So I pulled one of my amps off the shelf to play with it for a day. The amp is a early LK72 Scott. I thought I had it all working but when I pulled it down I had a problem which resulted in a hilarious troubleshooting experience. On the phono setting, the gain was wrong between channels. One side was like 6 to 10 times the amplitude of the other and I spent nearly all day trying to figure out why. I learned something (or realized something I already knew, and I'm going to learn a lot more before I'm finished with this. So I spent all day trying to figure out why the channel with the lower gain wasn't performing like the channel which sounded great! I replaced nearly every resistor in the phonostage with the best matching resistors (channel to channel) which I had in my stock. None of them measured substantially off and I had my doubts about replacing them. I replaced each capacitor 2 or 3 times. I took the pins for the suspect triode out of the socket and cleaned them individually then put them back in the socket and still had low gain on that channel. I swapped tubes and tested them 2 or 3 times. Tubes tested good each time, swapping them produced the same result, the same triode section producing way too little gain. At this point I've been focusing on this 1/2 of a 12AX7 for about 6 to 8 hours and the frustration is mounting. I'm ready to rip the socket out of the chassis and replace it, with little confidence in that course of action. Deep breath. Chill out Tony, this is supposed to be fun... Late that evening I found my culprit, My thinking was off, and I learned a lot. You already may know where my thinking was off. I had been focused on the triode with the lower gain. As it turns out that triode was working as designed. (or close enough to it) The fault in this case was found by turning up the amplitude on my signal generator. I noticed that the "good channel" developed several distortions in the wave form, while the Lower gain channel amplified the signal and put out a nice clean sine wave. This is where it came to me. I remember reading about how feedback networks lower distortion, but also lower gain. I thought to myself, "I had better pull up a schematic and see if this thing has any feedback around this stage." It of course does have feedback around the stage, and not only that, it has a PEC with the feedback components inside! FURTHERMORE!!!! It doesn't just have phono-stage feedback components inside, the PEC contains the feedback components which taylor the RIAA and NAB tape equalization curves for the two inputs. An isolated section of the tape/phono switch changes the EQ setting for whichever input is selected by the other sections of the switch. They are ganged. I have a broken lead coming out of one of these PECs and therefore not providing feedback around the stage for one channel, resulting in higher gain, (not actually good) and distortion, as well as RIAA non compliance! I thought about this for a day or two, and decided what I want to do. The PEC is done for. I'm not going to try and replace it. I'm not going to try and rebuild it. I'm going to remove the NAB tape equalization setting and the switch, from the feedback loop, and replace the feedback components within the PEC with components for a dedicated RIAA stage. This is where I come to the end of my Knowledge and abilities and I ask the experts for help. I took the liberties of drawing up the schematic as it stands and another of how I think it should be wired "without" the NAB tape section. I would love it if someone could explain to me why I'm completely wrong or what would be vastly better than this and why. Here is the phono-stage drawn as wired. The PEC feedback components are drawn in a box toward the bottom, and the switch is shown in the RIAA position. I want to eliminate the switch and go RIAA on bot inputs. This drawing shows the PEC and the switch eliminated, and only the components for RIAA left in the network. Please check my work here, I could be full of crap. Basically I want someone who knows way more than I do to verify that this will produce reasonably close RIAA compliance, or to show me what I did wrong, and perhaps suggest an alternative course of action, or some reading material on the topic. I lack the skills to design and model ultra tight EQ tolerances and I've never encountered a problem quite like this. Thanks in advance for your help, and happy holidays.