Does anyone has a clue what causes this?

[Tatzmann]

Weird "slapping" noises...

A while ago i played my Laney AOR 100II,
4xEL34 .

It has just your basic 100w poweramp, like in 800s and so on.

Whatever, i was playing the thing and as it
was time to really crank it up it started to
make i'd describe it as a "slapping" noise,
sounded like a mix between a whip and
a gunshot, a crack and pop so to say.

Im pretty sure that it emanated from the amp
itself, because i tested with a different amp
afterwards and there was no crack&pop.

The noise was amplified through the speakers.

Fuses didnt come.

Unfortunately, the next time i played it the noise
was gone, so it cant be replicated.

Idk, bad solderjoint maybe or just a loose socket, bad tube? I somehow got the feeling that maybe a signal carrying cable moved through the vibration and caused this.

Thx

 

[Pete Farrington]

I suspect flashover, either within a valve, at the valve socket, inside the OT etc.
Repeat it enough and it’ll arc and blow a fuse etc.

 

[Tatzmann]

I had arcing from anode to probably heaters before in another amp, it was popping loudly sporadically and after a short while the blue sparks came flying and the HT came.

This was definately something else, atleast soundwise.

What would you suggest to do if it comes
back, i guess changing powervalves first?

You are probably right, thanks!

 

[william vogel]

I recommend you set up for an assessment. As soon as the condition occurs again, stop, remove the chassis and restart the amp and observe the internals to attempt to catch a glimpse of the components. If it’s an arc occurring you should be able to see it in dim light. A conductive spot in an insulation area may not be visible but each time it occurs will lessen the dielectric strength of the component and the problem will get less intermittent. You might want to lower the fuse rating to help protect against damage but this could lead to making the amp unusable because the fuse opens under normal operation. It needs to be known that fuses open at higher than rated current and at a time interval for the given current.

 

[Tatzmann]

Thanks William, great advice!

I did the lights out trick a couple times
before in the past on different devices,
that definately could lead to an observation
of the problem.

 

[william vogel]

Here's two charts of the time/current for both fast blow and slow blow fuses offered by Littlefuse. This is Slow blow.


This is Fast blow
https://www.marshallforum.com/media/fbfuse-jpg.14099/

 

[Biff Maloy]

Loose speaker?