Occasional popping sound - pictures and initial troubleshooting [resolved]

[Mike1590]

A2 is measuring 0V (0.6 mV, to be exact).

[Paul Birkeland]

The brown stuff is flux.  If you've used rosin core solder, that's totally fine to see there and you can use alcohol and a toothbrush to remove it. 

0V at A2, an unlit LED at A3 and 0V at A3 and high voltage at terminal 5 would normally point to the A4/5 solder joint, but you say you have both halves of the tube glowing and the solder joint looks OK.

If the 22.1K resistor between terminals 4 and 5 isn't well connected, that would cause this problem also, as would a loose wire between terminal 5 and A1.  If you happened to break off the plastic guide pin on the 6080 and you inserted it in one specific way that's not correct, you could also end up with high voltage on terminal 5, but this also seems unlikely based on the information provided here.

[Mike1590]

Ah sounds good. I will try to scrub some of that off once I get my amp working right again. I will check on those other connections tomorrow. You are correct that the 6080 guide pin is still in place, so I don't suspect that's the case either. I'll keep ya posted once I check on the connections you described and have had a chance to touch them up/re-measure voltage. I will also double check both halves of the tube are glowing by putting the chassis right-side up. Thanks again for your guidance on this!

[Mike1590]

I couldn't wait to tinker til tomorrow, so I played around a bit more tonight. Good news! The A3 LED now glows and the voltage on terminal 5 has gone down. There is still a low and constant buzzing in both headphone earpieces, but I think I'm getting closer to finding a fix. I did try tapping around with a chopstick a bit and didn't hear the same popping that I was hearing before - but having said that, I didn't mess around for too long, so it's very possible those sounds still exist.

Terminal 5 - 82.4V
A3 - 1.578V
A2 - 0V
A1 - 81.9V

If you have any more thoughts, I'd love to hear 'em! Otherwise, I'll plan to test for the popping sound a bit more tomorrow and report back.

[Paul Birkeland]

For a low buzzing sound, I would check that all the 220uF capacitors are well connected and soldered to the terminal strips.

[Mike1590]

That did the trick!!! No more buzz and now I can move that smaller tube all around during playback without any popping sounds. You’re the man, Paul! Thanks so much for walking me through everything.

Edit: And most importantly, thanks for giving me the confidence to keep tinkering before throwing in the towel (and a bunch of money).

[Mike1590]

Darn, I jumped the gun. During music playback there was a loud pop and the left earphone completely cut out. At that time one half of the small tube also went out - presumably the one connected to A3 since that’s the one I’ve had trouble with. Should I try bypassing the LED and using a wire to bridge the pins like you suggested before?

Edit: I was able to confirm that the side of the tube that went out is the side connected to pin A3.

[Paul Birkeland]

If half of the tube stops glowing, it's not the LED.

[Mike1590]

Ah okay. So should I just start by checking back through the locations we already discussed then? Any other possible culprits that weren’t mentioned earlier in this thread?

[Paul Birkeland]

The tube glows from AC voltage fed to it by the green twisted pair of wires.  If one half of it stops glowing, the connection between A4 and A5 still isn't quite right.

 You can remove all of the other wiring in the amplifier besides the green wires and the incoming AC wiring from the power switch and IEC module and the 12AU7 will still light up.

[Mike1590]

Good to know. There is a tiny bit of solder on the white plastic socket between pins A4/A5. I’m wondering if that could be the cause of my problem? I tried to snag a photo of it but it’s nearly impossible to get an accurate looking shot of it. Is there a good way to try to clean that out without reheating the solder and trying to use a solder pump? As you’re aware, I don’t have a ton of real estate to work around in, so I don’t know how possible that attempt would be…

I may just try to suck as much solder out from A4/A5 pins and start that connection over again…

[Paul Birkeland]

A4 and A5 need to be connected, so if there's a speck of solder between them, that isn't hurting anything.

[Mike1590]

Should I have the actual pins be soldered together? I had it configured this way before I started this troubleshooting process, but removed that solder a few days back. It looked like the pins were separate in the instructions, but since I originally had them soldered together, it makes me think I read that somewhere in the instructions while I was initially working on my build… either that or I just made a mistake, which is most likely! 

[Paul Birkeland]

It's also possible that you have some dirty tube pins that are giving intermittent contact, and these can be cleaned by simply pressing the tube into a piece of steel wool a few times.  There's a very remote chance that the 12AU7 has an intermittent internal connection, but this is extremely rare and typically comes up when we test the tubes prior to shipping.

[Mike1590]

This was something else I was considering, so I’m glad you mentioned it. The tube that gave me issues this morning is one I bought secondhand online for tube rolling, NOT the one y’all had sent me in my kit. I will try my amp again with the tube Bottlehead included. I’ll also clean the pins on both tubes, just to be safe.