Bottlehead Forum
Bottlehead Kits => Mainline => Topic started by: MikeSattler on October 06, 2023, 05:41:13 PM
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Finished up my mainline kit the other day and it sounds excellent except for a very faint hum. It sounds more similar to 120hz than 60 hz and it's not affected by the volume or any of the front panel switches. So far I've tried a few things to no avail. Moved it to a different room, tightened up the transformer, reflowed the solder for as many of the ground points as I could find, and cleaned the tube pins. Voltage and resistance checks are spot on. What should I try next?
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Is it the same level in both channels?
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Yep, right in the center of the image.
Also forgot to mention - I replaced the buss wire with the ground breaker diode mod and tried a cheater plug to lift the ground connection.
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Can you tell us more about the source you have hooked up to it?
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Seems unaffected by having anything plugged into the RCAs. When I moved it to another room I just plugged the amp straight into the wall with nothing else hooked up to it and it’s still there.
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Perhaps it wouldn't hurt to post some build photos.
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Okay thank you for looking! Here's a Google photos album with the build: https://photos.app.goo.gl/oEUx4thV1rVWwXZh6
The underside of the chassis got a bit of overspray from the powdercoating. However, I sanded off the powdercoat in all the spots where it looked like a chassis ground connection was made. Other than the obvious ground lugs and lugs 3 and 7 on the attenuator is there anything else to make sure has a proper ground?
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What's the DC resistance between the "E" terminal on the IEC power entry module and the bare metal of the plate holding the attenuator switches up front?
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Getting 37.4 kohm
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The paint on the bottom side of the plate is causing issues. I've circled some points that need to be scraped on the bottom side of the plate to establish proper earthing. I would scrape a few of the holes around the standoffs for the front attenuator plate just to be extra sure that grabs earth nicely.
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Thanks for that, though I think that shouldn't be an issue anymore. I sanded off those spots as well as the ground lug at the headphone jack and the lug by the center tube socket, so they should all have a good connection to the plate.
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The E terminal of the IEC power entry module and the plate that the attenuator switches mount to should show continuity. Since they don't, you'll have to track down where this interruption has happened and correct it.
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Maybe my weird reading from terminal E has something to do with the diodes I installed? The first time I measured from there I got 37 kohm. Now i'm getting 7.6 mohm. If I measure ahead of the diodes, at the primary ground lug, I get 0 ohms.
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Well, I removed the diodes so I'm getting 0 ohms from terminal E but the hum still persists.
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Do the rest of the locations circled show continuity to E? I had forgotten about the diodes, that will throw off that reading.
My suspicion is that there's an earth reference that's just missing and resolving that may fix your hum.
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Yep, looks like all the locations are getting good continuity.
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What are your IA, OA, OB, and Kreg voltages on each side? Just out of curiosity, how long did this kit hang around before being built? Sometimes in a humid environment, the LEDs can absorb moisture over time and then experience damage whilst being soldered. Generally this takes 6+ months of exposure to a humid environment. This issue will manifest as a problematic voltage.
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Well, something is definitely up because a few of these readings are higher than when I ran the checks a few days ago. Listed all the voltages below from the manual plus the few extras you asked for. This build did sit for quite a long time before being finished. I started it a few years ago and had some big life events and it just fell off my radar for awhile. It's been sitting in my basement in a room with a dehumidifier that keeps it around 55 humidity so hopefully nothing got damaged.
Power supply
* 284v
* 6.3v
C4S Side A
* IA - 283.5v
* OA - 220.6v
* OB - 144.3v
C4S Side C
* IA - 283.5v
* OA - 219.8v
* OB - 143.3v
Reg board side A
* Kreg - 10v
* Breg - 219.8v
* -Reg - 0v
Reg board side B
* Kreg - 10.1v
* Breg - 220.6v
* -Reg - 0v
Terminal 20 bias reading - 144.4
Terminal 30 bias reading - 143.5
The power supply and IA readings were both really close to 275v when I first ran the checks. I also set the bias a few days ago right at 145v and those have dropped as well.
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OK, so your voltages are doing what they are supposed to, which is helpful. Can you procure a three light outlet tester and plug it into your outlet? An open ground may also create the issue you're experiencing (as could other powerline issues).
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Okay, shoot I was hoping the higher voltage on the +275 terminal was out of spec rather than trying to hunt down a phantom grounding or powerline issue.
My outlet tester is showing a ground so I don't think thats the issue either.
Just out of curiosity, is it possible I'm just being too picky? I mean, the hum is very very faint, enough so that it's hard to hear with any other noise in the room. I just assumed something was up because I never had that issue with my Crack or some of the other amps i've used. Is that just normal operation for the amp?
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Does it change with the high/low impedance switch? What headphones are you using to pick this up on?
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I first noticed it with my HD800's. Then I tried a few lower impedance headphones (portapros and IE600's) and it's more noticeable with those headphones.
When I first posted it didn't seem like the impedance or balanced switches had any effect, but now after resoldering a bunch of joints and sanding those ground points I am noticing a bit of a difference. It gets slightly louder when switching from low to high impedance and it almost doubles in volume (still faint though) if I switch from balanced to unbalanced while using the TRS input. Hum stays about the same volume when using the XLR output and changing from bal to unbalanced.
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I would try taking the amp to a different location (work is usually easiest) just to gather a datapoint as to whether it's environmental or not. Stuff like this can creep through your powerline and get into the signal path, and we can certainly help with ways to combat that if it ends up being the issue.
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Okay sounds good, i'll take it to a friends house sometime this week and check back. Thank you for all the help so far!
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Unfortunately no luck on moving the amp to a different house, noise is still there.
However I may have somewhat narrowed down what's going on by accident. I still had my portapros connected to the headphone jack while the amp was flipped upside down and I was about to desolder the regulator board to reflow those joints. I heard a loud hum out of the portapros when I moved the soldering iron close to the amp (even though it was unplugged and powered off obviously). Put the portapros on to hear the hum while moving the soldering iron over the internals of the amp and its the exact hum I'm hearing when the amp is on. No clue what that means but the hum seems to be the loudest when my soldering iron is close to the output transformers?
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Your soldering iron puts out a strong AC field. That radiated field what is getting into the transformers. You might want to double check that the drain wires that connect from each transformer to the safety ground have been well connected to the safety ground screw, and that the insulators under the all transformers are properly installed to insulate the transformer from the chassis. If they are not there is the possibility that the radiated field from the power transformer (which is, by design, a very small magnitude for a transformer of this size) is getting into the output transformers via eddy currents generated in the chassis panel and up thru any mounting screws that are not isolated from the chassis.
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I would also double check the DC resistance between the earth lug at the back of the chassis and all the headphone jack terminals with black wires on them (set to unbalanced).
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Thanks Doc, I'll check the transformer mounts tonight an make sure everything is properly insulated.
I think the ground lugs on the output transformers are working because I'm getting continuity from the earth lug to the metal housing of the transformers.
Thanks as well PB, measuring the headphone output ground wires with the switch on unbalanced I'm getting .1-.2 ohms on all 3 headphone jack ground connections (which is what I get measuring any of the ground lugs).
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Not sure if it matters or not but I just noticed that I get a little extra resistance measuring ground on the main transformer. If I clip one lead to E and then measure the top bell screws or the side of the transformer where it's not coated I get like 2-4 ohms instead of the usual .1-.2 for ground connections.
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2-4 ohms should still be fine. You might just try tightening the mounting screws a little. It's difficult to know whether all the connections are as they should be from an ohm meter reading since they should read close to 0 ohms from the transformer to the safety ground if properly isolated and connected to the safety ground, and would also read close to 0 ohms if they happen to not be isolated and are touching the chassis right at the transformer. Because of this a visual inspection is probably best to make sure that all the transformer mounting washer hardware is installed in the right order so the transformers "float" where they are mounted and only connect to the chassis electrically at the safety ground.
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Oh my gosh... Doc I think that was it! I pulled off the transformer and the fiber washers were completely deteriorated. A few of them were crushed as I had already tried tightening the transformer down and the metal posts holding the power supply board were touching the chassis. I just ordered a new set of washers from digikey and I'll put them in once they arrive. Thank you both for the quick replies especially over the weekend, this was driving me nuts and hopefully this fixes the problem!
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Okay well scratch my last post... it's still there. I put in all new fiber washers on all the transformers and verified that they're isolated from the chassis by disconnecting the ground lug near E and they don't show continuity on any of the screws.
If anything the hum is actually louder now, I have no idea what else to check for here.
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Any other suggestions for things to try/check for? I was doing a bit of googling and found a few instances where people had mentioned faint hum on their newly built Mainlines. One guy said it just went away after a few hundred hours of usage. If it's something that will may go away on it's own would it just be the tubes settling in or something else?
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There is a possibility that a couple hundred hours of run time could help.
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Okay thanks I'll give that a shot
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Just circling back on this with a follow up that resolved my issue. It looks like it turned out to be a noisy 6c45 tube. Originally I didn't think it was a bad tube because it didn't sound like microphonics. However, I think it was oscillating and making a hum in the low 100-200 hz range. Just swapped in some new reflectors I purchased off ebay and it's dead silent now. Thanks for all the help with tracking this down!
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Thanks for coming back to let us know what resolved the issue. With Russian tubes in particular, we have found that emission and noise performance will stabilize with some break in time (which may suggest they don't get much burn in after production), so it may be worth leaving the amp running with those tubes in for an extended time to let them cook before discarding them.
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Also worth giving the pins of the offending tubes a good cleaning.. John
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Thanks guys! Yeah I cleaned the pins with a q-tip on a dremel and some metal polish. I also put about 200-300 hours on them but it seems the one tube just doesn't want to quiet down. Out of the 4 6c45's I have now it is the most microphonic by far so I think it's just humming away no matter what.