negative feedback potentiometer issue (resolved)

[kwijabo]

last winter i completely re-built my 'pool style amp as a 2xEL84. when doing it i connected the heaters to all 4 EL84's so if i wanted to go to 4 it wouldn't be that tough, but didn't put any screen or grid resistors on them. and also i added a 100k pot in series with a 10k resistor in the feedback so i could mess around with different feedback values. after a winter and summer of problem-free use i decided i was going to populate the other 2 output tubes.

so i did. but turning the volume up past 3/10 would send it into a crazy motor-boating like oscillation i'd never experienced. narrowed it down to a mechanical issue and not a layout issue but didn't know where. after a day of seemingly getting nowhere, i turned the negative feedback pot full tilt (therefore, 110k in the feedback) and it went. unsoldered the pot and put a fixed 100k in its place and no more problems! amp sounds great. pot must be intermittently connected up at the end of the taper.

why this was so tough to figure out was because it only happened when the pot was up at the end of the dial. furthermore when i only had two EL84's in there it was set at 62k so i was never in that area.

anyways, figured i'd share this problem, resolution, and the rebuild 'pool.

video of me demonstrating the problem:
https://www.youtube.com/watch?v=8U4TSxoUxDI

and a picture of the amp as it is today. still need to replace the pot with another speaker jack.