I've posted many times about trying to debug this plexi clone amp I've built (see also https://www.tdpri.com/threads/how-t...-down-weird-overtones-in-plexi-build.1025094/ , https://www.tdpri.com/threads/not-sure-if-my-plexi-build-has-problems.984480/ and https://www.tdpri.com/threads/marsh...riations-get-breakup-at-lower-volume.1041560/). I've been trying to figure out over the past 11 months why the distortion sounds harsh and buzzy and I've made close to no progress. I'm losing my mind. At first I built it with a bunch of mods, but I've rolled it incrementally completely back to stock. Here are all of the things I've tried to figure this out. Throughout every change, the buzzy distortion persisted, and I heard no difference (in the harsh buzzyness). Take circuit completely back to stock, including removing tube rectifier Shorten leads as much as possible, make sure tube pin leads are not too close to each other Check all voltages in the amp Swap out all tubes for a brand new set Use my audio probe to listen to the signal at various points in the circuit. Built an attenuation circuit into the probe capable of attenuating the post power-tube signal to a listenable level, so I can listen to the signal post power tube and pre output transformer. Chop sticking Try using the 8 ohm impedance selector instead of 16 Completely disconnect NFB from circuit Change the NFB wires to use shielded wire Change the input wires to use shielded wire Change the effects loop wires to use shielded wire Try upping the value of the post-phase inverter "stability" cap Try swapping in a different power transformer Try swapping out each filter capacitor for a larger value Try taking the filter capacitor for the preamp tubes completely out of the circuit, and connect it via alligator clips, in case it was too close to the signal caps. Take out most of the signal caps and figure out which end is the foil-end, mark it, and reorient appropriately Switch V1 from shared cathode to split cathode Take the effects loop out of the circuit and instead connect the treble pot directly to the PI section Try varying levels of NFB, from lots to completely disconnected. Buy an oscilloscope and poke around with it Take amp to a professional tech to debug I really don't know what else to try.. The amp sounds fine right up until it starts to break up. Then it gets this horrible buzzy noise, which I think might be parasitic oscillation. But I've tried everything I can think of to alleviate it, and I cannot figure it out. I've tried probing the circuit, and to be honest, I think I may be hearing the start of the oscillation way back in like the first or second tube stage. It's only once we get to the post-phase inverter stage that the oscillation starts to break up, which makes the buzzy sound really noticeable. I really built this amp so that I could play it overdriven, not clean, so I want to fix this so badly. Does anyone have any clue what else I could try to figure this out?