Intermittent motorboating issue 72 vibrolux.

[earthmud]

About a week ago I was playing at home and all the sudden I hear a loud putting sound that started in one pitch and then wound down to a lower pitch and kept put putting in the same pitch. I hit the standby switch turned it back on and it was gone. Tonight about a week and a half later I was at band practice and when we finished a tune it started doing it again. I hit the standby and again it was gone when I flipped it back on. The other guitarist in my band is pretty knowledgeable with amps. He's built his own 5f4 clone and does most of his own amp repairs. When he heard it he said it was definitely motorboating.
My questions are this, can tubes cause motorboating? I've had the amp for about 4-5 years now and have become quite familiar with what a microphonic tubes sounds like. If it is a tube then it's causing a motorboating issue. I changed my phase inverter about a month ago from a Mullard 4024 to a Blackburn Mullard ecc81. I've been doing a lot of preamp tube swapping lately but the tubes in it now are the ones that have been solid for awhile. The amps electolytic, and filter caps were replaced when I bought it. From what I've read it's most likely a cathode bypass cap that has drifted out of spec right? Can tubes cause a motorboating sound( I promise it really is a motorbating sound? Since it's an intermittent issue tube swapping to see if it's gone will be pretty difficult. The only other thing that has been done lately is the chassis was pulled to solve a vibration rattle issue(The chassis was vibrating against the screen mesh on the top. A couple of small pieces if velcro on the lip of the chassis solved most of the rattle). The next time it motorboats is there anyway i can troubleshoot while the amp is on and making the noise? I've tried to include as much relevant info as possible so sorry if it's long winded Any advice is welcome.

Thanks, Jason

[pdf64]

It is almost certainly an electrolytic power supply filter cap in the doghouse, developed high ESD.
Do any of the controls affect the intensity or frequency? Pete.

[earthmud]

Since it's an intermittent noise I have not been able to see if the dials have any effect. I'll fire it up this morning to see if I can make it start put putting and check if any of the knobs make it worse. If I can't should I pull the doghouse and look for any visual signs like bulging caps? I know not to touch anything. Thanks for the response.

Originally Posted by pdf64

It is almost certainly an electrolytic power supply filter cap in the doghouse, developed high ESD.
Do any of the controls affect the intensity or frequency? Pete.

[Blue Strat]

Intermittents are really tough. If there are ANY electrolytic caps that haven't been replaced, just replace them. That might fix the problem and reheating solder connections is always a possible fix. Sometimes you have to forget about knowing exactly what the problem is and just do a lot of general things and hope for the best...unless you want to spend the rest of your life nailing the problem.

[earthmud]

I hear what your saying. I just hate to change anything else because when it's not motorboating it sounds great. I'll mess with it today and see if I can figure anything else out. Thanks J

Originally Posted by Blue Strat

Intermittents are really tough. If there are ANY electrolytic caps that haven't been replaced, just replace them. That might fix the problem and reheating solder connections is always a possible fix. Sometimes you have to forget about knowing exactly what the problem is and just do a lot of general things and hope for the best...unless you want to spend the rest of your life nailing the problem.

[phsyconoodler]

I worked on a vibrolux that had the same issue once.It turned out to be the output transformer laminates.They had come loose and it did exactly what you are describing.A quick squeeze with a large c-clamp confirmed the issue.when the clamp was on it was fine.Take the clamp off and eventually it would make the noise.
I whacked it with a big ball-peen hammer and it hasn't done it since.Laugh all you want but it tightened the loose laminates and the problem has been gone for 5 years now.
Now this is not necessarily your issue,but it does happen.the amp motorboated like crazy but only when it wanted to,or the temperature conditions were right.

[earthmud]

The OT was replaced a few years ago so it's probably not that but I did smack my amp last night to see if it would stop but it didn't. I always try a good smack first

Originally Posted by phsyconoodler

I worked on a vibrolux that had the same issue once.It turned out to be the output transformer laminates.They had come loose and it did exactly what you are describing.A quick squeeze with a large c-clamp confirmed the issue.when the clamp was on it was fine.Take the clamp off and eventually it would make the noise.
I whacked it with a big ball-peen hammer and it hasn't done it since.Laugh all you want but it tightened the loose laminates and the problem has been gone for 5 years now.
Now this is not necessarily your issue,but it does happen.the amp motorboated like crazy but only when it wanted to,or the temperature conditions were right.

[donnyjaguar]

Lack of de-coupling is the only thing that causes this. I agree, you need more capacitance in the power supply.

FWIW, it can be aggravated by someone changing the coupling capacitors to too high a value or otherwise messing with the filtering in the output section or increasing NFB. I guess what I'm saying is check for "mods" first.

[phsyconoodler]

Quote:"Lack of de-coupling is the only thing that causes this."

Yeah,right.Tell that to CBS.They often had motorboating amps and move a few of the wires that they seem to just 'throw' into those amps and it stops.
You can throw all the caps and decoupling resistors you want at those and nothing seems to work until you move the wires closer to where the blackface versions were routed.
Siverface amps either worked fine or they were plagued with issues.

[earthmud]

I played for about an hour or so today and no noises. This amp has been solid since I've owned and this is the first issue I've had but the lead dress on this amp is terrible. Maybe thats part of the issue. Heres some old pics before the electro's and filters were replaced. The only other thing done to this amp was the high frequency roll off caps were removed from the power tubes.

Originally Posted by phsyconoodler

Quote:"Lack of de-coupling is the only thing that causes this."

Yeah,right.Tell that to CBS.They often had motorboating amps and move a few of the wires that they seem to just 'throw' into those amps and it stops.
You can throw all the caps and decoupling resistors you want at those and nothing seems to work until you move the wires closer to where the blackface versions were routed.
Siverface amps either worked fine or they were plagued with issues.

[soldersucker]

brain fart post disregard..

[earthmud]

No worries. I have those all the time myself.

Originally Posted by soldersucker

brain fart post disregard..

[CDA]

Originally Posted by donnyjaguar

Lack of de-coupling is the only thing that causes this.

It is true that preamp stages that are cathode biased are more stable when "De-coupled" with bypass caps...

That being said.

I'd check the actual ground wires going from the turret board to the chassis up front..

If those get loose then there you have it.

Will do it every time.

Lift the cathodes of the preamp tubes above ground and bamo!

LF oscillator..


Could be 14 other things too lol

[Structo]

That lead dress is much better than a lot of silverfaces I have seen.