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Rattling sound in bass frequencies (Scott tube amp)

MadcapLaughs

MadcapLaughs

Active Member
Hello, I am hearing a rattling loose sound in the bass frequencies when I turn up my Scott LK 72 tube be amp up to moderately loud volumes, and it gets worse as you increase the volume from there. It is only happening in channel B. I have swapped the cables connecting to my speakers and the rattling swapped from the right speaker to left speaker. I also tried different pairs of speakers with the same results so I know it is the amp and that channel b in specific, my solid state sansui had no problem at all with my same speakers which are Altec boleros. Here are some additional troubleshooting done to no effect:

Swapped out all the 12ax7 tubes out with no change.

Reconnected all tubes to their sockets and tapped on all the tubes to see if there were loose filaments or anything rattling.

Tried a different electrical outlet and swapped the power strip that the amp was plugged into.

The amp has been thoroughly recapped and biased correctly very recently and had plenty of burn in time (if that's actually a thing) by the tech who worked on it.

Help me with some additional ideas as I am not sure how to isolate the issue. Everything else sounds great on this amp but the bass on the channel b sounds like a loose driver that is rattling in some shitty Honda with a subwoofer. Also this amp has this distortion in all preamp sections (phono, aux etc).
 
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Try swapping left and right channel output tubes and see if the sound moves to channel A.
 
New electrolytic caps can expose a tendency toward parasitic oscillation that might occur in bursts on large amplitude waveforms. Does this amp have G1 and G2 stopper resistors on the finals?
 
Mike, is this the effect also known as "motorboating" you're describing, or a version thereof?
 
No. Motorboating is usually a vigorous low frequency oscillation resulting from inadequate or failed power supply decoupling.
 
I have never experienced this failure myself but I have heard that the bobbins inside transformers can work loose over time. I wonder if you have a bobbin which has broken loose from the iron core and is rattling at bass frequencies. Can you listen to each output transformer with the end of a screw driver? (cheap mechanics stethoscope) That will tell you if it is happening right inside one of them and not the other.
 
Thanks for the suggestions everyone. Fortunately I decided to not bother with trouble shooting by myself and took it to get repaired. It's now in perfect working order again.... It was:

Traced the problem to a poor solder connection on the coupling capacitor C23 from pin 8 of V5 to pin 6 of V8. Repaired connection.
Replaced listed capacitors on V5 and V6.

i wouldn't have guessed that was the case but everything is all good to go. Thanks again, proper soldering is important when rehabbing these old amps!
 
The bad solder joint might have been disturbed when you swapped the cables, "fooling" you into thinking the cables where at fault, glad you found it. :thumbsup:

Mark T. :music:
 
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