I have a hum issue with my guitar amp without anything plugged in the input. Here's the thing, when I have the amp in standby mode there is no sound, now when I turned the power switch off the 'white noise' appears & slowly fades into nothing. There is no hum present at these two points. It's there whenever the amp is running. It's a 100w amp, laney gh100l, so if I turn the master volume to say 7 on the dial then the hum level goes up. All this while keeping the gain knob all the way down to zero as well.
Standing 5 feet away from the amp I can hear this hum. I have tried spare tubes, both preamp & power amp, only time the hum freq changed a little was when I had 6CA7 in place of EL34, rebiased and everything. Tried different speaker cables as well, no dice. In a band situation this hum might get drowned out but if mic'd then I have a feeling it might still get picked up. I don't gig at the moment so trying it out at a different place is also not an option due to me breaking my metacarpal & being restricted from lifting heavy objects for a few months.
Now there is one thing about my house wiring. We have two bedrooms, living room, kitchen & restroom. The mains board in the living room has three fuses, when I have all these removed, for some reason the refrigerator in the kitchen, water heater in the bathroom & air conditioner in my room still function while every other outlet in the house is disabled. I tried unplugging the fridge & AC from the outlet but the water heater is directly wired to a separate on/off switch, so I couldn't pull the plug on that. But unplugging the two devices didn't affect the hum either. I'll make a thread on the house wiring problem later, but I have a feeling this hum is related to a component failure within the amp, it's a 3yr old amp.
Standing 5 feet away from the amp I can hear this hum. I have tried spare tubes, both preamp & power amp, only time the hum freq changed a little was when I had 6CA7 in place of EL34, rebiased and everything. Tried different speaker cables as well, no dice. In a band situation this hum might get drowned out but if mic'd then I have a feeling it might still get picked up. I don't gig at the moment so trying it out at a different place is also not an option due to me breaking my metacarpal & being restricted from lifting heavy objects for a few months.
Now there is one thing about my house wiring. We have two bedrooms, living room, kitchen & restroom. The mains board in the living room has three fuses, when I have all these removed, for some reason the refrigerator in the kitchen, water heater in the bathroom & air conditioner in my room still function while every other outlet in the house is disabled. I tried unplugging the fridge & AC from the outlet but the water heater is directly wired to a separate on/off switch, so I couldn't pull the plug on that. But unplugging the two devices didn't affect the hum either. I'll make a thread on the house wiring problem later, but I have a feeling this hum is related to a component failure within the amp, it's a 3yr old amp.
Is this it?
http://www.freeinfosociety.com/electronics/schematics/audio/laneygh100l.pdf
Regards
M. Gregg
http://www.freeinfosociety.com/electronics/schematics/audio/laneygh100l.pdf
Regards
M. Gregg
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I haven't checked the -35V on the control grids, but by 50/50u caps you mean the big two that are 500WV,100uF each right?
In my case i'll be needing 100u 500V across each to check if they are faulty? I'll drain them first though, at the moment I'm getting a reading of 3V across one of them, the other one I can't seem to be able to measure. I haven't turned the amp on in two days, so they may have discharged, but anyways I'll need to get hold of a 500v cap first before I can check anything.
Yup, that's the one. I have that service pdf with me. There is one thing about that diagram, on page 2 of the pdf near the lower area of PCB 9050_2, at PAD 51 & 38 the +ves of the cap that are going there are printed to be 2x50u/500V, but the ones in my unit are 100u/500V each. So is that a misprint or am I missing something? I guess they may have changed values in the past 19yrs since that diagram was made available..
There is something odd I noticed about one of those caps. They have some glue or epoxy thing on them but one of them seems to have cracked & lifted up a little. I've attached pics of them. The Cap1 is the one that has cracked glued thing, the Cap2 looks fine. Is this normal?
Also do I scrape this glue off or heat it off? It seems soft enough to peel with a knife/blade, but it's located in hazardous region
Attachments
I can't edit the previous message, but an update to my reply to JonSnell Electronic, the control grid voltage I'm getting is -33V. Actually it's -42V but my DMM gives an error of -9V in HV so I guess -33V is the actual value. And I figured out what you meant by 50/50u caps. I'm a little slow sometimes 
Does the hum go away when you plug something in? If so, you've described every amp I own/bought/built, hifi, guitar, pre, phono, etc. (unless the input jack grounds itself when nothing is plugged in.. those don't hum.) I think I have mains voltage issues at my home too, though.
Someone here told me it was normal, as long as it goes away when a source is plugged in. Not sure how true that is...
Nope. Plugging something in introduces normal hum sometimes depending on where I'm standing from the speaker cabinet, feedback or emi kinda hum though.
So essentially the amp's you've built & come across hum on their own but don't do it when a source is plugged in? I've always found that pickups can introduce noise depending on shielding, wiring, the cable quality & amount of gain dialed in on the amp. This kinda noise usually goes away when the instrument is unplugged.
Sorry, I saw Amplifiers>Solid State/Tubes and went right in to post a thread. Won't happen againGuitar amp threads belong in Instruments and Amplifiers per forum directives. Please see sub headers below forum titles for details.
Moving to proper forum.
Thanks for moving it in this subsection, I wouldn't know otherwise.
Then for now, you can assume it's post-rectifier noise and no external sources are to blame. 100Hz can be a result of too little/faulty filtering, a non-twisted high current wire carrying 100Hz in close proximity to signal wires, or just bad design. Do you own an oscilloscope or know someone who does? It would help to see what comes out of the PSU.
The 'sharpness' (added harmonics) comes from the fact it's no longer a sine wave. This is what comes out of the rectifier at 100Hz (assuming it's full-wave):
The 'sharpness' (added harmonics) comes from the fact it's no longer a sine wave. This is what comes out of the rectifier at 100Hz (assuming it's full-wave):
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I used to have a OS but not anymore, don't know anyone that has one either. There are no twisted wires that I remember seeing, they seem to be using straight non flexible single strand wires in most places. They are bent at right angles at places but no twisted pair. I'll get a 500V cap first tomorrow and see if adding it in parallel to the one that I attached a pic few posts above helps lower the hum. It's odd to see that glue/gum lifting up like that, there's more of that stuff elsewhere in the board but none has cracked. Also I read somewhere that laney was using TL072 chips that were rated for 50°C , these would overheat & cause hum. Replacing them with 140°C rated seemed to solved the issue. I'm not sure how to check these for specs, but for the time being those would be the last thing I'd consider gone wrong.
Anyways, in the meantime I dug out my old laney lx35d SS practice amp. It seems to do the same thing. It's worse on the drive channel of it. I had this one since 2007. Don't use it much, I think I'll try and see if a full recap on it helps in some way for it as well.
Thanks for the help everyone.
Anyways, in the meantime I dug out my old laney lx35d SS practice amp. It seems to do the same thing. It's worse on the drive channel of it. I had this one since 2007. Don't use it much, I think I'll try and see if a full recap on it helps in some way for it as well.
Thanks for the help everyone.
Update:
Finally I got 100uF/500V Nichicon caps. Had to be imported, no one stocks 500V caps of any brand around my place or online around me. 450V are though plenty available. I tried them across the original caps, not much of a difference. I replaced the ones in the amp with these new ones, considering I paid around $14 to get them, so I might as well use them. Are Nichicons good? As in will they last about 15yrs or something? I got them from elemen14/Farnell UK.
Anyways, as to the hum problem. I had an old voltage stabilizer in storage that was used for an old refrigerator. I used that to see if would help, well it did make the hum a touch quieter, or maybe I'm imagining it.
Also I made a mistake when telling you guys about the freq of the hum. With the master volume below 7 on the dial it sounds closer to 100Hz hum, but beyond that it sounds like this 50Hz hum http://users.skynet.be/fa046054/home/P22/track50.mp3
If you listen using head/earphones with the volume up, then that is what is coming out from the speakers with the master volume all the way up.
Another thing is I'm getting static sounds coming from the speakers. They aren't extremely loud but noticeable. I can make these static noises reappear in two ways:
1. By trying to measure the plate voltage at the power tube socket, while the amp is running.
2. I have a pc, laptop & old home stereo that I use as monitors connected through a extension board. If I have the laptop charger connected & I connect the home system aux input to the pc audio out, then these odd static noises start appearing on the audio system. They go away the moment I disconnect the laptop charger from the laptop.
3. I hear it in the night sometimes when there has been a power out.
Now when I'm using my amp, I go direct to the wall, no extension cord or anything, no pc/laptop running either. Recently I think I've started to get these static noises more often. What can cause these? I'm guessing this is a definite mains problem.
I tried other outlets in the house, unplugging everything from all wall outlets, still no improvement.
I think this is a case where the building has poor/old wiring.
Finally I got 100uF/500V Nichicon caps. Had to be imported, no one stocks 500V caps of any brand around my place or online around me. 450V are though plenty available. I tried them across the original caps, not much of a difference. I replaced the ones in the amp with these new ones, considering I paid around $14 to get them, so I might as well use them. Are Nichicons good? As in will they last about 15yrs or something? I got them from elemen14/Farnell UK.
Anyways, as to the hum problem. I had an old voltage stabilizer in storage that was used for an old refrigerator. I used that to see if would help, well it did make the hum a touch quieter, or maybe I'm imagining it.
Also I made a mistake when telling you guys about the freq of the hum. With the master volume below 7 on the dial it sounds closer to 100Hz hum, but beyond that it sounds like this 50Hz hum http://users.skynet.be/fa046054/home/P22/track50.mp3
If you listen using head/earphones with the volume up, then that is what is coming out from the speakers with the master volume all the way up.
Another thing is I'm getting static sounds coming from the speakers. They aren't extremely loud but noticeable. I can make these static noises reappear in two ways:
1. By trying to measure the plate voltage at the power tube socket, while the amp is running.
2. I have a pc, laptop & old home stereo that I use as monitors connected through a extension board. If I have the laptop charger connected & I connect the home system aux input to the pc audio out, then these odd static noises start appearing on the audio system. They go away the moment I disconnect the laptop charger from the laptop.
3. I hear it in the night sometimes when there has been a power out.
Now when I'm using my amp, I go direct to the wall, no extension cord or anything, no pc/laptop running either. Recently I think I've started to get these static noises more often. What can cause these? I'm guessing this is a definite mains problem.
I tried other outlets in the house, unplugging everything from all wall outlets, still no improvement.
I think this is a case where the building has poor/old wiring.
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