Tube amp hum

[Beo]

I'm trying to lower the noise floor of my amp, a Mesa Boogie F-30. It has two JJ EL84's and four JJ ECC83's matched tubes, all replaced just one year ago. The noise dropped a lot with the tube replacement, but was not eliminated.

RG has some good info here, which I followed:
http://www.geofex.com/ampdbug/hum.htm

So, I'm getting 60Hz magnetic induced hum from the amp's power transformer. How do I know this? I shut off all other household electronics/lights, and tried various room locations. The hum changes volume as I turn direction and increases greatly as I approach the amp (using just guitar and cable). My guitar is well shielded and grounded and my cable is okay quality (and rang out okay with multimeter). Without a patchcord into the input jack the noise drops to a very low level (input grounded I assume). Inserting any cable (even a short patch cable) jumps up the noise floor. Both the gain and the master volume knobs on the amp increase the hum volume. I've tapped my tubes and I'm not discerning any particular sound or noise, however, I have not swapped tubes. I don't see the point, as from RG's advice, if both volume knobs increase the hum volume, then the hum is coming in before the gain stage. This leads me to suspect the input jack. Note however that when I rotate or wiggle a plug in the input jack, I don't get any crackle or change in noise level.

So, am I on the right track? Should I open up my amp and check the input jack and wiring? Or should I bring my amp in to a tech, as the problem could still be many other things (electro caps, pregain tube, etc)?

Thanks for any advice,
Travis

[amptramp]

You may be able to tell hum originating in the transformer from the buzz you get when an electrolytic capacitor goes bad.  The hum will be mainly a sine wave pure tone at about a B natural.  The buzz will be a little more noticeable because it contains all harmonics.  Normally, the power supply rectifier diodes conduct for a short time when the rectified voltage exceeds the capacitor voltage.  In old amps with tube rectifiers, the value of the input capacitor was limited to limit the peak current.  The average current has to equal the peak current times the duty cycle at which the rectified voltage exceeds the capacitor voltage.  The voltage at the capacitor will charge rapidly followed by a slower decline until the next time the rectified voltage exceeds the capacitor voltage.  This gives a close approximation to sawtooth wave with a short increase to full amplitude and a long dropoff to the next charging event.  If the capacitor loses capacitance (which it will do as it ages), the discharge will be more rapid and the voltage excursion will be greater but it will not be a sine wave - it will have all harmonics.  The charge / discharge equation is:

CV = Q = IT

where C is capacitance in farads
          V is voltage change in volts
          Q is charge in coulombs
           I is current in amps
          T is time in seconds

[DougH]

Does it do it with multiple guitars? May be a bad bridge ground in the guitar. That drove me nuts for a while with one of my guitars. It had similar symptoms.

[PRR]

> The hum changes volume as I turn direction and increases greatly as I approach the amp

So it's picked-up in the guitar.

It may be impossible to eliminate since the guitar pickups MUST pick up magnetic fields.

Hum-buckers will be less sensitive to hum sources further-away than the width of the pickup; that was their original point. They also have more lows-mids output and less highs output, a very different sound, so may not be an option for you.

> Inserting any cable .. jumps up the noise floor

Of course. The open cable is a hum antenna (even when shielded).

How about a shorted cable? If hum rises with a shorted cable, there may be some ground fault in the input jack wiring. However this is clearly less than the pickup in your guitar.

The power transformer would radiate more hum if your wall-voltage is way high, like 130V.

[Beo]

PRR, you were right, inserting a shorted cable had little effect on the baseline noise level. So, not the amp input. It must be my guitar. Unfortunately, I don't have another electric to try.

This will be hell to figure out. I rewired my strat earlier this year with the Seymour Duncan "Everything Axe" mini-humbucker set, replaced all the pots, switch and jack, and lined the full interior with copper tape. I have the 2-4 positions wired to split the humbuckers to try and get that notch quack sound. The volume pot is a push pull that adds the bridge pickup so I can have dual bridge-neck HB.

http://www.musiciansfriend.com/accessories/seymour-duncan-everything-axe-single-coil-electric-guitar-pickup-set

I guess I'll need to double check all my pickup wiring and grounds, and maybe shield each individual pickup. At the end of the day, my wiring might be fine, and these pickups may just be overly susceptible. Any suggestions on how to troubleshoot pickup wiring for hum?

[Jorge_S]

This will be hell to figure out. I rewired my strat earlier this year with the Seymour Duncan "Everything Axe" mini-humbucker set, replaced all the pots, switch and jack, and lined the full interior with copper tape. I have the 2-4 positions wired to split the humbuckers to try and get that notch quack sound.

Triple check that your copper tape is grounded. Check it with a continuity checker, all of it should be connected to ground, that's the only way it'll act as a faraday cage.

You shouldn't be getting all that hum with good humbuckers.

[DougH]

Does it still hum on the quack settings? Are you under flourescent lights?

[runmikeyrun]

I've found that high gain amps are very difficult to control hum... everything is being amplified so much that even a slight hum, what would be considered acceptable on any other amp, is amplified so much that it's almost impossible to get rid of. 

You can try a different pickup, or check the shielding/grounding in your guitar.  If your guitar doesn't cause hum in any other amps, then it might be because your amp has so much gain.  Worst case you can do what I did, wrap your pickups in a non-ferrous metal like copper or aluminum sheeting and then grounding it.

Your other option is a noise gate (either before the input or in the fx loop) to kill your signal when you're not playing.  This is assuming it's only a problem while you're playing.  This is my solution on my high gain amp... my guitar is VERY well shielded and still hums with that amp, but on none others.

One last thing- if your input jack doesn't have shielded wire from the jack to the board/1st input stage then try that.  Sometimes your hum is induced there.

[Beo]

Does it still hum on the quack settings? Are you under flourescent lights?

Yeah, hum is much worse on 2-3-4 SC positions. 1-5 HB positions are significantly better.

You can try a different pickup, or check the shielding/grounding in your guitar.  If your guitar doesn't cause hum in any other amps, then it might be because your amp has so much gain.  Worst case you can do what I did, wrap your pickups in a non-ferrous metal like copper or aluminum sheeting and then grounding it.

Your other option is a noise gate (either before the input or in the fx loop) to kill your signal when you're not playing.  This is assuming it's only a problem while you're playing.  This is my solution on my high gain amp... my guitar is VERY well shielded and still hums with that amp, but on none others.

One last thing- if your input jack doesn't have shielded wire from the jack to the board/1st input stage then try that.  Sometimes your hum is induced there.

I want to get the best baseline noise level with my guitar, cable, amp set-up, so I can then start optimizing the pedals I've been building. For example, I'm considering rewiring my big muff with shielded. I'll start tonight by checking out my guitar internal grounding, and also compare using my small SS Fender practice amp. I have a local music shop where I can try out my guitar on several amps, and even try their guitars on my amp. Thanks for the advice.

[pinkjimiphoton]

with 2 el84's as output, i'd imagine the amp is not biased as class ab, but probably class a,.....in that case, if a single ended output transformer is employed, you can get more hum. an ab transformer actually "bucks" hum too a bit.

i agree tho, check all the grounds on your guitar. one mistake shielding can make a diff between a faraday cage and an antenna.

[J0K3RX]

I had the same problem and I ran a dedicated line for my music equipment (you could have something causing a ground loop in your electrical somewhere, doesn't have to be turned on?).. I also got a Furman line conditioner. Another thing that causes hum is lighting dimmer switches... High gain amps get a lot of noise, maybe think of getting a Decimator G noise reduction pedal?

[Beo]

Well, I triple checked the grounding, and added some more tape to ensure connections in the jack cavity, but no improvement. I decided to dismantle the wiring to see if I could get improvement... took away the notch position splits, disconnected the tone controls, disconnected the bridge on push-pull, and finally disconnected one pickup at a time. In the end I had just the neck pickup direct wired to the jack, no volume or tone. I think these are just high output sensitive pickups. Is it unreasonable to think that decent shielding should get me close to the baseline noise of a grounded input? My second amp channel is much higher gain, and the buzz is so bad I can't really use it.

[J0K3RX]

Maybe you've already said what kind of pickups you have but I will ask again.. What kind of pickups do you have, make - model...?

[Beo]

Maybe you've already said what kind of pickups you have but I will ask again.. What kind of pickups do you have, make - model...?

They are seymour duncan mini-humbuckers: JB JR bridge, Duckbucker middle and Lil'69 Neck. Here's a link:


http://www.musiciansfriend.com/accessories/seymour-duncan-everything-axe-single-coil-electric-guitar-pickup-set

Next, I'm going to try shielding each pickup individually with copper tape.

[J0K3RX]

Sounds to me like you may have some sort of strange "phase/polarity" problem between all 3 pickups?  Single coils are (north and south - up and down) and humbuckers are (north and south - left and right)... Maybe try taking out the duckbucker all together and see if the problem persists?

[DougH]

I would *really try* to get another guitar somehow plugged into your amp. Take your amp to a music store or something. First make sure it's not the amp. Then if it is the guitar- call Seymour Duncan- seriously. They can advise you best on wiring those pickups and where you may have missed a step that caused a grounding problem.

[Beo]

I would *really try* to get another guitar somehow plugged into your amp. Take your amp to a music store or something. First make sure it's not the amp. Then if it is the guitar- call Seymour Duncan- seriously. They can advise you best on wiring those pickups and where you may have missed a step that caused a grounding problem.

Yep, this is what I need to do next. Over the past two days, I've re-shielded and re-wired my guitar. I have all signal grounds going to a common ring, and connects to the shield with just one wire. I've shielded the pickups themselves as well, with the pickup shield tape connected to the body cavity/faceplate shielding. I've left the jack cavity unshielded, but I have shielded cable to run from the jack to the volume pot. Overall, my wiring and solder connections are much cleaner, and I'm sure I have grounding done right, using star ground for signal, and connecting to shield at just one point.

End result... better, quite a bit better actually on my clean channel. Still not so great on the lead channel. I feel better about going into my local store and trying my guitar with other amps, and other guitars with my amp. I'll follow up with what I find. I'll also take some gut shots of my guitar wiring and shielding tonight.