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Dirty-South Blues Harp forum: wail on! >
Why do some amps hum and others don't?
Why do some amps hum and others don't?
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LSB
130 posts
Jan 21, 2012
8:23 PM
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My Harpgear Double Trouble, which I love, hums fairly loudly when it's at idle, while my Wezo-45 has no perceptible hum. I imagine there is a "simple" technical reason, having to do with design, why some amps have a pronounced hum and others don't?
Last Edited by on Jan 21, 2012 8:25 PM
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scum
16 posts
Jan 21, 2012
9:04 PM
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Are they tube amps?
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FMWoodeye
202 posts
Jan 21, 2012
9:07 PM
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I think it's an electrical issue.
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Willspear
37 posts
Jan 21, 2012
9:42 PM
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One thing that can help with noise is using a power conditioner. I generally have one I run my amp and pedal board from.
Single ended amps often have pronounced hum
Any 6v6 based I have owned has had audible noise. Nothing major but it's there.
Microphonic tubes can bring strange noise to the party.
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rbeetsme
590 posts
Jan 22, 2012
4:42 AM
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...because they don't know the words!
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Tommy the Hat
572 posts
Jan 22, 2012
4:56 AM
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@rbeetsme
Damn...you beat me to it! I don't care what anyone says...that's some funny shit right there! lmao
(ok, wish me luck...I'm going for the captcha!)
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Tommy
My Videos
Last Edited by on Jan 22, 2012 4:56 AM
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5F6H
1068 posts
Jan 22, 2012
6:30 AM
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@ Billy Shines - Both models mentioned are grounded at the AC receptical/cord. Models without grounded AC cords in the US usually have a ground switch to eliminate this hum, or you can flip over the 2 prong cord in the wall socket.
@ Willspear - power conditioners have negligible affect on hum specifically. 6V6 amps are no more prone to hum that any other tube types...BUT many 6v6 amps are "single ended" with one power tube. These types of amps are less efficient at rejecting filament hum because the only have one "end" to the output transformer. Push pull amps (a tube at each "end" of the output transformer) are filament hum cancelling, when wired with a 2 wire heater set up, with correct polarity.
Incorrect bias conditions (too hot) can cause hum.
Noisy preamp tubes can hum, it's always worth trying an alternative tube of the same type, or see if a lower gain tube drops the hum any. 6v octal preamp tubes (6SC/L/N7)are more prone to hum than 12v noval preamp tubes (12AX/T/Y/U7), which are normally wired as 6v humbucking in amps these days.
Any amp left in operating mode with a cord plugged in the input but no mic will hum, as the cord acts as an antennae for hum. Amp hum should be assessed with the amp on, but no instruments plugged in - to rule out the hum coming from instrument wiring.
How the amp was built (layout of components, wires, how the amp was grounded) are most common causes of hum...but difficult for the layman (& even for a lot of amp techs) to eliminate. At the construction stage there are numerous potential causes, and 2 main types of hum - low pitched filament hum (120Hz in US, 230Hz in Europe & a higher pitched buzz, most likely from layout/grounding.
Most amps hum a little it's only really a problem if it can be heard on stage, or picked up by a recording/PA mic & interferes with the performance (if this is the case it needs to back to the manufacturer to be assessed as whether it is excessive or normal). If you can just hear a little hum with the amp set to playing conditions, with your ears right by the speaker & no background noise (like in a room at home)....then you will probably just have to live with a little hum...as most people do.
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www.myspace.com/markburness
Last Edited by on Jan 22, 2012 6:33 AM
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Jim Rumbaugh
653 posts
Jan 22, 2012
7:57 AM
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1) When you are comparing hum between 2 amps, make sure both hav ethe input set to the same gain. (for example, does the hum go up with the gain turned up)
2) Are both amps the same age??? In my youth, replace the the filter capacitor in the power supply was a common repair. This is the first capacitor after the rectifiers. It's job is to store and dispense the power to the circuit between 60 cycle peaks. If it is weak or too low a value, hum can be heard.
Here's an electronic nerd comment. In the past, some "hi-fi" tube amps would have the filaments powered with D.C instead of A.C. to lower hum from A.C. filaments.
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HarmoniCollege March 24, 2012
theharmonicaclub.com (of Huntington, WV)
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LSB
131 posts
Jan 22, 2012
7:58 AM
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Mark, thanks much, that's what I was looking for. I was writing another post to clarify, with some additional info, at the same time you were posting, so I'll edit that and add it below, so folks have some additional specifics that I didn't make clear in my original post. Also adding a couple of thoughts in light of your comments, at the end......
Example: I have 3 tube amps -
VHT special 6 (6W, 1x6V6 + 1x12AX7)
Harpgear Double Trouble (18W, 2x6V6GT + 1x12AY7)
Mega Tone Wezo-45 (45W, 1x5U4 + 2xKT66 + 1x12AT7 + 1x12AY7).
All are essentially new and when plugged into the same electrical outlet in my house - one at a time - volume at zero, no instrument cables plugged in, here's what you get:
VHT - Low power setting, noticeable hum. High power setting, more hum.
Harpgear - (only one setting) More hum than the VHT when the VHT is at it's highest power setting.
Mega Tone - Virtually no Hum.
The original owner of the Harpgear amp said he contacted Brian Purdy, maker of the amp, and Brian told him the hum was normal. Just surprises me that the hum is so significant - it is quite noticeable from 20 feet away in a room slightly larger. I love the amp and the hum certainly is not a deal breaker. I only use the amp at home and I doubt you'd probably not hear it much, if at all, on stage. Although close miking the amp could be an issue if one wanted/needed to do that. Not a problem for me at this time however. Perhaps at some point I'll try another preamp tube just to rule out a noisy preamp tube as a contributing factor.
Mark, thanks again for the detailed explanations, much appreciated.
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