In preparing for my Marshall mashup amp, I settled on a PPIMV. A drawback of this setup is that the NFB voltage coming back to the phase inverter will be affected. I started toying around with the idea of a variable NFB setup with a 10K resistor in line with a 100K pot, so that as I turn down the master volume, I can adjust the NFB pot to compensate. I had built 2 other amps with a NFB pot, and I had put in a 0.82uF cap to block the DC from the phase inverter so the pot won't be scratchy. I didn't have any instability with this setup, but reading on this topic in various forums, the issue of phase shift crept up. There were mentions of lower frequencies going out of phase due to the capacitor in series with the resistor, hypothetically causing oscillations and other nastiness. Now, I have no formal training in electronics, and the last time that I did anything in a classroom with electronics was basic physics in college 30 years ago. I tried to read about phase shift in a R-C circuit, and my brain locked up. My takeaway was that voltage and current will be out of phase in a series R-C circuit, and it will get worse the lower the frequency. In a vacuum tube amp, however, there shouldn't be much current at all in the NFB circuit for that to have an effect, and I have 2 real-life examples. Anyway, tonight I desoldered the 0.82uF cap in one of the amps with the NFB pot and busted out the oscilloscope. Looking at the output of the phase inverter, I saw absolutely no difference in the waveform with the cap in circuit or out as I turned the frequency down on the signal generator. The output transformer crapped out at around 50Hz though. I didn't write any of this down, so this is not science. Any thought?