Motor Boating - some research

General Tech Discussion - Anything amp-ish goes!

Moderators: CurtissRobin, Daviedawg, Graydon, zaphod_phil, rock_mumbles

Posts: 10
Joined: Mon 02/12/18 6:19 pm

Motor Boating - some research

Postby roadshow » Tue 01/15/19 8:02 pm

I just built an 18W'r, and I had a real issue with what I learned was motor boating. Apparently, in decades past this was a common problem (and still is :D ), and I'd like to post what I've learned today, links included below, some real good stuff. ... ectronics)

MOTOR BOAT CAUSES ... 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. ... 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. ... oblem.html

Motor boating explained and how to troubleshoot, GREAT VIDEO!!:
0 x

User avatar
Super Duper Admin
Super Duper Admin
Posts: 1119
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA

Re: Motor Boating - some research

Postby JMPGuitars » Tue 01/15/19 9:57 pm

Search for parasitic oscillation (or variations) in the forum here and you'll see plenty more discussion on the topic:

search.php?st=0&sk=t&sd=d&sr=posts&keyw ... id%5B%5D=4



It's not always capacitors, it can be as simple as "chopsticking" to fix the problem.

0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design -
Guitars / Amps / Effects -

Superior Amp Tech
Superior Amp Tech
Posts: 476
Joined: Fri 01/08/10 2:00 am
Location: Scotland

Re: Motor Boating - some research

Postby Daviedawg » Wed 01/16/19 3:11 am

My penny's worth is that the one time this has happened to me I chopsticked and found it was a lead dress issue. Nice and simple that time.

1 x

Posts: 10
Joined: Mon 02/12/18 6:19 pm

Re: Motor Boating - some research

Postby roadshow » Fri 01/18/19 12:26 pm

Found this great article on the subject.
You do not have the required permissions to view the files attached to this post.
0 x

Return to “By the Bench”

Who is online

Users browsing this forum: No registered users and 5 guests