Combo crackles inside cabinet but not outside?

[jamesmafyew]

Bizarre issue. I have an Ampeg VT-120 I've posted about before, which I've taken on as a pet project. I like the amp a LOT where it is now.

It's had this constant but irregular crackle in the background since I got it. I didn't notice it much at first because it had so many other issues, but it's still there. I've replaced every electrolytic cap in the amp, every plate resistor, every coupling cap, every power supply dropping resistor (for other reasons, not for this problem), and the standby and power switches -- so I know it's not those. Swapping tubes does nothing. I can't isolate it to a particular tube stage, it's like it's coming from everywhere in the preamp. The only way to make it stop is to take out all the preamp tubes except the phase inverter. Even if the last tube before the phase inverter is out, if there are other preamp tubes before that still in, it will make the noise (albeit at a lower volume level).

I can turn up the crackle with either the gain controls OR the reverb control (even with no pan connected). The only thing that all these areas have in common is their ground path -- the bottom of the reverb pot, the gain controls, and the input circuit are all grounded to the neg terminal of first stage's filter and are AC-coupled to the chassis through C55 (which I've tried removing and altering the value, no change). The other parts of the reverb recovery stage are grounded elsewhere. Just the reverb pot is grounded to the input stage. I tried moving it but the amp hummed.

The WEIRD thing is -- it only makes this noise when the chassis is in the combo cabinet, RIGHT SIDE UP. It does not make the noise when the combo is turned on its side, does not make the noise when the chassis is right side up OR upside down OUTSIDE of the combo cab. ONLY inside the combo cab, right side up. There is no shielding or metal of any kind in the cab, just wood and tolex.

I think I'm going insane. Any ideas? I've attached an audio file of the noise recorded from the FX send.
vt120-noise.mp3

[madkatb]

Just a suggestion but try cleaning/retensioning the preamp tube sockets. Vibration can do some weird things.

[jamesmafyew]

Yeah you're right -- and I should have mentioned that I've already done that. I used DeoxIt on all sockets in the amp, and retensioned all of them too. This crackle happens with no signal applied, nothing plugged in the front of the amp. I've looked for arc traces on the preamp sockets, but can't see any. The sockets are black, though.

[g1]

Have you tried a complete resolder? A shaky connection (or several) can create issues that appear only when the chassis is oriented a certain way, also only when in the cab., with the extra vibration from being close to the speaker.
Shotgunning all the solder connections is often easier and more reliable than trying to locate the exact bad joint, when other connections may need resoldering soon anyway.
Connectors or ribbon cables could also cause similar symptoms.

[jamesmafyew]

Yeah I did a complete re-solder very early on, which fixed a few other issues it had. I replaced one of the four ribbon cables because it was torn when I got the amp, haven't done anything with the other three.

EDIT:
I just got it to do the crackle out of the combo cabinet, so it's not that... it was right side up when it did it. Still won't do it when it's in the cabinet on it's side.

[Enzo]

Make sure the mounting hardware for the transformer is tight. ANy motion between bolt and chassis can cause noises... really. Make sure all pot and jack nuts are snug too.

ANy other chassis related screws - snug them up.

[jamesmafyew]

Funny that you should say that... the power transformer screws are actually epoxied to the chassis, presumably because they were loose or something and they were stripped out (?). I don't know. There's epoxy all around where the PT touches the chassis, and no perceivable movement there when I push on it.

[Enzo]

Hey, I'm a funny guy.

[olddawg]

Funny that you should say that... the power transformer screws are actually epoxied to the chassis, presumably because they were loose or something and they were stripped out (?). I don't know. There's epoxy all around where the PT touches the chassis, and no perceivable movement there when I push on it.

!!!! Get a dremmel tool, grind that stuff off, sandpaper where the PT and the chassis make contact, and replace the screws with slightly bigger ones or drill out the holes and use bolts, washers, and lock washers.

[Enzo]

I usually recommend balling up your fist and whacking it hard. God's own rubber mallet. But with the chassis out, I think an actual rubber mallet would be best. Get one, or a hunk of 2x4, and whack the end of teh chassis while it is running. Does the whacking trigger the noise?

[jamesmafyew]

I banged on it all afternoon and couldn't seem to make anything happen. Could be I wasn't banging hard enough. I just took the PT off in the last hour and wow there was a lot of junk under there. Paper scraps, not-fully-cured-poorly-mixed epoxy, glue, a whole mess. Looks like it was vibrating or something and a previous owner decided to try and pad it? This one's inscrutable. Could this be causing my issue?

[jamesmafyew]

Update:
I cleaned off all the gunk under the PT and bolted it down properly. No change to the noise. I think I might be getting even more crazy. The amp makes the noise in V3 (reverb) and V1 and V2 when it's right side up, ONLY in V3 when it's on its right side, and ONLY in V1 and V2 when it's on its left side. I'm thinking maybe this is a shielding issue. The top of the chassis (where it's bolted to the combo cab) is unshielded. I wonder if shielding that would make a difference...

But why are the other dozen or so amps in the room not picking up this interference, if that's what it is? gahhhhh

[olddawg]

Have you tried operating the amp in a different location? Like someone else's house? Does where your at have an outlet that's grounded to a cold water pipe? Are you using any triad light dimmers? Is anything arcing near you? 15kv towers behind your house? Do you have access to a scope? Can you see it in the signal path?

[jamesmafyew]

No I haven't used it anywhere else. The wiring in all the buildings around here is mostly very old, but this place had all the wiring re-done about 3 years ago. I did the water pipe ground myself. No dimmers, but some CFLs. I have no idea if anything is arcing nearby, but I do know the line voltage varies on a daily basis from 114 to about 125... not unusual though.

I use a software scope hooked up to a lineout on my dummy load -- I can see it there, at the speaker outputs, and I can see it at the line out from V3, and when I use the cap/probe/battery amp setup I can see it all over the preamp. I suppose I could rig up something to look at the a/c lines and see if it's coming from there.

Edit: were you suggesting the cold water pipe ground could be an issue? Because I do have that.

[Chuck H]

Seemingly ignored based on the responses.

This crackle happens with no signal applied, nothing plugged in the front of the amp.

Not that I have an idea yet but I thought this detail was significant since in this light there's no need to suspect that speaker operation is vibrating loose parts, components or tubes.

I'm guessing the amp still crackles worse when in the cabinet? If yes then try placing some kind of temporary shield on the chassis where the cabinet shield would be if the amp were installed in the cab. And does this make the crackle like it is when the amp is in the cabinet??? Also, I'm thinking about the proximity of the reverb pan and it's cables. I know you said it does this with the reverb unplugged, but is that with the cable removed from the pan or the amp?