I tinkered with the feedback some and ended up pulling it back out. It sounded fine as I wired it, couldn't tell you why. Tried it again at V2A and V1B and it sounded bad at either point and would definitely start oscillating at V2A at some point I didn't bother measuring. It didn't sound good there anyway. At this point, I decided I needed to move the output transformer to the top of the chassis because I hated the way it looked with all the empty space under the cage. One I started it back up—motorboating like crazy, so I just gave up on it. It sounds great without it.
The big news from today was that I figured out why it seemed like I had so many microphonic 12AX7s... I had a bad .047 Mallory 150 cap in the tone circuit. I kept tapping joints and parts around V1 and hearing the typical plink racket and almost tried swapping the tube socket until I decided to spread out to the tone controls. Tapping that cap made a hellacious noise. Crazy. Swapped it out and totally fine there now.
Also decided that since I ditched the NFB I'd try fiddling with the tone circuit a little (why waste a nice hole ready for a switch?). The amp is brighter, in general, and pretty scooped at mid-ish tone settings so playing with some parts I had on hand, and looking at the tone stack calc, I clipped in a couple low value caps in parallel with the 250pf on the treble control and settled on 750pf for a total of 1000pf as a switchable option. This very noticeably shifts the midrange dip backwards, like 2-300hz vs. somewhere closer to 1k, and gives a little more nasally tone that's really nice at higher gain settings. For cleaner stuff, left out is best still. Nice flavor option I think. Maybe I'll change my mind tomorrow. I really need to stop tinkering.
Anyone see anything glaringly terrible with this version of the schematic?
Topic: Fizzy Distortion in Background of new Push Pull 6V6 Build (Read 10820 times)