Bottlehead Forum

Bottlehead Kits => Crack => Topic started by: Shadowfax on September 10, 2019, 08:06:10 PM

Title: New Build -- 98% excellent, 2% issues [resolved]
Post by: Shadowfax on September 10, 2019, 08:06:10 PM
Hello, I have just completed (finally) my first Crack build and it is 98% excellent with about 2% problems. At this point, the amp is quite usable in spite of some problems and I spent many hours today re-listening to some of my favorite tunes for the first time. I will post my listening impressions on another thread, but would like to focus here on the 2% problems that I am still having.

Last night I powered up the amp for the first time connected directly to my iPhone's mini jack output using a cable that I have been using elsewhere so I believe to be good. I connected a cheap pair of earbuds to the output. When I turned the amp on, I think I heard a pop, not too loud. I don't know if it came from the earbuds or something else. I don't know if that was significant or not or if it related to the issues I am having, but I mention it in case it is. I did have the headphones plugged in when I powered up though and I am not certain what the volume pot was at, but I think it was at minimum but cannot be sure. I did not see any smoke or see any problems, so I pressed on.

I listened to one track and it sounded very good even through cheap earbuds. Then, when listening to the second track, a scratching sound started and sometimes it turned into an electronic sounding tap-tap-tap, or pop-pop-pop.  The noise would come and go, but it was often enough that it was a problem and would be present once or twice on each track. I found that the volume of the noise did not change with the level of the pot, but was always at the same level and was even present at the same level with the volume pot set all the way to zero. The noise appeared to be initially in the left channel only, but later it was definitely in both channels.  I unplugged the iPhone and the noise went away. At this point I suspected the iPhone output jack and I believe I remember having similar noise from my iPhone when testing an Objective headphone amp, so I blamed it all on my older iPhone that I rarely use the output jack on anyway.

Today I connected the Crack up at my desk to my laptop through a Schiit Modi Multi-bit DAC with an EITR USB interface. Everything sounded great through a pair of Sennheiser HD-6xx headphones. Noise was gone -- at least I thought. Then, the noise came back after several hours of listening to music. Same noise, same scratching and tapping as before, even though I do not have the iPhone in this setup (but it was sitting next to the amp on my desk). Noise comes and then it disappears again. It is only present for 10 seconds or so, and then it goes away for 10 or 15 minutes or so, but I did not time it.

This problem does sound similar to user benzxc was describing in "Hissing when connected to laptop" and his solution was to let the 6080 burn in for about 50 hours and his noise seemed to be going away. I will wait and see if it starts to diminish on its own after burn-in before I start to do anything radical, but I am open to suggestions or ideas. FYI, since someone will ask, I did do all the ohm-meter checks and visually checked every solder joint before I powered on.
Title: Re: New Build -- 98% excellent, 2% issues
Post by: oguinn on September 10, 2019, 11:05:04 PM
It sounds like interference from the phone. Try moving it away or putting it in airplane mode. There’s also a mod in a stickied thread in the Crack forum here that consists of replacing the IEC ground wire with diodes that might be useful.

That plus cooking your tubes for a handful of hours are probably enough to solve the issue
Title: Re: New Build -- 98% excellent, 2% issues
Post by: Shadowfax on September 11, 2019, 05:10:39 AM
Jameson, Thanks for the advice. I will try running with my phone off today to see if it makes a difference and then see if cooking the tubes will get them to settle in. If those do not fix the problem 100% then I will try the diode fix. Thanks, I will let you know how it goes.
Title: Re: New Build -- 98% excellent, 2% issues
Post by: benzxc on September 11, 2019, 06:32:44 AM

My problem resolved itself after burning in for a few days. And just FYI, I also ordered new tubes prior to the resolution but they arrived after it was resolved. When I plugged in the new tubes there was weird crackling noises as well, although different from the original problem. These disappeared after a while on its own as well. Not sure what the reason was but both sets of tubes work fine now with no noise at all. Just wanted to throw this out here for your reference.

Title: Re: New Build -- 98% excellent, 2% issues
Post by: Doc B. on September 11, 2019, 07:03:18 AM
One thing to bear in mind is that "new" tubes are often 30 or more years old. No vacuum seal is perfect, and over time a bit of gas can infiltrate the glass envelope. Running the tube for a while will heat things up enough that the getter will activate and adsorb the gas. Also the filament or cathode often needs some run time to optimize the exposure of the emissive materials. Pins can also oxidize over time, and cleaning them never hurts. All this said it's easy to dismiss the noise from bad solder joints or outside interference as a "bad tube", so one should also explore those possibilities when tracking down the cause.

With time one can learn to distinguish the subtle differences in the noise these different issues create. But if one is unfamiliar with troubleshooting tube circuits it's a good idea to be thorough in your diagnosis.
Title: Re: New Build -- 98% excellent, 2% issues
Post by: Shadowfax on September 11, 2019, 09:30:39 PM
Well, that turned out to be the easiest repair in the history of repairs:

   Step 1) Move phone to other side of desk.
   Step 2) Enjoy the music.

Later on, I did test moving the phone back next to the amp, maybe 6 inches away, to see if the noise would return and it did come back within a minute or so. I moved the phone 4 feet back to the other side of the desk and the amp was noise-free for the rest of the day -- fairly conclusive that the cell phone was the source of the problem. I still plan to add the speedball upgrade some day and will still look into the diode fix at the same time, but for now, I declare this build to be at 100% excellent! 

Thanks to everyone for the quick and great advice:

Jameson, you nailed the problem on the first try -- batting a thousand!

Ben, all the work you did in troubleshooting your unit and your advice helped me zero in on my problem quickly. I will continue to burn in the tubes and listen for improvements as they settle in.

Doc, Thanks for the pointers on subtleties of tube behavior -- good advice. I did have to look up the difference between adsorb and absorb though -- thanks for the Chemistry refresher.
Title: Re: New Build -- 98% excellent, 2% issues
Post by: oguinn on September 12, 2019, 02:03:52 AM
Even a stopped clock is right twice a day. Glad you got it sorted!