Right Channel Hum After ~50 Hours Burn In [resolved]

[Sixth.Gear]

Hello all. I have a crack I built myself approximately a month ago with only a few minor issues. Last night the amp was working great, with no hum. I turned the amp on today and was greeted with a right channel hum after approximately 15-20 seconds warm up time (about the time the coils in the 6080 start glowing). The hum does not change in intensity with volume, except moving to both channels and slightly louder when maximum volume is selected.

Troubleshooting I've done:

- Checked, re-checked and re-checked the re-check all voltages and resistances. Voltages are within the 15% threshold specified in the manual, and all resistances as spot on.
- Checked all grounds.
- Tightened all hardware (screws/nuts on both sockets and the power supply)
- Tried a different socket away from any possible interference
- Tried unplugging source (no change)
- Tried jumping RCA jacks to ground
- Re-soldered all connections
- Re-seated tubes
- Tried turning the amp on and off in different configurations (headphones in & volume off, headphones in & volume slightly up, no headphones, etc)

The other thing I noticed around the 10-15 hr burn-in point was the microphony in the large tube. If any vibration was made around the amp, I'd get a nice echo and/or higher frequency hum that suck until I tapped the tube. It's been on and off sensitive, sometimes to the point where typing on my keyboard about 2 ft away from the amp would create noise in the headphones.

What sucks is I've been waiting for the amp to fully burn in before starting the speedball upgrade. I was planning on starting the build/install this week but I'd like to solve this hum issue first before adding more changes/layers of complexity. Any suggestions? I've thought about trying different tubes, however I don't want to drop more money/time on getting new tubes that might not necessarily make a difference.

Also, what is the proper configuration to turn the amp on and off in? I've been turning volume down to 0 before turning off and wait for the tubes to warm up for ~10 minutes before increasing volume from 0.

Thanks in advance!

Edit: Did some more chopstick testing and found that now I'm getting some serious hum whenever I put a chopstick near the wire connecting the volume pot and terminal A7. The hum is substantially louder now as well now that I've poked/prodded around the A7 wire. It also changes frequency slightly once the amp is powered off and trails off.

[Doc B.]

The place to start is reheat the connections on that A7 to volume pot wire. Also check the connections to the ground tab on the RCA jack on that channel.

[Sixth.Gear]

The place to start is reheat the connections on that A7 to volume pot wire. Also check the connections to the ground tab on the RCA jack on that channel.

Reheated/flowed both connections on either end of the A7 wire, and re flowed the RCA jack wire and ground tab. Symptoms have changed slightly, with the hum being quite a bit louder and now having static-like sounds. Headphones pop when the power switch is turned off, and the hum persists for ~5 seconds fading out as the large tube heaters lose the glow.

I'm completely miffed as it was a working unit, and has been progressively getting worse now that I'm monkeying with it even though I haven't changed placement of wires and have only re-flowed connections.

[Doc B.]

Try cleaning the tube pins.

http://bottlehead.com/smf/index.php?topic=8888.msg84089#msg84089

[Sixth.Gear]

Try cleaning the tube pins.

http://bottlehead.com/smf/index.php?topic=8888.msg84089#msg84089

Cleaned as indicated, no change, however the tube pins were quite dirty.

[Doc B.]

It could be a tube going south, particularly if it was overly microphonic to begin with. Which is difficult to assess without trying a different tube. You can also gently rock the tubes in the sockets a little to see if the noise cuts in and out.

[Sixth.Gear]

It could be a tube going south, particularly if it was overly microphonic to begin with. Which is difficult to assess without trying a different tube. You can also gently rock the tubes in the sockets a little to see if the noise cuts in and out.

Unfortunately no change while gently rocking the tubes side to side. I'm open to looking at new tubes if one of them was likely a culprit. I was eventually going to try my hand at rolling, just didn't want to start right this second before getting the speedball in. Which tube should I look at replacing first, the 6080 or the 12AU7?

[Doc B.]

If the 6080 was really microphonic that might indicate that it was going bad.

[Sixth.Gear]

If the 6080 was really microphonic that might indicate that it was going bad.

Finally got my replacement 6080 in (Tung-Sol 6A7SG), same exact hum and volume. Any suggested next steps aside from starting to pull connections/start from scratch?

[Doc B.]

You could try changing the 12AU7 next.

[Sixth.Gear]

You could try changing the 12AU7 next.

12AU7 replaced with an RCA Clear top, same result. Even swapped the 6080 with another tube, same result.

[Doc B.]

I think the next step I would try the chopstick test again, since that was the thing that gave you some insight about the problem before. Maybe with the new tubes you will have less microphony and be able to hear subtleties in the noise floor a little better.

[Sixth.Gear]

I think the next step I would try the chopstick test again, since that was the thing that gave you some insight about the problem before. Maybe with the new tubes you will have less microphony and be able to hear subtleties in the noise floor a little better.

Chopstick tested again, with the A7/pot wire being extremely sensitive to even a wooden chopstick being placed near it. Completely replaced and re-flowed the wire, with no changes. Re-snugged all grounding points as I could hear a change in tone when I pressed my finger against the base plate. When testing the A7 wire, I also noticed the further I moved the wire from the terminal strip and wires coming into A9 & A4/5. Photos attached of the socket and surrounding wires.Completely miffed at this point, and definitely frustrating as I was finally enjoying it. Thanks again so much Doc for taking the time to help with this one.

[fullheadofnothing]

Hard to tell from the pictures, but is 3L soldered?

[Doc B.]

The end of the red wire at A7 looks good. Where it is attached at the other end to the center pin of the pot looks it might a little light on solder in the photo. Or it could just be the angle of the shot. You might also tuck that red wire over a little closer to the pot and farther away from the red and black twisted pair that feeds A4 and A5.

I would also be inclined to reflow the lower center terminal of the front T strip, T3L, since that is a point of contact between signal ground and chassis ground. From the angle shown there is no solder filling the hole.