Good advice. I switched the oscillator to LED bias (red), and wired in an external 12AX7, in case there was something wrong with both my 6BM8s. At idle, the LED showed oscillation. When I played, it stopped. However, if I turned down the volume on my guitar enough, I got tremolo. That explains why scoping never showed any oscillation. I was scoping the signal path, and when I played, it stopped oscillating. So I moved it back to the 6BN8 and started playing with the preamp. First off, the first stage plate voltage was way too low, so I raised that to spec. It still lacked headroom, so I raised it some more. As you can see from the schematic, the spec is pretty low. Then I replaced the bias resistor with a 4.7K and the bypass cap with a .68. This both improved the overall sound and made the trem work at all volumes. Thanks.