Finally did get it working the other day. The LTPs were clean with resistors for tails (post 19). It looks like the CCS impedance at RF pushed it into instability since these are RF-capable tubes. With 1k added in series with the CCS’s it cleaned it up. Whatever it actually was had to be way up there - it is a 100 MHz scope. Adding a base bypass to the CCS almost worked - it would power up clean, but the oscillation could be started by driving it hard. Less audio band noise with it there, as well as the .01 on the rail so those stay. The 1k fixed resistor will take a few volts away from common mode headroom but this won’t ever be driven with more signal than a 5532 op amp can produce so I’ll live with it.
This is where things ended up, stable. On to the Baxandall tone control circuit - going to try a pentode gain stage to have more loop gain when it’s in full boost. It won’t need the non-inverting input, so that particular source of trouble won’t be there. If it oscillates it will (hopefully) be just a feedback/compensation issue. Which will at least be low enough in frequency to observe.
This is where things ended up, stable. On to the Baxandall tone control circuit - going to try a pentode gain stage to have more loop gain when it’s in full boost. It won’t need the non-inverting input, so that particular source of trouble won’t be there. If it oscillates it will (hopefully) be just a feedback/compensation issue. Which will at least be low enough in frequency to observe.
