Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: aravenel on July 28, 2022, 10:25:52 AM
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Hi all,
I built a Crack about 2 years ago (great instructions!). I used it happily for about 18 months with no issue before installing a Speedball kit a few months ago. The speedball kit passed all resistance and voltage checks when done, but after using for a month or so, I started getting intermittent drops in the left channel which got worse over time. I finally put the amp on the bench and started troubleshooting. I noticed several strange voltage readings and am not sure how to proceed troubleshooting--I fixed a couple of questionable solder joints, but that does not seem to have resolved anything and the voltages remained the same. The problematic voltages I am seeing are:
Small board:
OA: 176v (vs 60-90 target)
Large board:
OB: 174v (vs 75-100 target)
Terminals:
1: 177v (vs 50-100 target)
7: 174v (vs 90-115v target)
Are there any suggestions here? Clearly those voltages are way over target, but I am not sure what to test next to try to trace the issue.
Many thanks for the help--this forum has been a great resource!
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Are both halves of the 12AU7 glowing? Are both socket LEDs glowing? (if one is not, please do not attempt to resolder it)
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It does look like one half of the 12au7 is no longer glowing, and the LED at A8 appears to be out as well.
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Inspect the connection at A4 and A5. One is loose.
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I looked at A4 and A5, and neither looked loose or like a poor joint. All the same, I reflowed them both, but unfortunately readings are all still the same, and the LED and half the 12au7 are still out :-\
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Can you post a pic of the 9 pin socket wiring? It's also possible you may need to pull out and reinsert the 12AU7 a time or two if the pins are a bit oxidized. Once you get the second half of the 12AU7 to light up, your voltages will likely snap back into place.
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For some reason I cannot get photos to upload here, so I put a photo on imgur: https://imgur.com/a/8tAukL1
I am tempted to unsolder and clean every connection here and wire it back up... Think I have some extra LEDs in case I fry one taking it apart (had that issue when building!).
I removed/reinserted the tube a few times and that didnt seem to make a difference, so I am supposing I have a bad joint in here somewhere...
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Any LED that you solder twice is automatically suspect, as it doesn't take much heat to melt them. What kind of solder are you using?
Can you post a pic of your 12AU7 that's only half glowing?
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I also just went back and re-did the resistance checks--I wonder if something else is going on. Notably, I am seeing open circuit at terminals 7, 9, B3, and B6, all of which are supposed to be 2.9k-ohm.
I wonder if there more than one problem going on here... :-[
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Any LED that you solder twice is automatically suspect, as it doesn't take much heat to melt them. What kind of solder are you using?
Can you post a pic of your 12AU7 that's only half glowing?
Yes, I burned out one of the LEDs when I first wired it up (pre-speedball) and had to order more to replace the one I burned up. Lesson learned... I do have a few more though if you think I should replace some out of caution.
I am using tin/lead electronics solder--this stuff here: https://www.parts-express.com/Electronic-Solder-60-40-0.8mm-0.031-1-lb.-Spool-370-032
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And here is a photo of the 12au7 glowing--can (barely) see what is the left half in this image glowing: https://imgur.com/a/36njovJ
This picture is taken from the right side of the amp, so this would be the front half of the tube if you were looking at it normally.
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I would pull a little more of the wire through terminals A4/5 so you have some to crimp over A4.
You don't see 2.9K because the 3K resistors are removed when the Speedball is installed.
It's also possible you may need to try a different 12AU7, just to rule that out. If you remove the 6080, you can check proper operation of the heater circuit with a 12AT7 or 12AX7 as well, but I wouldn't run the amp with those 9 pin tubes and the 6080 installed.
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OK. I will try pulling more wire through and making sure I have a good joint there.
Is there any way to test the tube short of swapping it? Hate throwing (not inexpensive) parts at things if I can avoid it...
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You can measure the DC resistance between pin 4 and 9 on the tube itself and pin 5 and 9. They should be about the same.
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OK--against the tube itself I am getting open circuit b/t 5 and 9, and a ranging value that doesn't seem to want to settle even after a few seconds between 4 and 9. Given that, I think I will try swapping the tube and see if that fixes it...
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Sure, you could give that a try. It may also be that the pins are a bit oxidized. Just pressing them into a scotch-brite pad will often renew the pins a bit.
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Ah, good tip, that let me get a steady reading at least. Still open circuit b/t 5 and 9, reading 5.7 ohm b/t 4 and 9. Sound like a bad tube?
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On a very, very rare occasion the connections to the pins can break off inside the tube. I think I've seen this maybe twice in twenty years, so it's not impossible, but still extremely likely. Don't ask me how much time I spent trying to clean those pins before looking closely inside the glass and seeing the break!
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Pulled out a magnifying glass and dont see any obvious breakages in there, but all the same... It seems like the obvious things have failed, so might as well try swapping the tube, at least rule that out.
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Update: New 12AU7 tube came in yesterday. Voltages are now in line, and a listening test has the left channel back to where it should be. I much appreciate all the help--super excited to have my amp back up and running!