Hello all, looking for some advise before I take my amp to the tech. I was playing through my SVT skunkworks when all of a sudden loud feedback noises started coming through the speakers. It sounded like feedback through a sweeping filter. I turned it off then back on a few minutes later and it's still there. Any thoughts? Thanks
Enjoy the benefits of a premium membership!
Upgrade Account
-
TalkBass has been independent since 1998. Add your voice.
Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
Join free Log in
Want zero display ads or expanded classifieds tools? Compare plans.
Feedback from SVT head
- Thread starter jasmangan
- Start date
My first guess would be a microphonic preamp tube. VERY CAREFULLY DO THIS OR YOU CAN DIE! Open it up, turn it on, then take a wooden spoon or a chopstick or anything wood with no metal on it, and lightly tap each tube. A microphonic tube will usually make itself apparent when you do that. Try all tubes, pre, driver, power amp till you find a culprit. If you don't find a culprit, then you're better off taking it to a shop. But I'd bet money it's the first preamp tube.
A microphonic tube is certainly a possibility. Try taking the head off the cab, move it away to the full length of the speaker cable. Does this make it stop, or affect the noise? If so, that would point to microphonics. You can try to isolate which tube is responsible by swapping in known good ones, or even swapping tubes of the same type between positions.
If not, I'd suspect a bad power supply filter capacitor. Those caps prevent the audio signal from feeding back through the power supply to a high-gain stage earlier in the amp. This can cause all sorts of weird noises: sometimes it is very low frequency and it goes BUP-BUP-BUP, (often called "motorboating") while driving your speakers past their excursion limits; it can whistle or whoop (which sounds like your problem); it can even be ultrasonic. You called your amp a "skunkworks", weren't those made two or three decades ago? That's a typical time frame for caps to start to fail.
It could also be as simple as a bad connection. It happened while you were playing, vibration could have finally broken a cold solder joint or loosened a connector.
Bottom line, though: if you don't find a microphonic tube, and you're not experienced poking around live high voltage circuits, protect yourself and your amp by taking it to a good tech!
If not, I'd suspect a bad power supply filter capacitor. Those caps prevent the audio signal from feeding back through the power supply to a high-gain stage earlier in the amp. This can cause all sorts of weird noises: sometimes it is very low frequency and it goes BUP-BUP-BUP, (often called "motorboating") while driving your speakers past their excursion limits; it can whistle or whoop (which sounds like your problem); it can even be ultrasonic. You called your amp a "skunkworks", weren't those made two or three decades ago? That's a typical time frame for caps to start to fail.
It could also be as simple as a bad connection. It happened while you were playing, vibration could have finally broken a cold solder joint or loosened a connector.
Bottom line, though: if you don't find a microphonic tube, and you're not experienced poking around live high voltage circuits, protect yourself and your amp by taking it to a good tech!
Ya, if you didn't find a wonky tube, it's best to let a tech deal with it. Rickdog is probably right that it's a bad cap.
