Anyway, since then the amps have been working ok until I did some tube rolling. If I use a 12BH7 or E80CC, I started to get an oscillation. While monitoring the bias, I notice that if I turn the volume up on my preamp past a certain point, the bias goes up by about 10% and the amp oscillates. If I turn down the bias, the “pitch” of the oscillation lowers and then disappears.
So it’s like an input sensitivity. The 12BH7 and E80CC have a bit more gain but even with a 12AU7, if the volume is loud enough, the amps will oscillate.
I realized I neglected something about the NFB and I wonder if this could be the issue: the schematic takes the NFB (6.8K/680pf) to the 16ohms tap. The OTs I use has a single 6ohms secondary tap (and the speakers are 4ohms).
I soldered a 8k2 in parallel with the 6k8/680pf network to bring it down to 3K9 ish but now as soon as the the tubes light up, the oscillation appears right away with no signal at the input...