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Vintage SVT noise

Discussion in 'Amps and Cabs [BG]' started by slap5string, Jun 2, 2012.

  1. Hello all! As many of you have read from my previous thread - I have just purchased this 73 SVT in incredible shape and since its my first tube amp i'm not sure what is considered normal. Is a certain amount of "low level" hum expected? Seems to be just shy of conversation level but is enough to be annoying. The tone is of this amp is awesome and all switches, inputs, knobs work great - no other issues besides the hum and fan noise. Is there a way to minimize the hum and a way to reduce the fan noise?
  2. Has this amp ever been serviced? Hum can be an indication of failing power supply capacitors, so if this amp has never been recapped, it's due.

    Of course, there are other causes of hums. You've adjusted the little hum balance trim pot on the back of the amp?

    When nothing is plugged into the input, not even a cord, tube amps should not hum.
  3. Well, I spoke with the tech and he said he re-capped it and replaced the 6550's and replaced the power cord with a 3-prong. It does hum when standby is switched off and nothing plugged in.
  4. Im not aware of a hum balance trim pot. Where is it located?
  5. At the front, take out the front grill.
  6. Yep, brain fart, I was thinking of the V4 when I said on the back.
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Those fans are noisy. Changing the fan can fix that. I had a pointer in the last thread that will help. Some of the fans on these amps can be dissassembled cleaned and oiled. I would check this out first.

    As for the hum, there are a lot of possibilities that range from a bad tube or component, as has already been mentioned a power supply cap, an external influence such as bad house wiring or a nearby fluorescent light, you instrument pickups, or the calibration of the amp. I would take the amp back to the tech and ask about the hum to see if the level is normal for your amp.

    The easiest thing to try on your own is to adjust the hum pot at the front of the power amp chassis. The front panel will need to be carefully removed. Your tech can show you how to safely pop off the panel. There is a pot marked hum. Turn it to minimize the 60 Hz power line hum. If nothing changes and the hum is loud, the pot could be bad OR it could be something else in the amp.

    The other pots behind the panel are for adjusting the power tube bias and the balance between each set of three power tubes. If the balance is off, the amp will hum. These adjustments should be performed by your tech.

    I type too slow. Some of this has been covered.
  8. So hum, of any kind, should not be present at all? I practice in my basement which we just re-wired (new service panel and dedicated outlets for the band gear) but we do have a drop ceiling with flourecent overhead lights. As for the fan noise - If I hold my hand over the grill the fan noise improves greatly, but of course, then there is no flow. Could the holes in the grill contribute to the noise?
  9. nysbob


    Sep 14, 2003
    Cincinnati OH
    Fan noise is part of the amp. I generally play it loud enough that it's not an issue.

    Mine hums a little more than it used to, but again... once the playing starts moot point.
  10. I pulled the front cover and adjusted the hum balance and presto! The hum is all but gone. The fan is really noticable now! Weird thing is, if I cup my hand over the grill in the back, the fan noise goes away (but so does the air flow ;)) I need to find a replacement. What a wide range of tones this amp is able to produce! I tried my Musicman with rosewood fretboard and my USA Precision w/ maple fretboard and I don't think there is a sound I can't pull out of this rig. Now the weak link is definately the pair of GK 410's I'm using. I need to be careful with the tone settings cause it's easy to make these cabs fart out.

    Anyone have a Refrigerator cab for sale? lol
  11. When I first got my 70s SVT I noticed the fan being loud as well, but I've never even noticed it in a gig or practice situation, only if I'm sitting alone with the amp at home.
  12. Seans


    Jul 4, 2008
    Herefordshire, UK
    Sorry but I don't get this general quote that the fans are noisy, my 70 has its original fan and apart from the noise in the fact it's moving a great deal of air, mines not noisy. Does the fan still have the little blue plug?, this is a seal which keeps the oil in and also acts as a bearing rest.
  13. I havent had a chance to pull the rear grill yet. Im gonna jam it for the first time at rehearsal tonight so it'll be my first real chance to see if really is noticable in a band setting.
  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The fluorescent lights can be a source of hum but they don't have to be. Modern ones can have filters on the tubes and the ballast can be designed to eliminate radio frequency interference. Since you have solved your hum issue by adjusting the hum pot, the lights probably aren't an issue. Keep it in mind if you are at a different location and experience problems.

    The holes in the grille could be causing some noise. On my 71 SVT, there is a cutout in the grille where the fan is. There is a fan guard on the outside of the grille. Even still there is a lot of noise. Seans is right, a lot of the noise is related to air movement but some can also be from the fan itself. If the fan can be oiled, it may reduce the noise. A modern fan is engineered to better reduce noise due to the motor but they've made great strides in reducing air flow noise.
  15. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    You're lucky to have a quiet fan in your amp. I've found that fan noise is an issue with a lot of these amps. I consider air flow to be part of that noise. A better fan will conduct the air with less noise. My 71 SVT was bad till I changed the fan but my SVT-VR was louder, like a vacuum cleaner.

    Although some models have the oiling plug on the fan, many don't. The fans are sealed. Some of these amps have also had their fans changed so you never know what you'll find till you inspect the fan.
  16. Seans


    Jul 4, 2008
    Herefordshire, UK
    I was lucky enough recently to find a NOS Rotron for mine on ebay, interestingly it was there unsold for 3 separate listings until I found it. At present the original is going fine, but just in case.
    Making sure the front grill cloth is clean and breathable will help with noise too, mine was painted black which blocked the whole thing up.
  17. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Nice Sean. Does your NOS Rotron have the oiling tab like your original? My Rotron was a later model and doesn't. I think that the ones that you can oil are better fans.

  18. Seans


    Jul 4, 2008
    Herefordshire, UK
    David, yes it's exactly the same, I don't have pic's as it's still in the USA, as the seller wouldn't post to me over here, so it's at a friends place for now. But here's my original, the plug does two jobs, one, to act as an oil seal with two internal O rings and two, to act as a stop or bearing rest, this plug is very easily misplaced, once gone, it's like a car engine without an oil sump plug and not long before the whole thing goes bang.

    Attached Files:

  19. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Thanks for the pics. Looks like it is very good condition.

    Do you run your amp at 220VAC or with a transformer at 120VAC? I'm wondering what voltage your fan is seeing. Normally the SVT fan is wired to the AC mains on the same circuit as the amp's power switch.

    An interesting point is that with some 50/60Hz AC fans, they run a little slower when operated at 50Hz. This makes them a bit quieter because they are moving a little less air. That can be an advantage.

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