Low-volume, high-pitched left channel hum that increases when I tap power tube

[tusing]

Hi!

I recently bought the Crack with the following mods, as described by the seller:

• The box is well glued, sanded, and has many many layers of black lacquer.
• The internal wiring is pure silver (99.99%) throughout the signal path. The silver wire is sleeved with pure cotton, then heat shrink.
• Speedball upgrade installed
• Added a choke
• Added the Cree Schottky rectifier mod.
• The plastic knob has been replaced with a machined solid aluminum knob.
• Finally, I used a 21 stepped attenuator rather than the stock pot.
• Stock tubes, but optional great tubes below.

There is a very light buzz/hum in the left channel that is driving me insane. It's high pitched and the ringing either increases or disappears when I tap the power tube.

The hum happens even with the volume turned all the way down and the RCA jacks disconnected.

What steps should I take to eliminate this hum? I am capable of using a soldering iron but I have no clue where to start.

[Paul Birkeland]

You could return it to the seller. 

If tapping on anything causes changes, this tends to indicate a poor connection somewhere in the amp. 

I would start be performing the voltage checks as directed in the Speedball manual.

[tusing]

You could return it to the seller. 

If tapping on anything causes changes, this tends to indicate a poor connection somewhere in the amp. 

I would start be performing the voltage checks as directed in the Speedball manual.

Unfortunately, returning to the seller isn't an option.

I turned over the Crack and resoldered most of the joints I could reach. Not all of them - wasn't able to reach some of the joints for the front tube - but I did reflow A7 which seems like has been a source of hum for a lot of people.

I suppose this decreases the likelihood of this being a connection issue?

I have a cheap power tube on order that will arrive by Friday night. Let's see how that does.

[adydula]

Curious as to what type of tubes are in the amp?
Alex

[tusing]

Curious as to what type of tubes are in the amp?
Alex

The default Phillips 6080 and a Siemens Silver Plate 12au7. The problem is not with the 12AU7 as I've a few of these.