I posted this over at EL34World first but that topic seemed to lose focus of the issue. I figured I'd try again here. I finished this 5e3 build last week. Standard layout without Standby switch (Robinette's drawing) with the addition of his 3-way feedback switch, rectifier protection diodes, and power tube screen resistors. The amp worked on first power-up, had to fix a couple things and now it sounds great. Except I've got a noise I can't chase down. I don't know if I should call this a hum or a hiss: https://www.dropbox.com/s/rary9vnla8j9mkl/5e3humhiss.m4a?dl=0 .. (There I've got the tone at 12, I turn the Normal Vol up from 0-12 and back, and then the Bright Vol up and down the same way). The best way I can explain it is it sounds like the crowd at a baseball game on TV when no action is happening. Just this mellow breathy static. It behaves as if V1 were microphonic and I think that's all part of this issue, but it must be coming from the lead dress or a component, not the tube itself. I've put 3 different tubes in V1 and a light tap on the tube pops loudly through the speaker. A tap on the chassis or chopstick of the wires going to V1 produces a quieter version of the same sound. It occurs when the volume is turned up on either the Bright or Normal channel, I assume it's something before V2. That narrows it down quite a bit. I also assume some component isn't properly grounded. But all of the components in that section look good to me. I've gone over the input jacks a dozen times and they look wired correctly to me. I can post a photo of the jacks if needed. It did trick me into thinking it was the Bright Cap at one point but I removed one leg of that cap and the problem persists. I swapped out the tone pot just in case, that didn't change anything. Solder joints look fine to me and I've re-flowed most of them just to be sure. Rather than rebuilding that whole section, is there a most likely culprit in there other than the jacks? Is there a "best" order to troubleshoot the components? I've never heard a sound like this before so I'm lost.