Quad + 20/20 = groundloop or hum?

[Kupke]

Hi,

I've been building up my rack from scratch since I want to get rid of the annoying hum residing in it.

Atm the moment i'm running just the Quad with a 20/20 into an American Audio power conditioner. The Quad is controlled by a Rockman Midi Octopus.

The hum is huge when I put my 20/20 in ground mode, so I put it on lifted. Then there's still quite an annoying hum when i turn my volume on the guitar halfway down. (I can get rid of it mostly with my noise gate, but that's not really solving the problem).

So on another forum it's been suggested that I shortcircuit a cable in the amp-switch jacks of the Quad, so I can see whether the Midi Octopus is causing the hum. Another one would be to cut the ground from one end of the signal cable running from the Quad to the poweramp, to see if that is causing it. (I'm not really an expert when it comes to subjects like current and groundloops, so bare with me)

I'm basically looking for more suggestions asap, since I want to get this rack working by friday evening. I presume someone must have the same experiences with this combination in a rack.

BTW, I've dismounted both rack units (not the powerconditioner) from the rack to see if it was a rack induced hum. That doesn't seem to be the problem.

Any help is welcome!

Thx,

Ralf

 

[Kupke]

as a follow-up: I've been running some tests, and the major hum seems to be coming from the Quad itself. When i run a Pod XT Pro into the 20/20, there's barely any hum. The only hum I get is when i'm not muting the strings.

When i mute the strings with the Quad connected to the 20/20, it basically causes the same hum.

I also tried shortcutting the channel selection jacks, which resulted in the same hum, so the problem isn't the midi switcher either.

So if it really is the Quad, how would I go about fixing it? Is this a transformer problem i'm having?

 

[disassembled]

So you have tried hooking the quad directly to the 20/20? Maybe the quad has a bad preamp tube.

The quads usually have a low level hum to them but it isn't as drastic as what you're describing.

Does the reverb level affect the hum? Does the amp hum without a guitar plugged in? Have you tried different cables? Different guitars? Is a guitar cable laying on or near a power strip? Are there electrical intereference items in the vicinity (like tvs, fridges, microwaves, etc)? Are you standing too close to the quad?

Assuming the problem is ONLY with the quad then I suggest taking as much out of the signal chain that you can (like pedals, long cables, rack cases, rack effects, wireless stuff, channel switching equipment, etc) and only work with the bare minimum until you isolate it to maybe a single tube or maybe even a single channel. Do you have a mixer or headphone amp that you can plug the quad straight into?


Start with the easy stuff like unplugging unecessary gear, then work your way into inspecting certain areas (like 1 preamp tube at a time) and then move on if you deem it to be good.

Greg

 

[Kupke]

So you have tried hooking the quad directly to the 20/20? Maybe the quad has a bad preamp tube.

First thing I did. Out of the rack chassis. A tube could be the problem. Would turning the volume control on a guitar have an effect on how a tube behaves though? If the guitar volume know is anywhere in between 0 and 10 scale wise, the hum would be loudest at 5.

The quads usually have a low level hum to them but it isn't as drastic as what you're describing.

Does the reverb level affect the hum?

Reverb is turned off totally. When i turn it up, it doesn't have an added effect on the hum.

Does the amp hum without a guitar plugged in? Have you tried different cables?

When there's no guitar plugged in you hear the hum you always hear when you don't have anything plugged in. You can't really distinguish the other hum in the sound.

I've tried numerous cables in between both the Quad and the 20/20 as well as different speaker cables between my 20/20 and my cab. I've tried both grounded and ungrounded cables. Hum persists.

Different guitars?

Yes, same effect.

Is a guitar cable laying on or near a power strip? Are there electrical intereference items in the vicinity (like tvs, fridges, microwaves, etc)? Are you standing too close to the quad?

I've waved around both units while connected, power cables don't seem to have any influence on the hum. There is no other powered equipment nearby the units.

Assuming the problem is ONLY with the quad then I suggest taking as much out of the signal chain that you can (like pedals, long cables, rack cases, rack effects, wireless stuff, channel switching equipment, etc) and only work with the bare minimum until you isolate it to maybe a single tube or maybe even a single channel. Do you have a mixer or headphone amp that you can plug the quad straight into?

This is something I could still give a try. I can pretty much guarantee that the hum is only very present on channels Lead 1 and Lead 2 though. So it's got at least something to do with gain. The rhythm channels don't have the hum in it, or at least not as apparent.

Start with the easy stuff like unplugging unecessary gear, then work your way into inspecting certain areas (like 1 preamp tube at a time) and then move on if you deem it to be good.

Greg

How would I go about investigating tubes?

 

[spikenet]

you have the exact same setup as mine quad->20/20 with rockman octopus

I had a hum issue with the quad and was traced the the reverb tank leads had come loose. Also, make sure that the tank has the connectors on the right way as that can create lots of hum..

good luck!

 

[Kupke]

do you have any suggestions as to how i go about checking the reverb tank for hum? If the leads have come loose, would it be visible if i open up the Quad?

Thx,

Ralf

 

[spikenet]

do you have any suggestions as to how i go about checking the reverb tank for hum? If the leads have come loose, would it be visible if i open up the Quad?

Thx,

Ralf

yes it will be obvious. make sure you safely power it down to avoid and risks!

while you have it open check that all the tubes are seated to.