Hey, I just completed my build of a Ceriatone 67' Plexi Lead 50 (basically a Marshall plexi specced from 1967). I biased the tubes to get them sitting between 67% and 69% dissipation at idle. The amp was exhibiting some weird problems one of which kind of went away... I'm not sure what to make of it. These clips were recorded with the channels jumpered, each channel volume at about 8, with varying tone settings. Current "problem" - This problem still exists, but I'm not sure if it's a problem, or just normal for this kind of amp!! ¯\_(ツ)_/ Sometimes the amp sounds like it's distorting in weird ways, almost like a ring modulator or an octave fuzz. Sometimes when I hold notes, I'll hear weird overtones that modulate Here's a clip showing some of the spitty, fuzzy-like sounds. You can hear subtle ring-modulation like things going on: The problem also seems more pronounced when I turn my NFB dial down (in place of the NFB resistor, I put a 250k pot in, in series with a limiting 27k resistor). When it's at 27k NFB (from the 16 ohm tap), this problem doesn't really show up, and also the overdrive tone is much less overdriven. When I turn the NFB pot so the NFB is at more like 100k - 120k, the problem is much more present. The tone is more overdriven, but also comes along with these kind of spitty, modulating overtones. Note, the problem is a little less pronounced now, after I opened up the amp and made a voltage chart and put it back together again. When I record it, I barely even really hear it on the recording, though it sounds more pronounced when listening in person. Here are more recent clips showing it: https://soundcloud.com/itsgiusto/project-10-26-19-3-28-pm Is this sort of sound normal for a jumpered Marshall Plexi turned up to 8? Or do I have some sort of unwanted distortion going on? Former Problem - This problem seems to have gone away after I opened the amp up and made a voltage chart. ¯\_(ツ)_/ The volume was varying, especially after I hit a chord. It almost sounded like a compressor with a long attack time pumping - I'd hit the chord, then a half second later, it'd drop in volume. Here's a sound clip showing the problem: As you can hear, the longer I waited between playing successive chords, the more pronounced of a "pumping" effect I get. You can also hear some weird little crackling sounds sometimes when the volume pumps. Here are the voltages I've measured in my amp, with all knobs rolled down to 0, and nothing plugged into the input jacks: And here are the voltages Ceriatone has published as being normal for the amp: Some of the differences are accounted for the fact that Ceriatone's is split-cathode in V1, and mine is shared cathode. One thing that stands out to me in my build is that pin 8 on V2 is very high. I'm using a JJ short-plate 12ax7, since that seems to be good for cathode-followers. Another thing to note: the EL34 plate voltages have dropped since I took these measurements. The next time I went to measure, they were more like 406v.