JCM 800 help?

[nipsey_russel]

I've got a 50 watt JCM 800 1X12 combo, it's a single channel amp that had some mods done to it in the early 90s right after I bought it.

Anyway, at some point in the past 2 years, I noticed it starting to make this weird sound, like a zzzz, when I turn the volume up even to a reasonable level, particularly if it hit low, bassy notes and chords. I've taken the amp to a local guy several times, but he doesn't seem to have a clue what's causing the noise.

One thing I noticed today: there's a second gain stage mod, and when it's kicked on, via footswitch, it doesn't make the zzzz sound.

What'cha think guys? Any suggestions would be much appreciated!
(bearing in mind I'm not an amp builder myself, just a player)

Cheers,
Timmeh
http://www.angelfire.com/zine2/timmeh/

[objectivedays]

I have a 100watt JCM 800 head that is stock, but when I play bassy notes like palm muting, I also get some wierd noises. I'm not really sure I know what a "zzzzz" sounds like, but my is more of a crackle/static that fades out as the note trails off. Sorry I can't help with your problem, I can't even figure out what is causing my problem. Any ideas? Thanks.

[jaysg]

I've taken the amp to a local guy several times, but he doesn't seem to have a clue what's causing the noise.

One thing I noticed today: there's a second gain stage mod, and when it's kicked on, via footswitch, it doesn't make the zzzz sound.

What'cha think guys?

I think I should shut up and let a real tech like Richie answer, however, you may need to start with a different local guy. With clues like that, and hopefully a schematic from whoever did your mods, someone should be able to narrow in on the subcircuit in a few minutes. It sounds like a dying capacitor and/or cold solder joint, not a dying tube.

[Plexi]

Hmmm well, I'm with Jay..lol

""I've taken the amp to a local guy several times, but he doesn't seem to have a clue what's causing the noise. ""

If the tech had the chassis out and couldn't find it, it may be a vibration problem that is only when the amp is in the cab.

And since it seems to only happen when its in the other channel..
thats where i'd look for a loose connection..tube swap etc..

Since its only in the one channel or not in the mod,it helps narrow it down.
because,if it happened all the time..it could be preamp,or output tubes..to loose connections.dirty sockets..dirty jacks..to numerous things it could be.. even speaker..
Now you can have oscilations that can cause weird noises. But those usually show up when you crank the amp volume.. but the tech should be able to find or fix the problem. Or see whats causeing it. As many on here have seen, even the cab and handles,can cause buzzes..
Your clue of low bassy notes or chords at higher volumes..seem to lean toward a vibration problem. The certian fequency will cause it to vibrate more..like g note,or certian notes. Play them on the bass e string.till you see which ones effect it.
Which is most likely a loose connection, cold solder joint, or part that is vibrating at certain notes played.You also see this on input jacks..dirty contacts,certian notes make it vibrate,and cause it to make a static noise... You might take a wood dowel stick and do some board tappin to see if it shows up. hope this helps..

Richie

[nipsey_russel]

Your clue of low bassy notes or chords at higher volumes..seem to lean toward a vibration problem. The certian fequency will cause it to vibrate more..like g note,or certian notes. Play them on the bass e string.till you see which ones effect it.
Which is most likely a loose connection, cold solder joint, or part that is vibrating at certain notes played.You also see this on input jacks..dirty contacts,certian notes make it vibrate,and cause it to make a static noise... You might take a wood dowel stick and do some board tappin to see if it shows up. hope this helps..
Richie

Thanks Richie. F # definitely will induce the unpleasant sound, but only in the 1st "stock" preamp gain stage. When I switch to the high gain stage mod, it doesn't do it. Based on the history of this amp, plus what you and others have said, I'm thinking it's a part, like a cap, or a loose connection, etc. I've tried new tubes, and a new speaker, which had no effect. So I think I'm narrowing it down finally. 'preciate it!

Cheers,
Timmeh