RFI and Microphonics in Right Channel Only?

[mikenet]

Just put together my crack, plugged in some sacrificial headphones, and heard continuous popping on the right channel. Goes away completely if I shut off my WiFi router or move rotate it 90deg. If I tap around the unit, the glass envelope of the 12AU7 is *extremely* microphonic in the right channel, but not at all in the left. If I pull the tube out of the socket a tiny bit, I can rap on the chassis pretty hard and hear nothing...if I tap the tube..LOUD pops and ringing in the right channel.

I don't have another 12AU7 on hand to make a quick check of tube health, but the RFI issue puzzles me. Would a bad tube be sensitive to RFI? Not knowing much about tubes...I wonder if tube aging would make them more likely to rectify a bit of RF?

Or is there something else in my build I should be checking out that could be causing the rectification? Bad solder joints? Cooked LEDs?

Michael

[Paul Birkeland]

It's entirely possible that the popping could be coming from a marginal solder joint, especially since it is relegated to one channel.

A bad 12AU7 will measure above 100V on terminals 1 or 5.

How close is your WiFi router to your Crack?  A little bit of distance might not hurt. 

-PB

[mikenet]

The crack *is* very close to the WiFi router. Probably won't/shouldn't stay there permanently, just scratching my head as to why one channel would be so RFI sensitive and microphonic, while the other is *dead* quiet. So for now, I'm intentionally using the router to induce the problem for debugging :-). Want to make sure the base build is perfect before I drop the Speedball in (built the PCBs the first day, just wanted to make sure I got the base build right before installing it).

When actually running music through the thing (and the router off), the channels are well-balanced with perfectly-centered imaging, so it doesn't sound like one is operating differently when not aggravated by RF and/or physically vibration.

I'll try another tube first, just to rule that out (even if unlikely) before I demolish the thing. Turns out my dad was sitting on a whole pile of NOS tubes that he was just "going to throw out some day", so he mailed me a handful of 12AU7s to try. That'll at least remove one factor.

-Michael

[mikenet]

Just to check back in, threw in a 12AU7 that my dad sent. It's now dead quiet--can't hear the WiFi router at all, and the microphonics are eliminated. Both channels now sound identical.

Half of that 12AU7 that came in the kit must have been bad.

Hope my investigation can help someone else. Was kind of surprised the RFI (unintended rectification?) was caused by a bad/old tube.

Michael