amp coupling to guitar

[Lindz]

I have recently built a bigger 100 watt amp (SLO style High gain) using beefy transformers (Magnetic components "project" PT with 3" core and matching output transformer) and find it is really prone to guitar hum when I am close to the amp with my guitars - I know this hum/proximity effect is normal with all amps but the level is much greater the 50 watt or smaller amps I have built with smaller transformers. I am using mostly humbucker guitars, recent Les Pauls mostly that have shielding typical of those guitars and decent cables so I do not think my guitars are the issue

I assume that the bigger iron results in a bigger magnetic field so in general it makes sense, however not having any regular big amps with big iron (by regular I mean not built by me) to compare to I cannot decide if it is just the way it is or something inherent in my build that makes it noisy.

So far this is just on my bench, not in a cab and not otherwise shielded but the difference is quite noticeable to all my other amps and is to the point where I thought I might have an issue with something I did with grounding or the input jack, and have spent a fair bit of time reworking it but all checks out ok.

6 or 8 feet away rotated the away from the amp its got a little buzz but nothing crazy. But standing right in front a foot away when adjusting controls etc its VERY loud and sounds like someone just rang out a 60/120hz note on a bass amp. Of course no one really stands that close when playing but I think its pretty loud.

Assuming this is somewhat normal due to the magnetic field effect, I am thinking I might try some more serious shielding in the head shell rather than just a bottom plate to see if I can reduce it - i.e. steel sheet or steel screen on the whole inside of the shell especially on the front of the amp in front of the transformers all grounded the to the chassis etc - sort of faraday cage idea.

I have read a bit about the idea here and there (Oconnor's books and elsewhere) but am curious if anyone has tried it on a noisy amp and whether it was helpful

If any of you have any experience with shielding like this, what sort of sheet thickness and or screen type would be suggested?

I have come across a few places that sell somewhat expensive shielding tapes and plates and wonder if there is any great advantage vs going to my local steel supplier and buying something there. I get decent discounts and free cutting because I buy lots for my day gig so it is pretty easy to get that but just thought I would ask first rather than just spend money needlessly if it will not really do much

http://www.lessemf.com/mag-shld.html

I also assume it would have to be steel vs aluminum to best shield from AC/magnetic fields?

Any input would be appreciated as usual

thanks!

[Blackburn]

Regarding the hum you hear when standing close to the amp with your guitar slung... That doesn't sound at all abnormal and it should be louder since you're dealing with a lot of output.

Can you clarify about the hum you're hearing when not standing close? Do you know the difference between tranny coupled 60c hum and preamp hum? The tranny coupling hum can't be cured by mods, strictly tranny placement/orientation. A good way to test is to pull tubes (Even a rect.) and hear if your speaker is passing hum. Preamp hum is another animal and shielding helps and so does lead dress. Shielded wire on grids, etc. Aluminum helps out a lot vs steel because it isn't magnetic. Steel can interact with and exacerbate coupling between trannies.

[Lindz]

Thanks for the quick reply

Amp does have shielded grids and other wires in sensitive places but I never thought to try pulling tubes to see how loud it is that way at least not since first fire up and at that point I was more worried about smoke or shorts etc not a bit of hum.

It is essentially the same as any other amp I have made where if you are up close with a guitar it hums only in this case it is quite a bit louder. Move away its less, turn the angle of the guitar less again. Move far enough away at the right angle its not that different than my other amps albeit since its higher gain it is a bit noisier - nothing unexpected though if I am 8 feet away sort of thing.

If I do decide that it is not preamp hum and more related to the pickups picking up whatever field from the amp are you suggesting that aluminum would be more a effective shielding material than steel? I would have thought that to reduce a magnetic field you would want the shield to pick it up and pass it to ground..? Or are you suggesting aluminum would make a better shield as it would simply not allow the field to radiate (as much)?

I can see how it would be possible to exacerbate the PT and OT coupling and in this case the choke is large and pretty close to the PT so that may also be an issue as well if it can couple too. Amp has more 120hz hum than my other builds but it is also way higher gain so that does not surprise me. the lower gain crunch channel is pretty quiet. I will pull tubes later and report

[Structo]

Not sure about the aluminum vs steel but I too notice on my amps in a small room I get noise (buzz/hum) when standing too close.

And as you noted, if you rotate your body a bit you can find the quiet spot.

I think that the higher the gain, the more noise you will get when close to the amp.

Of course single coils are worse in this regard.

But I tend to play humbucker guitars these days.

Try to experiment in a larger room where you can get 10 ft away or so and see if it is the same.

There is a possibility that you may have to rotate your OT a bit.

Sometimes an 1/8" is all it takes.

But I would try a metal shield first before doing surgery.

[bruce egnater]

If you determine it is just proximity hum from the transformer, you can effectively shield it with mu-metal. I have done this many times in the past on the TOL100 amps. These used toroid power transformer that were supposed to reduce the radiated field. Ha.... all they did was narrow the field into a sort of figure 8 pattern but if you got in that field with your guitar, it was horrible. I would make a box around the transformer with mu-metal sheet. Foil is ineffective. It must be heavier gauge sheet metal and you must seal the transformer inside the box. Mu-metal is expensive and doing this is a lot of work but in the end result was worth it. You must enclose the entire transformer including the bottom.

[Lindz]

Hey Bruce

thank you for taking the time to reply. Its cool that you are posting here.. FYI.

I hope you will continue to contribute. There are a few pretty smart folks here to say the least. Same with a few guys at SloClonefourms.com which might interest you if you choose to lurk or post there. Jed, Joey, Zintolo (Roberto here) and some others are pretty smart guys

I do tend to think it is proximity hum though perhaps exacerbated by less than stellar layout and grounding. I am still not much past a paint by numbers guy but I am learning fast.

Being kind of a geek with more time and curiosity than sense I thought I would experiment on this a bit...

I ordered a few things this morning with the idea of comparing a few different shielding techniques

1. An EMF meter

2. some thin steel plate

3. some .004" Nickel alloy shielding foil from LessEMF.com

4. some thin aluminum sheet

Too bad you did not comment even earlier today - from the sounds of your comments maybe the EMF foil I got may be less effective than the thicker plate material. I looked at the MuMetal sheet and a few other things online and extrapolating from the data sheets published I thought the foil I bought might just work if I wrap the trannys or perhaps the inside of the headshell and get the proximity hum to a level I am more comfortable with like my Marshall 50 watt

I bought enough to wrap things a few times in spite of the expense so I can double or triple up the wrap if I have to which should mimic thicker material

Using the meter I am going to measure a few amps from certain distances and angles, then shield them in various ways.. entire cabinet, just transformers, partial cabinet just on the front etc and measure again. Might make an interesting thread instead of the stupid questions I usually ask around here.

I terms of me here I am largely a noob - lots of amps under my belt but not much electronics knowledge yet> I lurk and suck up the knowledge rather than contribute much so far

That said I do make cool faceplates using my snowboard printing equipment as well as some nice cabs with my wood shop..

http://ampgarage.com/forum/viewtopic.ph ... faceplates

The only other useful contribution I think I made here was regarding a tweaked Paul Ruby "zener" type clamp on a hot rodded el84 amp with power scaling - got rid of buzz when way power scaled back using a modified zener/diode "clamp" when apparently that was problematic for people

http://ampgarage.com/forum/viewtopic.ph ... by+scaling

no brilliance on my part just ignorant tinkering but it works pretty well

[Cliff Schecht]

If you determine it is just proximity hum from the transformer, you can effectively shield it with mu-metal. I have done this many times in the past on the TOL100 amps. These used toroid power transformer that were supposed to reduce the radiated field. Ha.... all they did was narrow the field into a sort of figure 8 pattern but if you got in that field with your guitar, it was horrible. I would make a box around the transformer with mu-metal sheet. Foil is ineffective. It must be heavier gauge sheet metal and you must seal the transformer inside the box. Mu-metal is expensive and doing this is a lot of work but in the end result was worth it. You must enclose the entire transformer including the bottom.

One thing worth note when doing something like this is to NEVER run a bolt directly through the toroid that touches metal (i.e. ground) on both sides of the toroid. This forms a direct short to ground on the secondary side and will quite literally blow something up if the fuse doesn't go immediately. With a metal shield around the toroid, you want to insulate the top of the bolt from the shielding metal. I'm sure you know this Bruce but I can't necessarily say it's common knowledge!

I've used one cheap (Antek) toroid and really dislike how thin the protective insulation is. Just a few thin pieces of plastic wrap and microns thick wire insulation are all that are keeping you from harms way. With a back panel on a headshell you're fine but it's still something that irked me enough to not order more. The ol' orthogonally positioned EI core transformers work just fine IMO (and I just eyeball them ).

[Lindz]

Cliff, my amp is not using a Toroid, I think Bruce was just mentioning that
as an example of an amp he tried to quiet down using shielding

[bruce egnater]

Correct, just an example. I did use a stainless steel bolt and never had any issues. The mu shield was grounded with no adverse affects though?

[Structo]

http://www.amazon.com/gp/search/ref=sr_ ... industrial

[Lindz]

Hey Structo thanks for the link. I should have looked yesterday at more places as I dropped 4x that much on a meter from lessEMF.. oh well. STill not all that expensive.

I guess if I fail as an amp builder now I can at least become a ghost buster.......

ha ha

http://www.ghoststop.com/EMF-Meters-Gho ... ng-s/4.htm

[Structo]

Yep, seems like there is a second industry where these EMF meters are used.

Who you gonna call?

GHOST BUSTERS!

[Lindz]

Worst case is maybe I will get lucky and using the meter will chase the ghosts and gremlins out of my builds, and I will not have to spend so much time chopsticking - ha ha