5F10 Fender Harvard Clone Pulsating (HQ Video/Audio Included)

[ellum68]

5F10 Harvard Clone, Pulsating Noise Issue - YouTube

Back in August I pieced together a 5F10 Harvard clone. It's dead on to the original with the exception of an adjustable bias and a twisted filament pair/virtual center tap. This thing used to be super quiet. Tonight I noticed a hum when I turned the volume down. I ran a sweep of the volume control and picked up a pulsating noise near full volume. I've tried a different pair of power tubes, a different PI tube, and removing the 12AT7 preamp with no change. Anyone have any suggestions?

[ellum68]

Figured it out. My 1-ohm cathode to ground bias checking resistor bit the dust and was showing open on my meter. I grabbed an old piece of 12 awg solid household wiring and it shut up right up when I grounded the cathode.