Sometimes it actually is a bad tube! (M3 weird hum, random snaps and pops)

[tpappano]

Submitted in case this could be helpful troubleshooting info:

This M3 had an obnoxious hum and produced random pops, but otherwise was functioning. Checked the filter caps for the cause of the hum, that was not it. Checked resistor values, etc., they were ok but the 12au7 phase splitter was showing an abnormally low plate voltage. Actually had to drag out the scope to see what was going on. The 6v6s were putting out rapid high level ultrasonic bursts resulting in an ac hum-like noise from the speaker. Long story short, the 12au7 (V3) was bad, bad, bad! All better now. Interestingly, tone 71 was missing and the problem there was its filter capacitor was *completely* open. After having changed about 20 lbs of drifted TG capacitors, this was the first one that had actually stopped being a capacitor!

[Joey_Gleet]

When I switched the 12 AU7 from that M3 to the other one, the hum went with it. Glad you're having fun.

[tpappano]

With all the "social distancing" I've pretty much been "Hammonding" all week. Yesterday rewired the RT3, connected up the 147 and installed tremolo and echo switches. Today fixed your M3 and put new keyboard felts in the BC. The BC is finally ready to come home now 8) Next up, re-cap the tg and amp in the M3 and defoam the upper manual and re-cap the tg in the D152. With the neighbors gone also a lot of "testing" going on! The pedal solo units in the RT3 and D152 turn them into real beasts, I hope all shaking of the building doesn't cause any roof leaks 8)

[KC9UDX]

Heh I didn't see this yesterday. I ran into the same thing on my Lowrey today (it wasn't the driver like I expected though)! Little did I know that the occasional thunder from the speakers was actually accompanied by lightning in a 6L6. Too bad I didn't get the lightning on video, it was pretty spectacular!