Losing left channel/high voltages post-Speedball install [Resolved]

[aravenel]

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!

[Paul Birkeland]

Are both halves of the 12AU7 glowing?  Are both socket LEDs glowing? (if one is not, please do not attempt to resolder it)

[aravenel]

It does look like one half of the 12au7 is no longer glowing, and the LED at A8 appears to be out as well.

[Paul Birkeland]

Inspect the connection at A4 and A5.  One is loose.

[aravenel]

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 

[Paul Birkeland]

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.

[aravenel]

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...

[Paul Birkeland]

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?

[aravenel]

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... 

[aravenel]

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

[aravenel]

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.

[Paul Birkeland]

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.

[aravenel]

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...

[Paul Birkeland]

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.

[aravenel]

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...