Ampeg SVT4 pro humming issue

[jackson bass]

hello new to forum not new to playing bass
i have had my svt4 pro for approx. 10 years now and recently it started humming any time the bass is plugged in I use a Jackson jpb-7 bass and never had the issue up until about 6 months ago. if i touch the strings the hum goes away but it's starting to effect the sound and tone.. getting really annoying. When i hear the hum if i grab onto the amp's chassis it goes away completely but i can't stand there holding the amp while i'm playing obviously... i'm wondering if anyone else has had this issue and if there is a simple fix i can do to get rid of it. I have plugged in my fender squire p-bass into the amp with the same result only alot louder hum. any help would be greatly appreciated

[skee616]

Grounding issue..most likely with your bass.

[jackson bass]

happens with all basses plugged in like i stated in first post...

[B-string]

Have you tried cleaning the input jack with DeoxIT or at least a decent contact cleaner (NO WD-40)? You have tried a new instrument cable? Also try a new power cord, I have seen the ground connection open on power cords.

[Jerrold Tiers]

Originally Posted by jackson bass

i have had my svt4 pro for approx. 10 years now and recently it started humming any time the bass is plugged in I use a Jackson jpb-7 bass and never had the issue up until about 6 months ago. if i touch the strings the hum goes away but it's starting to effect the sound and tone.. getting really annoying. When i hear the hum if i grab onto the amp's chassis it goes away completely I have plugged in my fender squire p-bass into the amp with the same result only alot louder hum. any help would be greatly appreciated

Originally Posted by jackson bass

happens with all basses plugged in like i stated in first post...

Going away if you touch the strings AND happening with all basses is not typical.

Typically it IS a bass problem.... and in this case it seems to be at least different with different basses.... for what that is worth.

Going away when you touch the strings or the amp chassis is really typical for a bass with poor shielding of the control wiring. That indicates a bad ground or bad shielding causing the bass to pick up hum from YOU. When YOU are grounded, you no longer induce hum....

If you can get it to go away by touching the strings, there IS a ground connection in the cable, and the amp is *probably* not the cause (not 100% a certainty) ..... it might be possible for the CABLE to be the cause.....

If the amp had a bad connection in the cable shield connection, it should hum all the time, and probably would NOT be fixed by touching the strings.

Did you try a different cable?

[jackson bass]

cable is a brand new monsterbass cable
i dont see how it could be the bass if it happens with all basses i've plugged into it
granted i have no other bass amp to plug my bass into to test this out but i just dont see how it would be the bass if both my basses produce the same buzz/hum
i have not cleaned the amps input jack but i have replaced the bass input jack recently (after the humming came about)

[B-string]

Originally Posted by jackson bass

cable is a brand new monsterbass cable
i dont see how it could be the bass if it happens with all basses i've plugged into it
granted i have no other bass amp to plug my bass into to test this out but i just dont see how it would be the bass if both my basses produce the same buzz/hum
i have not cleaned the amps input jack but i have replaced the bass input jack recently (after the humming came about)

The only thing that proves is you paid way to much for an instrument cable that muddies the high end? New cable from anyone can be defective. Your amp isn't close to a noise source is it (TV, CF lighting or dimmer). Try the amp with no instrument cable plugged in, if the amp has a problem it will still hum. If the amp no longer hums the trouble is with the cable, basses or a stray noise source (turn off all other electronics in the room may help find it).

[jackson bass]

turned amp on with no instrument cable plugged into output and hum still present

[B-string]

Try a different power cord (like from a computer) to see if the cord has just gone bad, plug direct into a different outlet and check again. If none of this helps you could have a bad preamp tube or more serious troubles a tech would have to sort out.

[Jerrold Tiers]

Originally Posted by jackson bass

turned amp on with no instrument cable plugged into output and hum still present

But it goes away if you touch the strings? Earlier you said it hummed with a bass plugged in... this sounds like it hums all the time but sometimes goes away if you touch the strings.

Odd..... must be a cancellation effect... those can drive you nutty......

A humming amp needs to be worked-on by a tech.

[B-string]

Or teach it the darn lyrics?

[T-Bird]

Hi.

Originally Posted by Jerrold Tiers

Odd..... must be a cancellation effect... those can drive you nutty......

A humming amp needs to be worked-on by a tech.

^These.

Could be a combination of filter caps going bad/leaking and bass shielding malfunctioning, or something way more difficult to diagnose.

Like Mr Tiers said, an random hum cancellation by more than one hum source can be a real PITA to find.

For the most difficult cases, I've started with installing a line voltage filter to minimise the power grid interference.

Regards
Sam

[Jerrold Tiers]

Ok, I got your PM, but figured I should reply here just to clear it up....

First, "hum"..... You mentioned two hums, one a "hissing hum" ONLY if you turn the amp up to 6 or 7 on both gain and "volume" (Master, I assume) with nothing plugged in ..... AND a different hum when you have a cord/bass plugged in.

Now, there is "hum" and there is "hiss" and there is a hum that is "hissy".

Hum is deep with a good amount of lows, with a definite "tone" or "timbre"..... "Hissy hum is like that, except that the lows are reduced, and what is left is all the harmonics... you can tell it has a 'tone" to it, but it is a "buzz", there isn't much if any real lows to it. "Hiss" is just that, with no particular tone to it, although setting the EQ can change it and almost give it a particular tone....


What I am getting here is that the turned-up /no cable hum is the softer, buzzy hum, and the one with an instrument is LOUD and has some solid lows to it, not just buzzy or hissy.

If that isn't the case, tell us.

While I'd like to see "zero' noise even if the amp is turned way up, I'm going to separate these two somewhat different "hums" and I am going to say the hissy, buzzy one when turned up is "normal".....doesn't mean I LIKE it, but I *understand* that one, could be tubes etc,etc. it isn't really unusual, and generally depends on EQ/tone settings etc,

So, lets concentrate on the one that occurs with something plugged in.

I very much doubt any power cable is going to do anything... you already found that out.

There are several usual causes for this kind of hum, that goes away when you touch or get near to grounded things, like the amp.....

In general, it means something in the bass or the cable or the amp input jack is not connected, broken wire, bad solder joint, loose screw, etc.

if you can touch the guitar strings and it goes away, "usually" it means the shield over the controls/wiring in the bass is not connected, and hum voltages picked up on your body are getting into the circuit. The strings are usually grounded through the guitar cable. When you touch the strings, you are then "grounded" and no longer have any hum voltage on your body, so no more hum.
That usually ALSO works if you touch the amp chassis, maybe even if you touch the screw on a light switch cover, or a radiator in the room, or nearly anything else that is grounded.

OK.... you did say you can touch the strings and it goes away....

BUT....

Usually that is with a particular instrument..... but you say it is with several..... Hmmmmmm

If the CABLE is bad, one of the two wires is usually broken.... if the center wire is broken, you get no sound, or weak sound....
If the shield is broken, you get hum..... LOUD hum..... but usually touching the strings usually does NOT work.... although touching the amp often DOES work.

OK, but you say various different cables all are same..... Hmmmmmmmmm

If the input jack is not grounded right, maybe broken solder etc, it is pretty much the same thing as with a bad cable..... Same things, and usually touching the strings does not do much if anything. Touching the amp chassis often WILL cut the hum. And maybe it is OK sometimes if the way the cable hangs jams the bad solder together and makes contact.

The problem I have with giving an answer here is that what you say is not consistent with ANY SINGLE one of these,........

It's just like a bad cable or jack, but you say touching the strings cuts the hum.

It's just like an unshielded controls problem, but you say it does it with several instruments and cables.

As I mentioned, it may be some odd cancellation thing..... although I admit I can't come up with a good reason for that.

Just to straighten this out..... can you tell us FOR SURE what "kind" of hum is in each case....?

Is either one a deep, bassy hum that is pretty loud?

if so, which one?

And when you touch things, or switch instruments / cables/ whatever, does it stay the same?

If I am way off on what I am "getting" from your information, correct me and let's go from there.

I don't want to send you off to a tech without a good reason, because it will probably just cause you money and time if the problem isn't one that needs a tech.....

[jackson bass]

after all the comments and feedback i'm starting to think that both my basses have issues... i'll be calling my local music shop on monday and try to see if i can take my bass in to test it on another amp to see if the problem is with the bass instead of the amp... hopefully it's the bass as you people have suggested then i can just replace either the wiring or the pickups and it wont be an invasive amp recovery like i'm hoping it's not. thanks for all the input i'll update as soon as i know more..

jerrold i wish i could describe the sound that i'm getting but it's hard to give a good idea of the sound on a keyboard, i see what your sayin about the amp sound without input plugged in that i can deal with because it's pretty faint. but when the bass is plugged in it's just too loud to even tell what i'm playing it corrupts the whole sound/tone. i wouldn't describe it as bassy but just a mid level hum/buzz hard to describe i guess sorry my info is so scattered it's just that the noises are so different on each different setup i try

[pedulla1]

If like you said the hum is still there with no basses plugged in then it's not your basses. Take the amp to a tech and get it certified.

[LegoBass]

Hey guys, I am having the same problem with my SVT-4 Pro. I know it is not any of my basses, cables, signal chain, power cables, or outlets. I have tried all differnet things.
I would say that while my basses are pluged in, a "hum" occurs that is very loud, not super loud, but when cranked you can hear it in the audience inbetween songs. (I don't run thorugh PA). When nothing is plugged in, there is a very high frequency "hiss", very simailar to your cable going out. The "Hiss" is also present with something plugged in. Is there any chance this could be the pre-amp tubes? Should I have it taken in a get a complete rundown on it? Thanks guys.

[JimmyM]

Originally Posted by LegoBass

Hey guys, I am having the same problem with my SVT-4 Pro. I know it is not any of my basses, cables, signal chain, power cables, or outlets. I have tried all differnet things.
I would say that while my basses are pluged in, a "hum" occurs that is very loud, not super loud, but when cranked you can hear it in the audience inbetween songs. (I don't run thorugh PA). When nothing is plugged in, there is a very high frequency "hiss", very simailar to your cable going out. The "Hiss" is also present with something plugged in. Is there any chance this could be the pre-amp tubes? Should I have it taken in a get a complete rundown on it? Thanks guys.

I'd try swapping tubes one at a time with a known good one first. If that doesn't work, clean the effects loop jacks with contact cleaner. If that doesn't work, take it in.

[LegoBass]

Thanks Jimmy. Taking it in or buying new tubes is not really the answer I wanted, but you gotta do what you gotta do.

[JimmyM]

Well do the effect loop cleaning first then If you don't have contact cleaner, you can just run a plug in and out really fast a few times in a row.

[LegoBass]

Okay, I will go try that first.


EDIT: Yea, that didn't do anything. I guess I am just going to have to take it in. Thanks for the help though.