Really weird noises from amp

  • Thread starter Thread starter EnGl
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EnGl

EnGl

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Here we go :

Today I biased my Peavey 6534 a little hotter , about 34 mA (Plate Voltage about 480-500). After all procedure I turned on amp and I played about an half hour. Everything was fine , no problems so far.

After 3-4 hours I wanted to play again. So I turned the power ON , I waited for few minutes and I hit the STANDBY ON button and bang ... Amp maked some reaaaaaaaaaally weird noises , like this :
https://www.youtube.com/watch?v=PH54cp2ggFk
But it was more aggressive with more emphasize on lower frequencies. After about 30 seconds - 1 minute this strange noise disappeared , everything was normal again. I was bit scared so I turned off amp and I turned it on again after about half hour - the same situation , weird noises appeared again and after minute ... BANG ! They are gone ! And I turned amp again off and on few times - again weird noises ...

IMPORTANT: This weird noises persist when guitar is not plugged. Only amp + cab. When I plug guitar - without changes.

My first thought - power tubes are gonna die soon or maybe preamp ? Strangest thing is that this noises dissapear after about minute when I turn on the standby.

Anyone can help ?

EDIT : few minutes ago I turned on amp , again , weird noises and few momments later - dead quiet and then I slight tapped the amp and the noises returned.
 
I'm not an amp tech but here are a few things I would try:

Make sure all the tubes are seated down real good.
Spray Contract Cleaner, like Dioxit, on the tube pins and work them in and out
Check for a micorphonic preamp tube (see below)
Bumping the amp effects this noise so maybe you have a cold solder joint or something is loose.


 
When I wiggled speaker jack in the amp connector this noises stops. So it can be faulty cable ? I dismantled amp and I looked on the PCB and all of the solders was looking good , and other parts don't looked burned or blackouted.

I messed a bit with this speaker cable and ouputs and everything is ok for this moment. When I turn on my amp I can't hear any suspucious sound , only sweet high gain white noise :)

Thanks man for help , when I will ever have any thoughts or when I will suspect any other noises I will try this few things :)

For now I will wait , maybe someone else will add few words.
 
my question is, how did you get the plate voltage reading? if you took a tube out of it's socket did you clean the socket? at bare minimum run the tube in and out of the socket a few times?

a microphonic preamp tube is like a squeal not a white noise so that's not going to be your issue. if you didn't use new tubes with a hotter bias i ask, how old are your tubes? maybe they're on their last leg and ready to drop out?
 
It's not the cable. I borrowed the other speaker cable from my buddy and nothing changed.

Peavey 6505 series have plate voltage in 480-500 range. I didn't measure it , I just rely on this readings , I don't wanted to bias amp too hot , 60-65 % plate dissipation is perfect spot for me so 34 mA is in this range whatever with 480 or 500 PV.

Also I just pull out the tube , put the measurement adapter with tube ( like this : http://www.compu-bias.com/images/Meter/ ... robe_m.JPG) into the socket and I checked the bias - that's all.

I will try cleaning pins and sockets , maybe this can help.

I also checked all preamp tubes - everything is fine.

My amp has RUBY EL34BHT for about more than year , they were inside when I bought amp in last year in may. But I didn't gigged with overbearing volume , I used my amp only in home with the volume about 1/10 so ... Tubes weren't hard abused.
 
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