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When I plug in the jack with the 10k resistor, its quiet. When I plug in a cable, with or without a guitar, its noisy. What is going on here?

Phil, with a tremendous amount of respect, I dont think its the guitars. I tried 3 different guitars all having humbuckers, which dont make noise on my other amps. They all make noise on the 18.

The buzz only happens when you plug a guitar in, and it reduces with the position of your guitars' controls. So it looks pretty definite that it's something being picked up by the guitars. If the amp doesn't buzz when you plug in the jack with only a 10k resistor (to simulate the resistance of a pickup) across, there's nothing wrong with your amp.

There are possibly some other reasons for why you haven't found this with your other amps, such as the level of background electrical noise at the time you were testing the various amps.

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Ok, so are you saying I just need to live with it?

Or better shielded cables and line or paint with conductive paint your control cavities
These are what I use when building http://www.stewmac.com/shop/Electronics ... lding.html

How loud is this hum?

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Fantasy Noise is what my shop of dreams is called and was the only way to beat my addiction, like many here I suffer from G.A.S.


Tele's make the best noise

Personally, I wouldn't, as I hate background noise in any amp. So I would start by shielding the guitars. Humbucker pickups reduce hum but don't prevent buzz getting in. Of all of my guitars, the one which is the worst for picking up buzz, has H/Bs, but no cavity shielding. I prefer to use copper foil rather than paint. Good shielding instructions are on http://www.guitarnuts.com/wiring/menu.php There is one possible amp mod that *might* also help, which is to connect a 0.1uF cap at each channel's pair of input jacks. Connect the cap between the jacks' ground connection and a point on the chassis right close by to the jacks. If buzz is being picked up by the shield of the guitar cord, the caps will run that buzz straight to ground by the shortest path, rather than letting it radiate around inside the chassis.

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Before modifying any guitars, the two places I would look at first would be the RG174 runs (which can short out very easily when soldered/resoldered), and the underside jumpers (which can occasionally drop off while reworking the components on the top).

Also. is the amp sitting on a bench or is it back in it's cab? Anything sitting next to it or around it that would induce hum into it? Florescent lighting, light dimmers, CRT's, etc. ? Some pictures would be helpful...

Joe G

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good point on the external noise. I often lay a sheet of metal over an exposed amp to test for noise. Or put in the cab with shielding on the bottom.

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Those are the same things I would immediately look for. However, the simple tests that were carried out and discussed a few posts back prove that the buzz is being picked up in the guitar and *not* at the amp. The amp itself is quiet.

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Why is it necessary to have the cap there to eliminate that buzz? Why doesnt the existing ground do that? Im just curious what the cap does.

If I put those .1uf caps in, do I then remove the existing ground that is next to the input jacks on the regular channel? Im wondering if having the existing ground and the caps going to ground will create a loop?

I never said that it's necessary, just that it *might* help. I don't recall the exact points where your amp grounds connect to the chassis. The function of the cap is to short any buzz riding on the shield of the guitar cord directly to ground, by the *shortest* possible route. If there is a ground connection to the chassis very close to the input jacks, then this cap isn't required. In that case however, as you have two sets of input jacks, it might still help for the input jack pair of the channel that doesn't have the adjacent ground connection.


No, because it only shorts the higher frequencies to ground and not the 60Hz. FYI, this trick is recommended by Randall Aiken, who is one of the best amp design engineers out there. However, as I tend to think the noise is being picked up at the guitar end, this trick may not provide any benefit at all in your case.

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Jon, grab some adapters and try plugging an iPod or a cd player or something into the amp, set the amp to your typical playing volume, bring up the volume on the ipod to your "normal" playing level. Buzz still there?

Joe G

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Hi Joe;
I plugged the ipod in, and very little noise and no squeal. I just dont understand how the guitars could be picking up that much noise. Especially when my Tweed Champ clone doesnt do the same thing when its dimed. ]

Any Ideas?

Tweed Champs have a very limited frequency response due to the low-fi OT they use and also the negative feedback. The Marshall 18W design tends to be quite bright and toppy, probably due to the EL84s as well as the hifi-ish OT. So your Champ is killing those frequencies that contribute to buzz and pickup squeal, which the 18W is actually emphasizing.

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Ok, that makes sense. But how come no noise with the ipod?


Is the only think to do shield the guitars?

A guitar is basically an antenna for picking up noise. An iPod isn't. Nor is a 10k resistor soldered directly onto a 1/4" jack. I think shielding your guitar will be the most effective cure. Also potting pickups to prevent squeal, if they haven't been already.

And it's generally a good idea to shield guitars anyway.

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Not trying to second guess you Phil, just throwing ideas out. It's just that I've never had a guitar shielding issue so bad that it couldn't be resolved by just moving the guitar away from the amp, computer, etc. The worst are those faux Strats that my students come in with, and even they don't buzz that badly
It just seems like a bad cable would be a more likely suspect or the inputs are wired inside-out (signal on the shield and visa/versa), boost stuck on, something simple.

Joe G

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I certainly have, and on a number of occasions. The problem is when the noise is coming in and being radiated through the AC supply of the actual room or building you're in. I've seen it happen in some houses, as well as on stage from the stage lighting dimmers. And you can't just then decide to leave the building or the stage.


+1 I would definitely recommend checking those.

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To go along with this thread is my new toy. I was in Winnipeg and dropped into Mothers Music.
They had this Squire Tele Custom with Duncan Designed P-90s. That and the blonde finish with maple neck and skunk stripe. Then I picked it up and never put it down. Light weight with a neck that fits perfect, don't care that its a squire, anything I don't like can be changed.
It has a major problem, any of my amps I plug it into, it turns into an antenna. The howel from that ax is stupid loud. To be playable, volume needs to be at 3 or lower. And this is on all my amps not just one. Turned off all cfl's in the house and it was a little quieter. I took a quick look last night and it has no shielding. That changes today. This is the first guitar I have ever owned that was this noisey. I prefer Fender style axes so I am used to single coil howel but have never heard it like this. Then again this is my first squire and one way to save money is to cut down on parts and labour.

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Fantasy Noise is what my shop of dreams is called and was the only way to beat my addiction, like many here I suffer from G.A.S.


Tele's make the best noise

That's what was leading me to think it may be something else, since JCR's humbucker-equipped guitars appear to be fine on other amps.
I've definitely experienced this $quire $hielding $hortcoming from my students' Faux Fender's

Joe G

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Its more accurate to say on my tube amps and each one reacts differently. Nosiest on the tweed, least but still objectionable is my Peavey Windsor Studio. Although any of the small solid states are much quieter, which sort of makes sense as they are sold to be used with cheap entry level.
But that faux Fender feels so good and sheilding isn't tough. The best part is I really like the sound of the pups, so new strings and sheilding under way.

_________________
Fantasy Noise is what my shop of dreams is called and was the only way to beat my addiction, like many here I suffer from G.A.S.


Tele's make the best noise