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Motorboating in Tremolux tremolo circuit

bern1

Well-known member
Joined
Nov 23, 2004
Messages
1,157
I was hoping somebody might be able to point me in the right direction. This was mentioned briefly on the other Tremolux thread but can anybody tell me where to start?

The problem is that when the tremolo circuit is engaged I can not turn up the intensity or the speed to any useful level before loud motorboating kicks in and really overpowers the effect.

I just had the amp serviced and the tech guy said everything was ok, just that some of these amps are different and have different characteristics, values drift etc .etc.. (This is a white Tremolux 6G9A circuit,) I took this to mean that he either did not know what to do or didn't want to if it was a laborious process. In any case the amp is otherwise working well, has new power tubes, newer filter caps and the preamp tubes were checked and seem to be ok.

I also have a brown tolex Vibrolux that has virtually the same circuit. On that amp the tremolo works fine, no motorboating and it works great.

Thanks in advance for any ideas!
 

MikeSlub

Administrator
Joined
Jul 15, 2001
Messages
14,895
From the web:

1. "MOTORBOATING"
Excessive feedback in an audio amplifier can result in a fluttering or popping sound that resembles the sound of a motorboat. This oscilation is caused by undesirable coupling between the output and the input of an amplifier or chain of amplifiers. It can also occur as a result of capacitive or inductive coupling in the wiring or be caused by the power supply. Because of the characteristic sound, this low-frequency oscilation is called motorboating.


2. Motorboating is almost always a signal that the decoupling capacitors in the B+ lines of the preamp section are going high impedance, not decoupling properly. Replace the B+ decoupling capacitors at least for the preamp. Since the other capacitors are old, also, consider replacing ALL of the electrolytic capacitors in the amp.

Since you indicate that the filter caps have been changed in your amp, it's other caps that need changing. :hank
 

ptrickamp222

New member
Joined
Aug 22, 2007
Messages
879
Look at a schematic and many a vibrato circuit is about 3 caps. Try changing those or is it the blackface circuit with the glass piece in it. I forget what it's called. It will be covered in black rubber and is part of the vibrato circuit. Something like a photoresistor. Good advice so far.......be careful!!!
 

MRG

Member
Joined
Apr 25, 2002
Messages
672

bern1

Well-known member
Joined
Nov 23, 2004
Messages
1,157
Thanks all for the ideas, I'll start checking them out.
Bernie
 
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