Hi, I am building an sort of hybrid 200W Hiwatt amplifier including the midrange circuit from Ampeg SVT. The amp has two channels with Hi/Low (You can find the circuit here https://photos.app.goo.gl/NBn0L5slkjQS2lM62) I just assembled the whole amplifier, and for some reason I have two issues .- There is an oscillation, like something like a chopper sound, which appears when the volume/treble/master increases. Also, when I play with the midrange rocker switch, the same chopper sound appears .- There is a dependency between the volume at each channel. I was expecting as I introduced a valve V3 to isolate both channel. Apart from that, the sound is very clean and there is almost no hum. Does anyone has an idea about how to solve this?