Buzz in left channel

[Nikodemus]

Hello all!

I am hoping to get some assistance with my Crack + Speedball, that I have just finished.

I first built the basic Crack, and listened to it for the remainder of that day. No problems, so I proceeded to install the Speedball. All voltages are within specifications also after the installation upgrade. There is small discrepancy between A&B sides, which I assume correspond to the left and right channel. My knowledge on the subject is not enough to interpret whether that is meaningful or not.

Small PCB:

OA: 73V
IA: 190V
B-A/B: 0,3 mV
IB: 187V
OB: 70V

Large PCB:

OA: 109V
OB: 110V
G: 0
B+: 188V

The amp makes music, and it sounds great, however there is a problem. After a minute or so of turning on the amp a buzz appears in the left channel, which gets louder as the tubes warm up. It sounds a bit like a fly in the earcup, but not really that loud. Enough to be really noticeable, though. The buzz cuts off every now and then for a split second, and does not change with respect to volume setting. First I changed both tubes, no difference. Next I reflowed all of my connections, except the components on the PCB's. The buzz still persists, and was not altered in any way by the new solders.

Any guidance would be greatly appreciated. I am new to this stuff, and am at a bit of a loss as to what should I try next?

-Tomi
 

[Nikodemus]

An update to the case. It appears that I had not been thorough with my findings before. As I mentioned, I tried new tubes to see if they are the problem. Changing input tubes didn't seem to affect anything. For output tube I have the Philips that came with the kit, and a Svetlana Winged C 6AS7GA. I now switched back to the Philips to double check. The buzz moved to right channel, and was slightly quieter. It sounded a bit different too, slightly higher pitch. How I didn't notice this before I will never know.

Could it be that I have two faulty output tubes? To have a fault in the amp which would cause erroneous sound in different channels depending on the tube type does not make sense to me.

[Paul Birkeland]

Could it be that I have two faulty output tubes?

This is very unlikely.

To have a fault in the amp which would cause erroneous sound in different channels depending on the tube type does not make sense to me.

You have at least one solder joint that needs to be reheated.  When you pull a tube out and put it back in, the connections inside the amplifier get moved ever so slightly, so the problematic connection will be jostled. 

I would suggest reheating all of the solder joints in the amplifier, especially those on the terminal strips.

[Nikodemus]

Thank you for your the response, Paul!

I was expecting such a suggestion... I suppose there might still be problematic joints, even though I re-did all of them once. I might have to take some of them apart completely and mechanically re-attach them too, to make an improvement.

Right, I'll go over my solders once more and report back with the results.

[Paul Birkeland]

The other joints that give problems are:

1.  Where the black wires meet at the headphone jack.
2.  3L
3.  The solder joints in the power supply where each 220uF cap attaches. A loose cap will make noise in both channels though.

[Nikodemus]

I have now re-soldered all the connections twice again, and gone over the terminal strips twice more. There were a few poor mechanical joints that came loose when the solder melted. I fixed those. However, the problem is unchanged. With the Philips tube I have a buzz in the right channel, and with the Svetlana in the left. This difference between the tubes is constant and repeatable.

I don't see how I can improve things anymore by reheating the solder joints, I already did that so many times with no apparent change, better or worse. Either it is something else, or there is something fundamentally wrong about my soldering which causes me to repeat a same mistake. Even if it was so, (and it is very much possible) with so many tries I would expect to get it right at least by accident, and perhaps make things worse somewhere else. If I would have a fixed a problem and caused a new one, I would expect the symptoms to change at least a bit. Since this is not the case, I am inclined to conclude that no effect has been accomplished with my efforts in resoldering. If there is no way to try and pinpoint the problem, I don't see much other options than a complete disassembly and restart of the project.

Too bad, I have really been enjoying the build, and the sound the amp makes is great. I had high hopes of having fixed the issue after fixing the loose joints.

[Doc B.]

If the left vs. right channel noise continues to be repeatable from each tube it is likely that the tube pins are oxidized or the tubes themselves are  noisy.

[Nikodemus]

If the left vs. right channel noise continues to be repeatable from each tube it is likely that the tube pins are oxidized or the tubes themselves are  noisy.

Hello Dan!

Thanks for chiming in! Indeed it is repeatable. I must have gone back and forth dozens of times by now, and each time it is the same. In the Philips I have pretty quiet, metallic buzz in the right channel. It is only noticeable when in quiet passages, but enough to be annoying. It can clearly be affected by tapping the tube or the amp housing, as it sometimes starts or stops when I do that. In the Svetlana I have a slightly louder, scratching sort of buzz in the left channel. It cannot be affected by tapping.

If it would really be the tubes, I would certainly welcome that as good news and just get some new ones. I was quite convinced that it must be in the solderings, though.

[Doc B.]

Before you look for new tubes try cleaning the pins on the ones you have. A Scotchbrite pad or fine sandpaper (320 grit) can usually cut thru any buildup without much effort. Some Deoxit will help keep the contacts clean.

[Nikodemus]

Before you look for new tubes try cleaning the pins on the ones you have. A Scotchbrite pad or fine sandpaper (320 grit) can usually cut thru any buildup without much effort. Some Deoxit will help keep the contacts clean.

Thanks for the tip! I will definitely try that.

Looking forward, in the case that cleaning/changing the output tube does not help. What options for corrective actions do I have?

I checked the grounding path, as per instructions in the sticky article. As far as my measurement accuracy goes (within maybe 2 ohm) everything seems to be fine.

I tried to record the noise I am experiencing. Rather pathetic attempt, I suppose, since the recording device at hand is my smartphone. Anyway, I was able to capture the noise emitted to the left channel by the Svetlana tube. You can find a recording here: https://www.dropbox.com/s/mu5f3mg1ilrn5me/svetlana_noise.wav?dl=0
There is a lot of random noise because of the way it is recorded, and that is not from the amplifier. You might have to turn it up quite a bit to hear the scratching type of noise. The Philips buzz was too faint to rise above the backgound noise, and unfortunately I don't have a recording of that for comparison.

I don't know if hearing the noise is helpful in any way, I am just trying to use all tools I have to get to the bottom of the issue. =)

[Nikodemus]

Another update to my saga.

I was able to test the amp with a third output tube, same kind as the JAN/Philips that came with the amp. Result was familiar, first everything seems fine but after a few minutes a faint buzz appears in the right earcup. Faint, but noticeable during quiet sections. At times it stays quiet for a few minutes, and then returns.

Convinced that the fault was within the amp, and most probably with my soldering, I proceeded to desolder, detach, reattach, and solder all wiring and components from the transformer, tube sockets, terminal strips, RCA plugs, headphone jack, volume potentiometer and mains plug. So practically everything. The end result of this is that nothing has changed. The buzz is exactly as it was before.

It seems I cannot figure this one out, unfortunately.

[Doc B.]

Have you listened with no source connected and the volume pot turned down all the way? Did you ever try cleaning the tube pins?

[Nikodemus]

Yes, I have listened with no source connected. I had an instance with an Elekit amp I built earlier, where the origin of a similar (but worse) buzz turned out to be my computer. Unfortunately it is not the case here. The sound is the same regardless of inputs or volume level.

I didn't try to clean the tube pins. Somehow since three tubes had exhibited similar problems I didn't think that would be it. To me it would seem more likely that the tube socket would be the culprit, causing a bad connection with all tubes. I will anyway try cleaning the pins next, and report back wit the results.