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Home recording studio - noise with guitar amps - who to call?
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jonpaul:
Dave NJ Guitar pickups and hum buck are a big topic, not all are equal.

Suggest to test near a strong 60 Hz mains field eg near large power transformer or tape head demagnetizer to see the hum susceptibility.

Then us a battery powered amp to eliminate ground loops etc.


Jon
BrianHG:
Try a grounding wire between the bottom of the pickups and the square metal plate at the back of the entry to the neck of the guitar.
BrianHG:
Wouldn't it be funny if all this problem is due to the 'Hot' and 'Neutral' being wired backwards on DaveNJ's wall outlet.
(Also potentially dangerous depending on how some old tube amp and radio designs do have a degree of coupling between one of the AC outlet pins and their chassis GND, but this would explain a lot.  Some old AC chords don't even have a polarity on these old designs.  You would need to get your amp re-furbished by a pro like Mr. Carlson who knows how to correct for this stuff.)
magic:
Not sure what you mean?

In any decent equipment, the secondary of the transformer should be grounded to protective earth, and that's separate from neutral even in America, right? :wtf:
And if the amp is grounded to neutral, there's going to be mains AC on the guitar and everything. I think that would be hard not to notice pretty soon >:D

But it is indeed an interesting question: is the problem a field in the air picked up by the guitar, or ground bounce in the amp and the guitar "picks up" the static ambient field of earth which looks like noise with respect to the noisy "ground".
BrianHG:

--- Quote from: magic on November 22, 2022, 09:45:40 pm ---Not sure what you mean?

In any decent equipment, the secondary of the transformer should be grounded to protective earth, and that's separate from neutral even in America, right? :wtf:
And if the amp is grounded to neutral, there's going to be mains AC on the guitar and everything. I think that would be hard not to notice pretty soon >:D

But it is indeed an interesting question: is the problem a field in the air picked up by the guitar, or ground bounce in the amp and the guitar "picks up" the static ambient field of earth which looks like noise with respect to the noisy "ground".

--- End quote ---
In North America where the 120v outlets have one contact to the fuse box GND (called Neutral) and the other side connected to 1 phase of a 240v supply (called Hot) to give us 120v.  This ground is also Earthed to the ground with a spike as well as plumbing and ducting in a house.  Normally in consumer equipment, there should be a class Y cap connected to the Neutral line to the amp's/device's chassis GND.  Some older equipment from the ~1940's had it directly wired, no series cap.  Though, in older equipment, AC plugs used to have no polarity direction meaning you rotate them 180 degrees and still plug them in.  This means the 'Hot' 120v would have a 0.1uf to the amp's chassis.  This means the guitar's pickups should hum if the plug's polarity is backwards and the guitar is placed on the ground as the amp's GND may be it's 0v reference, but, the ground below would look like a 120v oscillating electric supply.  (Placing the guitar in that conductive blanket, as in his videos when you hear that blanket make electrical contact to 1 or more metal surfaces on the guitar, having the noise vanish, means everything inside would reference the frame GND and isolate the pickups from the surrounding true earth GND.)

If you are un-aware of the North American power system, and you didn't know you can rotate the AC plug around 180 degrees on older plug which which didn't have a thick bulge on one side to polarize them, then I would not expect you to know this and about the class Y cap tying the AC plug directly to it's chassis or GND.

Also as the standards have changed back in the ~1970s, some electricians doing work in that period may have wired the AC outlets backwards and we have some AC outlet testers which specifically have an LED to tell you if you have this problem with your outlet.  If the amp was serviced with a new polarized outlet, it may have also been wired backwards as some old designs had the AC power switch on the wrong line and the serviceman didn't want to go through the hassle of re-wiring the AC input stage to fix the problem properly as it entails some additional work as Mr. Carlson Lab's electronics vacuum tube radio/amp restorations have demonstrated this exact problem with shoddy third party restores.  (Mr. Carlson Lab's is here in Canada, so he pays particular attention to the AC power and power switch issue with his restorations for safety reasons...  Without the re-wiring, turning off the radio/amp may mean energizing the chassis of the radio with 120v.)
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