Strange static noise

[aro]

I have a JCM 800 2204 clone which worked fine. A few days ago I made a few mods to it, basically giving it more gain. After that I played it for a total of about 3 hours in two different locations. I cranked it up to 9 through an attenuator and the tone was absolutely great.

Yesterday night I went to a third location with it, cranked it up the same way through the same setup, and right from the beginning the problems started. I would play an open A and the chord would face away real quick, and instead there would be a strong, constant kind of static noise. I would wait a little, the static would go away, play an A, and again, A would fade away and be replaced by static.

At some point the static would not go away at all anymore. Even with both master volume and gain volume turned to 0, noise would be still there. I turned it off and on, tried both Hi and Lo input, tried without an attenuator, and always the same problem. I gave up on it eventually.

This morning at home I opened it up. Visually everything looks good. I plugged it in through the same setup, cranked it up to 9 and played it for an hour without any noise at all. I turned it off and on, Hi and Lo, and no noise. Everything is just great... :confused1: Does anyone have a guess as to what in the world could have caused that?

[simonallaway]

This sounds like the problem I had with my P1 the other day, which was a dry joint on the preamp stage cathode.
The effect was like a noise gate kicking in on a sustained note, but all that was left was crackly-hiss. Playing a note again would make it come back. And much like yourself it would come and go. The forum here suggested it might be oscillation and that I should make sure the grid resistor on the output stage was right on the socket (which I did do), but then the problem went away....until it came back permanently ;)

So you might try re-flowing solder joints around where you made your mods. I ended up removing as much solder as I could, and completely redoing it. Or even chop-sticking might make it come back, which is a useful test.

[sluckey]

Location is everything.   

Don't worry about it unless you can get it to repeat at different locations.

[aro]

Location is everything.   

The truth is that it was a seedier location, so I guess you're right

I'll chop-stick my new solder joints for good measure.

[aro]

I finally got to play the amp at the same location and the noise was there again

Amp cranked up through an attenuator set to very low volume was perfectly fine at home for hours.

At this other place the amp cranked up through an attenuator that was also cranked up created noise. Since the attenuator level was the only setting that was different I thought it might be the speakers, but it didn't seem like it. Eventually I resorted to an old sophisticated debugging technique: frustrated, I slammed my fist on the amp head cabinet. And...noise disappeared... I started sparring with the cab and it was definitely reacting to my devastating blows. (BTW, the amp head was not on top of the speaker cabinet, it was on the floor next to it).

After I delivered a few good punches the amp threw in the towel and it started behaving exemplary. This process repeated about five times at short intervals, all in the beginning - static noise, then fists, then nice and quiet. Then after a few minutes, again, noise, fists, quiet. I ended up playing it for hours without any issues anymore after the initial fights. I kept punching it so that I can see if I can induce the noise, but nothing, no more noise.

I'm thinking that it must be one of solder joints from when I modded it, but then I probed them all with the amp on and there was no reaction. And why didn't it get noisy when I was punching it later on...? If punches remove noise, shouldn't they also induce noise?

Or maybe it was something that wasn't working in the beginning, before the amp warmed up properly...

The other thing, most likely unrelated, is that after the mods, which are designed to give it more gain, it sounds harsh, like a pedal that's clipping. I don't know if that's by design, or if it's a bug that I introduced.

[Leevi]

Remove the chassis from the cabinet, switch the amp on and wait until the noise comes back.
Take a chopstick and touch the circuit in different points and try to localize the problem in that way.
/Leevi

[Ritchie200]

Aro,

You may want to check your sockets first.  Make sure they are clean and tight.  Wiggle your tubes around with some cleaner dabbed onto the pins.  There could be some leftover forming lube on the sockets from when they were stamped out.  Location, humidity, bumping around in the back of your car, etc...  Do the easy stuff first!

Good luck!
Jim

[aro]

Remove the chassis from the cabinet, switch the amp on and wait until the noise comes back.
Take a chopstick and touch the circuit in different points and try to localize the problem in that way.
/Leevi

I did all that already.

[aro]

You may want to check your sockets first.  Make sure they are clean and tight.  Wiggle your tubes around with some cleaner dabbed onto the pins.  There could be some leftover forming lube on the sockets from when they were stamped out.  Location, humidity, bumping around in the back of your car, etc...  Do the easy stuff first!

Good ideas. I'll clean the sockets, and I'll take it to work in the trunk, just like yesterday.

[aro]

Well...I did a lot of testing, but I'm not sure if I have gotten any further. Maybe I have...

I took the amp for a ride to work and then played it at home. There was maybe 1% of the static I was getting at that other place. Both in duration and volume. I was able to get this noise only by slamming my fist on the top of the amp. Every time I would do that there would be some faint, but audible static together with the note, and then it would go away. I took the chassis out and probed lots of joints, but nothing caused noise. I rocked and shook the chassis with both hands while someone else was playing the guitar, but no noise. After a while it just didn't make any noise anymore.

I put it in the cold over night, then I cleaned all the sockets and played it again. Same thing, no noise, unless I hit it, and even then very little. Again, after playing it for a while I couldn't make it noisy, no matter how much I hit it.

In conclusion it looks like when it's warmed up there is no static. This conclusion might be wrong, but that's all I can say right now.

Happy New Year to all, and may all our amps be healthy in the new year 

[Willabe]

If someone hit me I'd make noise too!     


        Brad