Thanks for info, did you change anything for the 12AU7? Here is the schematic:
Oh some pics... lol not the most elegant. I wasn't sure about layout and how to run wires for low noise.
No changes it was motorboating around 12 o'clock. I read in another forum motorboating could be from too much gain. I tried a 12au7 and all was good. So I left it.
I don't see a PE pole, is your box connected to the protective ground?
Motorboating can be caused by the wiring; you may be able to avoid this in your next amp by optimizing it. Keep the inputs of the gain stages away from the outputs. A Mumetal shielded input transformer can also help and would simplify the setup considerably.
motorboating is typically caused by insufficient B+ filtering, or it can be due to low frequency oscillation. All wires emit magnetic fields, so keeping them short and tidy is always a good idea. High current wires like heater supplies should be kept away from sensitive grid wires. In this kind of a circuit, heater elevation would probably be a good idea. This means referencing the heaters at some arbitrary higher DC voltage like 30VDC instead of referencing them to ground (0 volts)
Motorboating is nearly always due to insufficient decoupling between the B+ for output tube and input stage. One solution is a voltage regulator, dropping B+ by 10 or 20 V. Alternatively, substantially increasing capacitance on the rail will "brute force" it to be stiffer. Magnetic coupling between wires running parallel is always worse at high frequency. If filaments operate on DC, magnetic coupling is virtually zero (only changing magnetic fields couple to induce voltage into nearby wires), but twisting of filament pairs is always a good practice.
I've been fascinated about cascode circuits for a while. Never gotten the chance to experiment with one though. From what I've read the 12au7 is particularly well suited for such circuits.
