Humming on one channel, possibly bad recitifer?

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It looks like the 5AR4 is almost dead because I saw it arc a few times this morning. I noticed that one of the channel is humming slightly after these incidences. I am not sure if humming was present before to say that it started after the arcing, but it's noticeable now. The hum goes dead when I pull the tube with all other tubes in place. I went over my wiring and it looks pretty identical to the non-humming side. Could this be due to bad 5AR4? I ordered a new one at any rate. Thanks!

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And here is another thing. Replacing the 12ax7 tube with another one significantly reduces the hum. I could hear very faint hum coming from both channels with the 12ax7, only audible with my ears close to the speakers. Any idea what could have caused the initial hum?

[erichayes]

The 12AX7 hum is probably from heater-cathode leakage in the tube. Since the two triodes are completely independent of one another, one side could have slightly more leakage and the hum was exacerbated by the failing rectifier. Normally, when a 5AR4 dies, it lets you know both visually and aurally in no uncertain terms.

Because the 12AX7 is the stage with the most gain, things like H-K leakage are going to be more noticeable than leakage in the driver or output stages. A heater-cathode short, on the other hand, will be noticeable in any stage.