With the pot on the back all the way off, the signal is pretty low so it sounds like the hum is gone. But if I max out the master and both gain controls, I can still hear it slightly.martin manning wrote: ↑Mon Aug 21, 2017 10:51 am Sorry, on the back panel pot I meant "up" on the schematic, grounding the grid. The behavior is as expected there.
Since the hum is stopped by grounding the input to Gain 2 that pot is okay, and that pinpoints the source as V1B. That leaves the tube, socket (poor contact perhaps), and the filament and socket wiring. If you have DC on the filaments it could have some residual ripple, but a short would upset the bias. What does the filament DC supply look like, and where is its ground reference?
Layout: http://www.ceriatone.com/ceriatone/wp-c ... t-2015.jpg
DC Schematic: https://img02.deviantart.net/2eff/i/201 ... bb05j4.jpg
You can see how the DC supply is set up there. It's created from the extra 5v tap on the PT. Three 4700uF caps, two diodes, and the regulator. The ground of the DC supply is right there at the regulator's ground at a local chassis node. Just like in my other builds (and they are quiet).
You can see it in the upper right corner of this photo: https://orig13.deviantart.net/8c5d/f/20 ... bkpvon.jpg