B15 Preamp issue

[hylaphone]

Hiya folks, just completed a single channel B15 pre w/ 12au7 CF stage, all crammed into a cute little enclosure.
Having an issue with the Treble control, when it is zeroed the amp breaks into what seems like a low Hz oscillation. Not a squealing or wailing mind you, more like an angry, prehistoric hornet.

I've triple checked my wiring to this schematic:
http://unofficialampeg.com/schematics/b-15-n68.gif

...and tried replacing the pot, no dice. Anything suspicious in that schem?
Thanks--

[soundguruman]

Since you only have 1/2 of the schematic built try 500K treble.
What amp are you using, and how is it coupled?
As usual, the layout of the circuit can make quite a bit of difference.
So show us the layout, and how you crammed it into a tiny box.

I suspect you are amplifying heater buzz from AC filaments.
So, try it again with DC to power the filaments. That will confirm the problem.

[hylaphone]

I should add, the onset of this noise is very sudden at the last 1/10 or so of CCW rotation.

Wouldn't heater buzz enter gradually?

I will try a different pot value.. and snap some pictures later.

Testing this thing through a SS power amp/PA speaker.

It plays beautifully otherwise, very quiet and convincing