Pop on left channel, intermittent [solved]

[alexroz]

Hello and greetings from Malaysia,

So I've built the Crack, at first without the Speedball. Resistance check was fine, voltage check was fine though a bit high but within the 20% allowed but I put this down to our voltage here which can reach 250-260VAC at times. I did test the transformer before installation and the output was a bit high for the 150V outputs, I got a 178V reading.

Without the Speedball installed
It starts up with a pop in the left channel after 10-15 seconds. Checked the voltage on the TRS on startup and it climbs past 20VDC (had the 20VDC range selected on my multimeter) on the left channel but then drops back down to 0VDC immediately after that. It plays music fine after this, both channels are balanced, there isn't any noise or hiss with or without signal present. Tubes glow, the LED's light up. I had about 2 tracks playing before there was a pop again from the left channel. Music kept on playing without any interruptions when the pop happened. Immediately shut it down and did the voltage check again after 30 seconds. All checked out except for that rise past 20VDC on the left channel on the TRS connector.

With Speedball installed
Exactly as above. So I know its not the Speedball.

I reflowed some joints, did a continuity check on all the wires. Everything seems to check out. Bad 6080 tube? I might be imagining this but I could have sworn I saw a small flash from the 6080 twice. The first time was on the first power up, it came on, glowed then went off and then came back on almost immediately. Never happened again after that first power up. Second time might have just been a flash but it didn't die or anything, the Crack was facing me and the flash came from the back left side of the tube. What do you guys think? Oh, I haven't actually left it on long since the build. I think I've managed a total of 2 tracks so far and probably a total time of less than 10 minutes, afraid it'll kill my headphones. Do you think the tube needs to settle in or something?

Thanks for any help offered

[Doc B.]

Try leaving the amp on for a couple of hours with no headphones plugged in. The 6080 may need to settle in. Then if you have some inexpensive headphones that you don't care about (they don't need to be high impedance) turn the amp off, let it cool down for an hour, and try the amp again with the inexpensive headphones to see if it still pops. Before you do all of this look for the other possible cause of popping - a missed solder joint.

[alexroz]

Hello again,

Thanks for the swift reply. Went thru the solder joints again, all appear to be good. Ok, left the amp on for an hour +. Turned it off, waited a minute, turned it on again and same thing. Now, while the amp was on, here's something I noticed:

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi1284.photobucket.com%2Falbums%2Fa573%2Fblackoctagon12%2FIMG-20140307-04579_zps1d76397e.jpg&hash=fa1c5d389d2cd36878810ea021a30f70032f76f1)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi1284.photobucket.com%2Falbums%2Fa573%2Fblackoctagon12%2FIMG-20140307-04580_zps435e462f.jpg&hash=dba6042e236119cd2307af7c28495da19b9d5c7d)

Why's the right side of the tube glowing a lot brighter than the left side? Is this normal? If so then I'm gonna go reflow every joint again but really am at a lost here if its not a funky tube.

PS: Sorry for the big pic.

[Paul Birkeland]

Can you carefully measure the AC voltage on power transformer terminals 4 and 5?

-PB

[alexroz]

On the AC scale, I got 5.9V on the 6.3V taps and 174V on the 150V taps.

[Paul Birkeland]

Ah, that's pretty good.

Can you let the 6080 run overnight, to see if that purple haze dissipates?

[alexroz]

I'll turn it on whenever I'm at my desk. A bit hesitant to leave it on overnight, seems to get really, really hot. Just turned it on. Even from cold that one side of the 6080 burns brighter than the other side. Will see what happens over the weekend and report back here next week. Thanks!

[Paul Birkeland]

Yeah, the brightness is just a bit more of the heater showing, that's not abnormal. 

The Crack will also run pretty hot, but by having the correct voltages inside the amplifier, you can rest assured that the proper currents are being drawn. 

[alexroz]

Hello again,

Ok, left the unit on, so now its been on more than a few hours. Did a bit of resoldering, redirected some wires. And I did some voltage readings, I checked the AC voltage and I was getting 245VAC. Below are the readings:

1 - 78
2 - 182
3 - 0
4 - 181
5 - 78
6 - 0
7 - 111
8 - 0
9 - 110
10 - 0
11 - 0
12 - 0
13 - 177
14 - 0
15 - 197
16 - 0
17 - 0
18 - 85
19 - 82
20 - 0
21 - 217
22 - 0

A1 - 78.5
A2 - 0
A3 - 1.56
A4 - 0
A5 - 0
A6 - 79.1
A7 - 0
A8 - 1.55
A9 - 0

B1 - 79
B2 - 177
B3 - 112
B4 - 78
B5 - 177
B6 - 110
B7 - 0
B8 - 0

Everything seems to be pretty close to what the manual says it should be but I'm a bit confused about terminals 18 to 21. The manual doesn't seem to say to check terminals 16, 17 and 18 but I included their readings anyway. They seem to be off from what they're supposed to be.

What do you guys think?

Thanks!

PS: I tried a different 12au7 tube with no change. Don't have a spare 6080, tried to pick one up over the weekend but was told its rare over here. Two shops I tried said that and told me that most amps don't use the tube so they don't stock it...

[alexroz]

Ah, just saw Doc's corrections. Think I was reading an old version of the manual...so my voltages are all fine...where does that leave me? Both channels climb to past 20VDC on startup. Left channel goes to 28VDC+- while the right goes to 23VDC+-. Manual says they shouldn't go past 9VDC...?

[Doc B.]

With the speedball installed the voltage goes a little higher at startup. As long as the voltages go down from there you are fine.

[alexroz]

Okie, just an update. The problem never went away so I finally got another 6080 tube and plugged it in, measured the voltage on the output and it only climbed to 13V instead of the previous 28V before dropping to 0V. Plugged in headphones and turned the amp on, no popping, just clean music. 

I think there was a short in the original 6080 that came with the amp because there were a couple of times lately when I saw a bright flash from the tube when the pop happened. And the pop seems to have gotten louder over time. So dodgy 6080 aside, I actually didn't have any trouble at all building the amp and also the Speedball...