So this issue is totally unrelated to my B-18N problem with seemingly unbalanced power tubes, this is a new issue I found today on my early B-15N. I'd better give the full story, in case there are important clues: Got this B-15N a few months ago and it functioned just fine, to a point... not perfect, but well enough. Some things I had noticed were that there was not much headroom on channel one (bass was breaking up at about 12 o'clock, halfway), a bit more headroom on channel two (bass would break up about 3 o'clock, seemed about right). 6L6GC were a visually matching set of Philips/ECG, not original to the amp, but not new either. Previously, a guitar player had been using this amp (that may be important). I opened it up and the big caps had been changed in 1995 (same with the speaker, and probably the PT). The big caps weren't quite correct (32uf/500v, in both spots where they should've been 20uf and 40uf multis), but apparently close enough. I ordered all the correct caps from Bruce at Fliptops. This amp still has the 2-prong plug and death-cap, so that'll be changed soon, but I'm being quite cautious meanwhile. The correct caps have now been installed, including the 50uf/50v cap (between the power tubes) that appeared to have leaked it's black oil all over my original schematic, at some point in its history (it was dried up). The end had popped off that cap... why they didn't replace that, I don't know. Some other caps around it had been replaced with orange drops (about 4), but that cap that runs between the power tubes (the 50uf/50v) was shot. However, the amp was functioning even with the dried up leaky cap. I'll assume that's because they put in a bunch of new caps, maybe that was the weak link and it opened up afterward? So that brings us to today. I noticed that the rectifier tube was actually not correct for this amp, the chassis (and schematic) is marked for a 5U4 but there was a 5R4GYA installed in its place. To me, that probably explains the headroom issues (which I still had, even with the new caps installed). If I understand correctly, the 5R4 uses less filament (2a) than the 5U4 (3a) AND the B+ will run a bit lower (about 25-35v, I think). Two possibilities, in my mind, the guitarist wanted a bit earlier breakup so a tech told him the 5R4 might give him what he wanted OR when they installed the new PT, a tech decided it might be less taxing on the new PT to use the 5R4 (or the other possibility is that the old rectifier just went bad and that's all they had in a pinch, but I doubt that). I happened to have a good old GE 5U4G recitifier, so figuring I have my new filter caps in there, I'm probably safe to return this thing to stock. I swap the 5R4 for the correct 5U4 and flip on power. Seemed fine (although I might have heard a little pop, could have just as easily been my imagination), so then I flipped off standby and still good, tubes began to get the usual light blue haze, played a few notes on my bass and then I quickly (within 5 seconds) get a bit of sparking inside the 6L6GC nearest the rectifier, right up at the top near the double ring getter, lightning show. I heard the popping also, through the speaker. So I quickly turned the amp off. The 6L6's seemed to heat up awfully fast, but that too could be my imagination... lots more 'cooling off' sounds than usual, after I shut it off. My hope is, that one of you guys will read this and know precisely what's up, or maybe had this experience before? My theories (which are admittedly not worth much) are: 1) The power tubes are old and marginal. They were fine with the 5R4 rectifier, but the 5U4 revealed the fact that my power tubes are on their last legs. (that's the easy fix, I have a very nice NOS set of Sylvania 6L6GC, but don't want to risk them til I fix this). 2) Someone changed something in there that means I can't really run a 5U4 in there anymore? This old B-15 is cathode bias, so I don't think it's a bias issue. All the caps in there now are either the original little guys, or they are correct value big ones... and all the resistors appear original. The replacement PT appears correct, just has the heavier wrinkle finish on it than the originals. 3) Maybe someone put some super funky PT inside an Ampeg PT can and potted it? This is highly unlikely, in my opinion. But I do suspect a tech was inside this thing in 1995, replaced some caps, PT, new speaker and possibly put those 6L6GC and the 5R4 in there at that time (btw, the preamp and PI tubes appear original and test good). As a note, I have checked for continuity between pin 2 & 3 on the 6L6GCs (no short there); I've also cleaned my power tube receptacles with Deoxit.