes away when you power the amp back up - if it does then the new 12AX7s might cure it? If not, then it's related to the output section/heaters/lead dress. Who rebiased the amp when you put the JJs in it? Do the original tubes drone as much?

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    Bump. I just got all new preamp tubes (Tung Sol 12ax7 from the Guitar Center), and the problem hasn't gone away. Here is a current recording. The mic is about a foot away from the amp and both volume knobs are turned up all the way (guitar is turned all the way down). As you can tell, it's hard even to hear yourself with all the static. I think the buzzing is clearer in this recording too.

    newtubesampstatic.mp3

    One thing I noticed is that after I flip the amp power switch (at the end of the recording), the static fluctuates (I don't know if that's significant). I didn't get the amp re-biased because the consensus between several guitar stores I asked over the phone seemed to be that I didn't need to. Is this a good idea? How much should I expect to pay for getting an amp re-biased?

    Note: I did have my guitar and a delay pedal plugged into the amp during this recording, although their volume knobs were turned down. In the recording in my first post, nothing is plugged into the amp.

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    Hiss:
    Where is the reverb knob set at? Just wondering if the reverb circuit is introducing the hiss. Does the hiss go away/reduce with the reverb turned right down?

    What happens when you run the amp without V1? Without V2?

    I'm not terribly familiar with this amp, but you can see on the schematic that the bias doesn't have an adjustment pot. So FYI, to adjust it, you'd have to change resistor value(s). But adjusting the bias, to my knowledge, won't get rid of hiss.

    Hum:
    If the output tubes are not matched for idle current, you can get a different idle current thru each side of the output transformer, and this can be heard as hum. Did you purchase a matched pair of output tubes? If the amp's bias circuit were modded to allow the bias of each output tube to be adjusted individually, you could adjust each bias so that both tubes conducted the same current, and not introduce hum in the output stage. But a matched pair of output tubes should be conducting the same idle current, given the same bias voltage.

    Any mods to the amp? It's also possible a bad tube can introduce hum. Does the amp sound fine except for the hiss and hum?

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    Thanks for the reply, Chevy.

    Quote Originally Posted by Chevy View Post
    Hiss:
    Where is the reverb knob set at? Just wondering if the reverb circuit is introducing the hiss. Does the hiss go away/reduce with the reverb turned right down?
    In the 2nd recording, the reverb was set lower than one. I don't think that's the problem.. turning that up only adds a little reverb to the hiss, haha.

    I'm not terribly familiar with this amp, but you can see on the schematic that the bias doesn't have an adjustment pot. So FYI, to adjust it, you'd have to change resistor value(s). But adjusting the bias, to my knowledge, won't get rid of hiss.

    Hum:
    If the output tubes are not matched for idle current, you can get a different idle current thru each side of the output transformer, and this can be heard as hum. Did you purchase a matched pair of output tubes? If the amp's bias circuit were modded to allow the bias of each output tube to be adjusted individually, you could adjust each bias so that both tubes conducted the same current, and not introduce hum in the output stage. But a matched pair of output tubes should be conducting the same idle current, given the same bias voltage.

    Any mods to the amp? It's also possible a bad tube can introduce hum. Does the amp sound fine except for the hiss and hum?
    I bought the power tubes matched from thetubestore.com. I will try to find out if one of the tubes might be bad. Is it safe to turn the amp on without all the tubes in place?

    I haven't touched anything except the tubes, and I bought the amp new, so it hasn't been modded. And it's a nice sounding amp aside from these noise problems.

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    It's safe to turn it on without the preamp tube(s) plugged in, yes, or without the power tubes plugged in, for that matter. All the audio signals are passed thru coupling caps at each stage, where that stage's DC is blocked, so each stage is separated from the others. DC current flows thru the tubes, and if they're not plugged in, no harm done; no path for the DC current to flow for that stage; also of course no audio signal will pass thru the stage without a tube. When you pull a tube, or reinstall it, do it with the power off.

    If you pull V1, and the amp goes dead quiet, I would guess that the hiss is coming entirely from the first 2 stages. But it seems that would still point to a hissy preamp tube...... I'm not an expert at this, just a few thoughts to perhaps help.
    If you pull V3 and the amp is still hissing, then of course it's coming from the output section....... but this doesn't jive with the fact that the hiss goes up with the volume control.
    Do a little experimentation on this and see what will kill the hiss; pull V1, test, replace it, then V2, test, replace it, then V3; let us know. There's little point in pulling the output tubes and seeing if the hissing stops, as there won't be any output!

    I guess it's out of warranty??

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    Hm... I tried pulling the 12ax7's out one by one. Each time the result was the same: There was no practically no hissing (although the buzzing was still there). There was no sound when I plugged in the guitar either (I'm guessing this was expected?).

    Then I tried switching them around a couple times, which did nothing (I didn't think it would since they are brand new tubes and the noise is the same as it was with the old tubes).

    I should mention that the reverb knob does increase the static, but only slightly.

    There's a five year warranty on the Blues Jr, so I should be covered if I can find the receipt. I want to have some idea what's wrong with it, though, before I hand my precious amp off the the guitar center folk.

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    No, the guitar would not produce any output with any of the preamp tubes pulled.

    So... pulling V1 got rid of the hiss... logically it seems that it's that tube stage or the power supply making noise; but I'm not familiar with why/how a well-capped power supply would "hiss". Perhaps someone else can chime in on this ? I've heard that carbon comp resistors cause hiss in the early preamp stages as well, but I kinda doubt Fender uses them on these new amps.

    The buzz is either unmatched output tubes as previously mentioned, a grounding/ground loop issue inside or outside the amp, possibly all compounded by a poorly filtered power supply.

    Can you go to GC and check out a different Blues Jr and see if it's got the same problem(s)? I'd suggest taking advantage of the warranty; specifically ask them to let you know what the problem is.

    Sorry I couldn't be more help...

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    Oh man, I don't know what is happening with my amp. Just now, I was playing around with my guitar and the amp started to make a sound like a tea kettle. I didn't believe it was the amp before I tried adjusting the volume. The low buzzing sound has been getting worse over the past couple months since I noticed it, but I hadn't heard this sound before. I really want to get it repaired since it's still under warranty, but I'll have to wait until Monday for the Fender staff to come back from holiday break.

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  11. #11
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    I'd bet on a bad ground or connection somewhere in the 1st stage. These new Fender amps are prone to cracked solder joints on the tube sockets, among other places.

    I'd take full advantage of your warranty. Don't start messing with this amp and void the warranty.

    Good luck...

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