Express build with intermittent noise

[rawnster]

I was giving the amp some serious playing time last night. Just a short time into it, and with it cranked above 10oclock, the amp started emitting some weird popping and crackling. The distortion was flubby and farty...if those are words. I hadn't heard this before. It sounded like something was shorting out or something. I pulled the chassis and checked voltages again. See below. I chopsticked for quite a while to search for any loose or bad solder connection. I couldn't find anything out of the ordinary. One thing I did notice was that I forgot to put back on the spur-type ground washer on the input plug when I installed the wood face plate. Perhaps, the input plug wasn't getting properly grounding, or so I thought. I played it for a bit before putting it back into the head cab. Sounded great. After getting it all back together, I played it some more with no ill effects. But then the popping/crackling returned. I thought that perhaps I have a back speaker. To test, I pulled out another 50 watt amp and cranked the crap out of it. No popping or crackling. So, the speaker is fine. I have no idea what going on here. Any advice would be greatly appreciated.

BTW: The noise doesn't kick in until I start to push the amp a bit. Low volume clean tones are great and with no added popping/crackling.

Thanks for your help!

b1=409
b2=395
b3=313
b4=294
b5=279

v1:
1=208
2=.6
3=1.8
4=3.3
5=
6=181
7=
8=1.4
9=3.3

v2:
1=262
2=7.0
3=3.0
4=3.3
5=
6=
7=
8=
9=3.3

v3:
1=192
2=25
3=38
4=3.3
5=
6=205
7=24
8=38
9=3.3

v4:
1=
2=3.3
3=408
4=392
5=34
6=397
7=3.3
8=

v5:
1=
2=3.3
3=408
4=393
5=34
6=398
7=3.3
8=

[rawnster]

Ok, excuse me if I'm slow, but it just dawned on me that I must have a cold solder joint. For, when I had the chassis away from the speaker, it sounded fine...no pops or clicks. When I placed the head back on top of the cab I got the noise again. The cab must be vibrating hard enough to force a bad connection to reveal itself. bummer.

[Wayne]

could be a bad connection between tube & socket as well.

W

[Buschman]

Get someone else to play while you chopstick. Also look really close at your socket wires for a stray single strand. I had this happen once and almost needed a magnifying glass to find it.

[jjman]

I once tossed a mullard 12ax7 since I thought it was causing this problem. Then it started up a few days later with another tube. It was the cathode to ground connection on the 6v6. Then I remembered that I forgot to take the garbage to the curb that week. After some garbage pickin' I found the mullard amongst some coffee grounds. Still using it.

[rawnster]

I've chopped sticked again for a while. Turned it up so I could hear any issue. The only thing I noticed was with the input plug. When I slightly move the "tip" portion, I get crackling similar to what I get when playing a guitar. It doesn't look wired incorrectly. hmmm.

[rawnster]

I re-floated one solder joint on the input plug. It looked suspect, I guess, but not heinous. Put it back together and played it for a bit with the chassis sitting on the cab. Sounded great; no snap crackle or pops. That couldn't have been the culprit. So obscure. Oh well. If the noise returns I'm tempted to rip the amp apart and rebuild the entire thing.

Question: There was some discussion about an Express's input plug and connecting to ground. A few people here ground the input plug to the pot grounding buss. I did not do this. It simply gets its ground via the chassis. Could what I'm hearing be a grounding problem of some sort?

Thanks!

[rooster]

I did continue the pot buss wire over to the input jack - too mich of a Fender guy, I suppose. I thought about this a long time - relatively - and I really don't know why KF didn't do this. He probably knew more than I do on the subject. Anyway, no issues here. With aluminum I really don't think there is too much chance for error, however it is done, ya know? YMMV, of course, and it still is something to think about.

It seems like you have run your trouble down, but I also think you might want to replace your shielded cable run to V1 off the input jack, or at least check it out. As someone else said, you may have a stray ground wire here? Lastly I would check out the Presence control hookup. Make sure things are good here, including the feedback wire run. Remember that guys like Alan Phillips suggest a separate ground path for this pot - separate from the pot buss wire. I do this, as well, maybe something to think about.

Loved the Mullard story. Been there, and it was also involving coffee grounds - and it never made the curb either. What's with amp guys and coffee anyway?

[jjman]

If it's clean and tight, and doesn't hum, I think your ground connection is probably ok as is. Many amps use the chassis for it's sole input-ground connection. I've used both in builds w/o problems. Sometimes it could be important however, to avoid hum.

[rawnster]

Yeah, the Mullard story is cool. I, too, love coffee. Living in the Northwest where we're considered coffee snobs, you gotta love coffee.

Thanks guys for the troubleshooting ideas and help.

Good point about the shielded wire from the input jack. I did as Buschman suggested and got out a big ole magnifying glass and looked it all over. My wife saw me doing this and just laughed. I'm sure I looked foolish. Whatever, we'll see if I let her play my amp, or "turn it down" the next time she quibbles. Anyway, didn't see anything inconspicuously touching something it shouldn't. Such a mystery.

[dehughes]

Still working? Did you rewire the input jacks yet?

[rawnster]

Still working? Did you rewire the input jacks yet?

Yes, and no noise so far.

[67plexi]

Last week I had this after 1 1/2 years on two Express builds
1. what I found I had used brass screws for my grounds
after time the brass corroded and made a poor contact black in color
I replaced all the brass screws with stainless steel with nylock nuts
and ran a jumper wire from the power tubes ground buss
Also I have switched to silver wire and solder.
No hiss or pops at full volume

Brass and aluminum chassis don't mix

[Structo]

The only thing I can add to this is something I read about before.
Might have been on the Aiken site.

But when you rely on the chassis for ground on jacks if the jack happens to come loose as they sometimes do, the ground contact will be sketchy.
They recommended having a wired ground to all jacks unless they need to be isolated.

[rawnster]

I ended up running a ground wire from the input jack to the ground buss wire. And, while I was at it, I completely rebuilt the preamp board with new parts, and rewired all of the preamp sockets. The only noise I get now is hiss. Thinking about replacing the resistors for the first to gain stages with metal film to see if this will quieten it up a bit more. Who knows.....