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SVT Hum

Discussion in 'Amps and Cabs [BG]' started by i_got_a_mohawk, Aug 10, 2005.


  1. right, i may never get rid of all the problems i seem to get with this amp, i turned it on for the first time in a while last night, about 3am, i couldnt sleep, i just made sure the volume was right down on my bass

    Anyways, ever since the wierd whirring sound i had, ive noticed this background hum, it was alot more present as i was playing quietly

    the thing is, its constand, it didnt increase in volume with how high/low the basses volume was, and no matter what volume the SVT was at (well, the volume dial, i wasnt actually playing it at full blast at 3am!) the hum was at the same level

    i really really hope this is just a microphonic pre tube ( my JJ's are in the post :hyper: ) , but its just the fact that the volume of the amp didnt affect the volume of the hum, what could it be ?
     
  2. biasing?
     
  3. yeah, ive not had the bias checked since i got it, ive been meaning to, ill have to sort that out next week, even if it isnt the problem, its something that should probably be done
     
  4. best of luck with that. Usually pretty quick and painless for the tech.
     
  5. Maverick Blues

    Maverick Blues Being a Thumper is all about ATTITUDE!

    Apr 28, 2005
    Richmond, VA
    Microphonics are a possibility, but keep in mind that's a matter of amplifying physical vibrations. If you have a preamp tube that's microphonic enough to pick up and amplify, say, the vibrations of the power transformer laminations coming through the chassis, then it's pretty sensitive. I'd expect it'd therefore pick up you tapping the chassis or the front panel or pretty much ANYTHING. If the SVT doesn't seem particularly susceptible to "handling noise," then I'd put microphonics farther down on the list of suspects. If you want to check this though, you could take something like a pencil and gently (fragile glass-enclosed vacuum, remember!) tap each of the preamp tubes. If one of them is wildly microphonic, it should reveal itself.

    The hum being constant regardless of volume setting is a valuable clue. It simply means the hum is being injected AFTER the volume control (or MOSTLY after). This would rule out any preamp tubes or circuitry before the volume control (forgive me for not being an SVT guru; when you say "volume" I presume you're not talking about a front-end gain control, which might not have much of anything before it).

    Depending on the tone, my first suspicion might be power supply hum. If it's a deep, mellow hum, your filter caps may be slowly drying out on you. A tech can check this by simply bridging them and seeing if the hum goes down in volume. It's been a LONG time since I've done any tube work, but I'd suspect a leaky rectifier could also be the culprit -- that'd be tube swap(s) to test. If it's more of a thin, edgy buzz, then you probably have other things going on.

    Keep psychoacousitics in mind: Might the hum level be the same as it's been for a long time but it just seemed surprisingly loud at 3AM when you were trying to be quiet? Not saying you shouldn't get it looked at, just that it may not have been a sudden component failure. Try her again in the bright light of day the way you usually use the amp, and see how it sounds/feels (which you've probably already done).

    Hope this is of some small help. Disclaimer: While I am a Certified Electronics Technician with radio/tv/audio endorsements and I used to work on amps and PA systems, it's been a looooooooog time since then... so please take anything I say on the subject with an appropriately-sized grain of salt. Altzheimier's, dontcha know. :)

    Hey, do us all a favor and let us know what you find out? Might help the next of us with a similar problem.

    Cheers,
    'rick
     
  6. Wow, cheers! ill check it out later today

    I dont think it'll be the power supply, the supply in the uk is usually pretty decent, i wont bother taking the top of the case off and checking the pre tubes, ill just see if its still there when my new set arrives next week :smug:

    still, i really want to get it serviced for piece of mind, so ill get a tech to check her out asap :)
     
  7. Maverick Blues

    Maverick Blues Being a Thumper is all about ATTITUDE!

    Apr 28, 2005
    Richmond, VA
    Hehe. This may be one of those situations where one person speaking English and another speaking "American" don't communicate quite so well. :) Apologies if I was unclear or didn't realize the UK terms were different.

    It sounds like, to you, "power supply" means the supply of power -- the quality of what comes out of the electrical outlet on your wall -- or perhaps the utility itself.

    I was instead referring to the section of your amplifier known (in America) as the power supply. It converts the alternating current from the wall connection to a pulsating direct current at a usually different voltage, filters it, and depending on the type of equipment, may also regulate the voltage to keep it stable.

    An amplifier stage usually has some sensitivity to changes in the voltage it runs on. If the filter capacitors responsible for smoothing out the pulsating DC begin to deteriorate (drying out due to age is common in really old equipment), the smoothing doesn't happen as well. Instead of a nice, even, quiet DC voltage, you start seeing a ripple. The more the filter caps deteriorate, the worse the ripple gets. At some point it gets bad enough that the various amplifier stages start amplifying (or at least passing) the ripple, right along with the signal. So you wind up with a hum with a fundamental of either the line frequency or twice it (excluding switching supplies, which are different beasts entirely).

    I hope the above helps clarify what I was thinking. But regardless, you're planning to get it checked out, so it'll all work out in the end. And I'd still like to hear what you find out.

    Cheers,
    'rick
     
  8. I'd like to know as well...never one to kid myself that I know too much about my amp, I always assume that hum like this is biasing...I'm glad to know (as I always kinda assume) that there could be other culprits.

    good luck Mohawk.
     
  9. Maverick Blues

    Maverick Blues Being a Thumper is all about ATTITUDE!

    Apr 28, 2005
    Richmond, VA
    Mon, you could very well be right! Your earlier comment about biasing was one of the reasons I want to hear what it turns out to be... when I read your post it was a, "Hmm, I wouldn't have thought of that!" moment. :)

    'rick
     
  10. cool. I love this place. meeting of the minds it is.

    too bad my mind rots more often than not in "off topic". :smug:
     
  11. i knew what you were meaning bout the filter caps, when you said power supply i thought you meant socket power, cus ive heard its a bit rougher around the edges in the us, which can show as excess background noise on tube amps :)
     
  12. something must be going microphonic, i cracked her right up and got some funky feedback, i threw the volume dial on the bass right off and it kept going, had to flick to standby

    pre or power tubes do you think :(
     
  13. top the hop off while she was on, check all the tubes, there was a thumping coming through when i tapped the first and second 12ax7 in the pre section, and with one of the power tubes there was a slight almost in audible noise whenever it was tapped
     
  14. Maverick Blues

    Maverick Blues Being a Thumper is all about ATTITUDE!

    Apr 28, 2005
    Richmond, VA
    At this point you should probably hope for someone who knows tube amps -- or SVTs in particular -- to weigh in. I'd likely be in over my head if I tried to tell you anything more specific without having some actual time with the amp.

    In my first post I was talking about a tube being "wildly microphonic" enough to pick up physical vibrations from the transformer... that'd be one pretty darned sensitive tube! Thumping when you tap some tubes might be pretty normal/acceptable if it isn't outrageously extreme. (I'd think you would get a little microphonic sensitivity out of some tubes that are working just fine. All the elements are just sitting inside those glass envelopes perched on wires, after all -- if you tap it, isn't something likely to move?)

    How about it, all of you tube/valve-wise folks? Would someone care to weigh in on this? Does this sound like a microphonics issue to you?

    'hawk, I'll toss one more thought into the bucket. Power supply filter caps not only smooth out the ripple from the rectified mains, they are also responsible for dampening out fluctuations caused within the amplifier itself. Bizarre internal feedback is one possible symptom of bad filter caps, and I've seen it myself on more than one amp I was repairing. Although I wouldn't expect this until the filters were so bad that the background hum was obnoxiously loud, I wouldn't count the possibility out yet.

    If you're expecting your new tubes pretty soon, you could wait and see if that fixes the problem. But it's sure sounding like your SVT could benefit from a visit to the shop.

    Sorry I'm not giving you something more useful here.

    'rick
     
  15. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Now I don't remember what "flavor" of SVT you have.... I will assume a normal vintage variety.

    Bias yes, can cause hum.... if too high or not balanced correctly between banks.

    Gain balance pot can cause hum, since if the two banks are off-balance the hum which is normally present may not get cancelled.

    Hum balance pot is intended to control hum that may be due to heater leakage.

    Microphonics, sure. But anything not controlled by volume is in the latter stages of the pre, or in the power amp. So V1 is probably not it.

    If you plug a SHORTED plug into the "external amp" jacks, that will block signal coming from preamp. If hum is same in that condition, it is from inside the power amp and might be bias etc.

    And, don't forget the cooling fan, which can cause a purely acoustic hum....
     
  16. Its an SVT II ive got (non-pro)

    i think the hum was pretty much exageratted last night because im a bit out of it atm

    there is just the problem of that hellish feedback atm, what ill do is plug bass or bass to sansamp into the power amp section, if theres no feedback, then im sorted, i would know its probably the pre tubes, if its still feeding back then its something to do with the power section

    i just thought id post on here to get a headsup on any problems or anything i should maybe tell the tech when i take it in

    its not the accoustic hum of the fan, its coming through the speakers, unless your meaning some sort of EM background interferance from the fan causing a slight hum
     
  17. on another note, i finally think ive gotten rid of an annoying buzzing that happened occassionally when i played a D, it was a loose washer :)

    still got the feedback problem, hah, great investment this has been :D

    ==========edit===============

    right, whenever i play a D there is a rattle from the power valve (the one that was putting a noise through the speakers when tapped), however, this rattle is not amplified, but i take it that means that tube is heading south

    i swapped the 12ax7 in the driver section for the first 12ax7 in the pre section, and it seems to have sorted out the problem, well, its not given me any feedback like that yet

    the only thing thats really bothering me is that rattly 6550 :(

    ==========edit again===============

    from further investigations, there was another rattly power valve, that doesnt make and sounds through the speakers, i thought it was the fan, but it wasnt, the fan seemed to becauseing the minor rattle tho, which i thought was odd, also, the power valve that made the slight sound through the speakers when tapped, isnt doing that anymore, its almost like my amps working fine ???????

    (have i spoken too soon)