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https://en.wikipedia.org/wiki/Motorboat ... ectronics)
MOTOR BOAT CAUSES
http://www.tdpri.com/threads/what-cause ... ng.210392/
Motorboating is low frequency oscillation like an amp with the trem on. Just without trem, I've had old Marshalls do it. You get a "bup-bup-bup" sound that sometimes pushes the speaker to full excursion.
It's a low. low frequency--lower than audio (and therefore a "putt-putt" instead of a hum)--build-up/discharge somewhere around the grid of a tube, probably in the preamp.
Motorboating is what happens when an electronic audio circuit, designed to operate at frequencies between 30 cycles per second and, oh, let's say 20 Kilohertz, ceases to function at those frequencies, but still produces sound, of a putt-putt nature, at around 1 to 10 Hz.
• could just be poor solder joints
• The usual cause is insuffient decoupling between PS nodes, so yes a cap job could well help it, if the electro's have started to dry out and their ESR increase
• It's murder trying to trace a bad cap, but that's usually the culprit. Best bet is to look for physical signs on the caps around the preamp tubes: bumps on the electrolytic cap ends or bad looking joints.
• way back in '67 in tech school, when we were taught troubleshooting on tube and transistor circuits - we were told that motorboating is always due to a bad capacitor. In radio circuits it was the cap in the AVC (automatic volume control)....a configuration not seen in guitar amps. Anyways, it proved to be true. Any radio that had motorboating was due to a bad cap (not the power supply filter cap).
In guitar amps I've never encountered motorboating but I believe (from reading various forum posts) that it does happen. People contributing threads said "it was a bad cap".
So, I don't know which cap in the guitar amp is bad but it's probably good to start off looking for a capacitor as the culprit.
https://www.diyaudio.com/forums/tubes-v ... o-amp.html
Oscillations can be caused by wires from output transformers too close to input wires.
Try disconnecting the cathode bypass cap on the left channel.
https://www.radiomuseum.org/forum/gow_t ... oblem.html
Motor boating explained and how to troubleshoot, GREAT VIDEO!!:
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It's not always capacitors, it can be as simple as "chopsticking" to fix the problem.