The edge of instability......

[Cliff Schecht]

Your body capacitance could be what's killing the oscillations. An easy test is to use something long and non-conductive to push on the tube. If it doesn't oscillate then it's the socket, tube or a solder joint. If it does, then it's probably a wire that needs to be moved 1mm.

[geetarpicker]

When I read about you guys testing the amp with the guitar cord plugged in (but without a guitar) and dimed I was wondering what was up. My amp doesn't oscillate when dimed and unplugged AT THE AMP as the jack is wired with the standard shorting connection. However yep it will oscillate just a tad with the guitar cord wide open, but the hum levels are almost so loud it's hard to tell what's going on at that point. And also in this situation if you put your hand close to V1 you can tame the oscillations. I did once or twice over the years get some oscillations with the guitar unplugged because the shorting connection had gotten a little dirty and/or the jack got loose enough to cause an iffy ground. I think the only oscillations worth chasing and taming are when you crank the thing with the guitar plugged in, guitar knobs off but and it's also worth checking that as you fade your guitar up it continues to stay stable. Hope that helps!

[redshark]

Glen, when I test for oscillations is with No cable plugged in....
I wouldn't test with only the cord plugged in and no guitar, that is just noise waiting to happen.
It's interesting that with guitar plugged in I get no oscillations, only if it's fully dime I get feedback I can't control but is musical and not noise.

[M Fowler]

At some point you need to call this amp done and oscillating at high volume is not going to happen in a playing situation, I never have an Express dimed

[redshark]

Yes Mark, I guess you are right.

I think I should call it done. See my post #12 and as I said if an original does it, mine can do it too and as you said I will never play it with those settings. I just wanted to fully stabilize it.

[Cliff Schecht]

Yes Mark, I guess you are right.

I think I should call it done. See my post #12 and as I said if an original does it, mine can do it too and as you said I will never play it with those settings. I just wanted to fully stabilize it.

You really don't want to fully stabilize it though. I did fully stabilize my first Express build (the rats nest one) and I learned pretty quickly that once you do get the amp stable, the sound goes to shit. You lose a lot of gain getting the amp stable and have to throw pedals in front of the amp to get the gain back.