Strange overtones on certain low notes.

[vavlehead]

Hey Guys!!
First Post.

I have been digging around amp chassis for 20+ years. Know all the safety stuff.

I'm a little stumped here.

So i have a ab763 Build. Only a few month old. Basically a Super Reverb with SS rec.

Sounds awesome. But it developed the following problem:

After it warms up.. On certain Low notes it has an "undertone" that sounds like a mix of arcing and buzzing. Not very loud but not faint either. Very annoying!!

Full tube swap completed ( power and preamp ). Speaker swap completed. Still there!!
High notes sound great with no hint of this problem. The amp is quite when not played.
It seem to me to be "mechanical"... I can produce the sound by knocking the cabinet. Although its at its loudest when I hit the low notes.

Any ideas would be helpful.

[jmaf]

Bad electrolytic capacitors.....probably time for a cap job.

[vavlehead]

Bad electrolytic capacitors.....probably time for a cap job.

"So i have a ab763 Build. Only a few month old"...

[acorkos]

probably a faulty solder joint...with the amp running, and taking proper precautions, poke around with a wood or plastic probe on every solder joint on the boards, tube sockets, pots, jacks, etc. be patient, you'll find it.

[Gibsonman63]

A simple test would let you know for sure. Pull the chassis and see if you still get the overtones with the chassis isolated from the cabinet.

Is it on both channels? If it is only on the reverb channel it could be the reverb tank getting rattled around too much.

[vavlehead]

@acorkos yep... doing that now.

@Gibsonman when using a separate cabinet the noise was still there but much more quiter... so yeah its vibration related at the low freqs.. I only use one channel... The normal channel is fully disconnected. it happens with reverb off and tank unplugged.

Again... it starts when the amp warms up nice and good.

[Gibsonman63]

Then it sounds like a little chop-sticking may be in order. Let it warm up good. Poke around (taking proper precautions) and see what you find.

[vavlehead]

I got it warming up now... I noticed something right out of the gate. The amp is just turned on.. still cold.

This "hollow echoing mechinal/rattling" sound in the background... only when I let the low G ring out... any other low note E,F,A tails off without any extra sound. Weird Crap I tell ya!!

[vavlehead]

Man it sounds like it should be a tube. Gonna swap again.

[km6xz]

Not really weird, the low note is hitting the mechanical resonance of the defective part. A faulty coupling cap, tube, carbon composition plate resistor, lots of things could be modulated by the mechanical vibration. You ought to use a signal generator if you have one, it will be much easier to sweep across the low band and find the resonant frequency of the fault.
What test instruments do you have available? A distortion analyzer or wave analyzer would really help, find the resonant frequency, and null out the fundamental and clearing hear the noise component and not the G note.

[vavlehead]

Not really weird, the low note is hitting the mechanical resonance of the defective part. A faulty coupling cap, tube, carbon composition plate resistor, lots of things could be modulated by the mechanical vibration. You ought to use a signal generator if you have one, it will be much easier to sweep across the low band and find the resonant frequency of the fault.
What test instruments do you have available? A distortion analyzer or wave analyzer would really help, find the resonant frequency, and null out the fundamental and clearing hear the noise component and not the G note.

Hey thanks!! Km6zx.

But dont we already know the freq... a G note??? Or am I confused.
I only have DMM for tools. And wooden chopstick.

[jmaf]

The overtones may come in from whatever part is able to act as a transducer: caps are transducers, cables are microphonic and tubes are always microphonic.... Those are your main suspects. Solder joints can do that too as mentioned above.

The chopstick as suggested by Gibsonman63 is your friend at this point. Go poking around, moving wires, tapping things - you'll likely find your microphonic part.

[protojim]

I have had similar issues with either a fractured solder joint to a filter cap but that was usually at moderate to higher volumes. It would make a horrible distorted noise riding along with any note you play.
Or
Are your tubes biased way cold? I remember getting that "noise" on the low C note in an under-biased amp amp with 6l6's. Total power dissipation of tubes was maybe 12 watts ea.
Sounded like pooh especially on the clean channel. I cranked up the bias to about 70% tot power dissipation of tubes or about 17.5 watts per bottle on 5881's / 6l6's and was like a new amp

[Jazz P Bass]

I would think that with a strange problem, the first thing that should be done is go "over" the amp.
Basic tech stuff.
Resolder critical paths, clean & lube the pots & sockets.
Check for proper voltages.
Put the amp on a signal generator with a dummy load.
Monitor the output with a scope.
If the problem occurs at the output then you go sniffing.
Break the circuit in half.
Is it there.
Yes.
Break it in half again, towards the input.
No.
Break it in half towards the output.
Localizing the problem is the key to repairing the amp.
Or you could guess.

[deci belle]

I gotta ask… How do I put the amp on a signal generator? I have an ab763 Showman (my problem is high notes). I don't know how to do this test. Please say exactly what to connect where, I have the equipment— but I don't know where to start! haha! Maybe valvehead doesn't know either!