Strange oscillation

[marcoloco961]

I need a little input from the crowd. Have had a home build bass amp up and running for a couple months or so and it was working super. At rehearsal the other day I am waiting for the drummer to get set up while the rest of the band was talking so I was practicing my part at a very low volume. So low you could talk over it easily. All of a sudden I hear a faint crackle starting in the speaker. When I turned it up to practice volumes, I have a super raspy distortion on the low notes so I baby it through.

I have unhooked numerous components and metered the values with nothing looking out of line. All voltages are the same as before with the possible exception of the PI grid to cathode voltage. , I have changed out pre amp tubes. I changed one tube socket that needed it anyway, re-did the input, re-flowed most of the solder joints, blah, blah.

I am suspecting a faulty cap of some sort. All have metered the correct value, but the symptoms lead me in that direction. The one symptom that I have noticed that stands out is if I really dig into a low string hard, I get not only distortion, but a blocking distortion effect. So much so that it starts loud and raspy, then decreases in volume until I have nothing, not even background hiss. Total silence for just a second, then the sound returns. I put a coupling cap in parallel with all three couplers and didn't see any change, but I had this problem before I reduced the value of the initial couplers. I am still thinking coupler or filter cap.

I haven't switched power tubes yet, but they both are sitting right where they were before the problem started voltage and amperage wise. The only thing I have done to it recently before the problem was sand off a couple long pot shafts. I did use a heat sink on them, and took it real slow, letting things cool before continuing, until I had enough off, but I suppose I could have damaged one. The oscillation just makes me want to look elsewhere.

Any thoughts? Filter Caps ARE Sprauge Atoms. <<<< Maybe this?

[tubeswell]

Crackles can be a symptom of power tubes on their way out. I'd change those first before you do any other stuff. (The amp was working fine up to that point I take it?)

[marcoloco961]

Crackles can be a symptom of power tubes on their way out. I'd change those first before you do any other stuff. (The amp was working fine up to that point I take it?)

Yup, it was weird. It had been working great, I was playing really soft and quiet then a faint "crackling" started, followed by distortion on low notes and then by the strange "cut off". When I said the thing goes quiet after digging in hard, I mean it loses ALL sound. DEAD QUIET. Not even the slightest background noise of electrons jumping all around. Nothing. Then after just a second the sound fades back in. It only oscillates once, loud and distorted to nothing then back to a not so distorted normal volume. The 6L6's are not new tubes by any means. I don't have another pair but I will throw in a pair of EL34's and reset the bias to see if it helps.