Help tracking down a hum. [Resolved]

[handrewmoore]

This is the second Crack that I've built, and I have a really faint low hum that starts about 10 seconds after turning the amp on. The hum does not change with volume. The first Crack I built was dead silent, so I know that this isn't normal. I do have a few things different with this amp though. I have the speedball installed, a stepped attenuator, neutrik locking jack, vhaudio 21awg unilock wire, furutech rca jacks. I guess I didn't notice the hum initially, I just got everything put together a couple of weeks ago, but it's all I hear now when I start up the amp.

What I have tried with the same results.

tightened the ground screws
shorting the rca jacks
moved amp to different room
reflow solder joints
rolled 3 different power tubes, I don't have any extra 12au7s

I'm guessing that it's either the driver tube or the stepped attenuator. I have one of the 5814a tubes that went out on special a couple of months ago and never tried it in my original crack that is now about 300 miles away. I'm going to order a new 12au7 tonight or tomorrow. The wiring for the stepped attenuator was a bit different than I thought it would be, I double checked it and it seems like it should be fine. The attenuator was the only thing I wasn't confident when wiring. The Crack plays great and everything else seems as it should be other than the hum. The resistance and voltage checks before the speedball all seemed within range and the voltage checks after the speedball seem to be ok too, I'll post the voltage checks below. If anybody has any other ideas, tips or things to try I'd be really grateful.

1 - 77.25
2 - 185.15
3 - 0
4 - 185.15
5 - 71.5
6 - 0
7 - 93.1
8 - 0
9 - 87.2
10 - 0

11 - 0
12 - 0
13 - 185.5
14 - 0
15 - 204
20 - 0
21 - 224
 

[Paul Birkeland]

Wiggle each of the 220uF/250V capacitors.  A loose one will cause what you describe.

[handrewmoore]

Thanks for the tip. I tried wiggling the 220uf caps but the hum stayed. I went ahead and pulled them and resoldered them. No luck. I also uninstalled the stepped attenuator and installed the stock pot but the hum was still there. I also replaced the 21awg wire in the power supply with the stock 20awg but that didn't change anything either.

I'm going to uninstall the locking jack and try the stock jack tonight. If that doesn't work, I'm going to swap the rca inputs with the stock rcas. After that, I can only assume it's the 12au7 or faulty part somewhere. I'm open to any other suggestions if there are any. Thanks again.

[Paul Birkeland]

Woah woah woah, removing caps and replacing them is not going to help your cause.

Look for loose wires now, my guess is that you have a loose black one. 

[handrewmoore]

Well, I thought I had found the problem. I went through the manuals step by step checking everything along the way. When I got to the speedball, I saw it, I soldered one of the 237ohm resistors in the wrong way. I can't believe I did it, I thought I was sure to double check orientations before soldering. Anyway, turned it around and soldered it back in place, checked the voltages then fired it up with my fingers crossed. But the hum kicked in as the tubes warmed up. Could running the amp for a few hours like this have damaged any parts? I assume this would have put stress on the 12au7. I have one on the way, should be here by the weekend. Thanks again.

[Strikkflypilot]

The resistors have no polarity (no + and - sides) and can be soldered in like You please.

[handrewmoore]

Well, I guess that would explain why my measurements weren't off.

[handrewmoore]

It was the 12au7, popped the new one in and silence.