A weird noise

[Leevi]

I'm finalizing my Plexi 50 and I noticed after some hours playing a weird noise that waves.
I thought that it is related to a bad/microphonic tube and changed the preamp tubes but it didn't help.
Any idea what could be a cause?
/Leevi

[Voxbox]

Hi Leevi,

Can you describe what "waves" means in another way?
 :wave: :wave: :wave:
Cheers

[plexi50]

Maybe you have a power tubes that is going bad. Volume drop and nasty distortion? Intermittant?

[Leevi]

Can you describe what "waves" means in another way?

The intensity of the noise is varying randomly

I'm thinking a possibility that the amp works as a radio receiver which
causes the noise?

/Leevi

[Voxbox]

What kind of noise is it?
Hiss, crackles, popping?

[Leevi]

It's more like hiss
/Leevi

[plexi50]

Is this what you could be hearing!

[Leevi]

Download and listen
Works at least with VLC player

http://www.kolumbus.fi/risto.kivioja/RikstoneAmps/PlexiHiss.MP4

/Leevi

[FYL]

Download and listen

Interesting. Looks and sounds a lot like the signal I got from PSR B0823+26 a while ago.

[plexi50]

This sounds exactly like a bad preamp tube. I have had this issue the past 2 weeks using some old tubes in one of my amps. Replaced the bad tube or tube socket and all should be quiet

[Leevi]

This sounds exactly like a bad preamp tube

That was also my first guess but after I changed the preamp tubes one by one
the problem was still there.

This makes it more difficult to troubleshoot:

I added today 1 Ohm resistors to the cathodes of the power tubes and started to test the amp.
First I listened the noise but after one minute it suddenly disappeared.
OK, I thought that maybe the cathode resistors have some relation to the problem and made another
test by removing the cathode resistors. I didn't hear the noise any more and now my problem is to
get it on again.

/Leevi

[Leevi]

I had a similar problem in another amp and I succeeded to found the cause for the problem.
There was a coupling cap (47n going to treble pot) which did't work properly.
It allowed DC go trough and had damaged the treble pot. This was the first time
I found a new Silver Mica cap behave like that.
/Leevi