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Hum increases with 3-prong cord

Started by Saransk, December 03, 2021, 11:24:59 PM

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Saransk

It may be coincidental but a couple of the restored guitar amps I've done seemed to have an issue with running a 3-wire AC cord with ground and removing the polarity switch circuit.
The latest is a Gibson GSS100 head that has a more pronounced hum after a cord and capacitor replacement.
I've had the same issue with a Vox Berkeley 3 head and a Fender 1st generation solid state Deluxe Reverb
The amps have the same type of transformer driven "totem pole" output circuit. Could it be that this type has an issue with the hard chassis ground with the 3-wire AC cord?

A very frustrating issue as I haven't added/removed any of the circuit grounds, the inter-board wiring was the same.
Thanks
Michael

Enzo

The nature of the power amp output stages is not likely the issue.  When you added the third wire ground, exactly WHERE did you attach it to chassis?

And for that matter, is the hum 60Hz or 120Hz?  (Assuming you are in the USA or Canada.  50Hz or 100Hz in 240v land)

Saransk

Usually attached to its own ground point, or to a lug on the power transformer
I'm sure it's 60 cycle

Michael

Enzo

Usually is fine in general, but how about on the GSS100?

phatt

Seems strange it's happening to many Amps?
Some other possible things to check
1/
Check you have continuity from power cord Earth to the chassis.
(maybe get your electrician to check house Earth)
2/
Try other wall outlets

3/ Are you using the same guitar? & Is it single coil? (as they are prone to hum)

4/
If using the one guitar, have you checked it is all grounded correctly?

5/ Are you running the guitar through a pedal board setup then into amp?
If so plug direct to Amp,, if it removes hum then you may have psu issues in the pedal board supply.
(you can unknowingly create a ground loop with some pedal psu types)

Many guitar amps do tend to have some background hum and buzz as they are all unbalanced setups.
Phil.

Saransk

I understand about the idea that a lot of things can cause hum but I just was wondering sing the whole idea of the polarity switch and the "death cap" was to ground the side that produced the least amount of hum.
Wiring an older amp with a new 3-wire cord, and using the standard wiring code, it might not connect the amp hot & neutral up to the side that hummed the least.

I've never tried swapping the black & white wires to see if that changes anything.
I also can't tell what, if any, effect that the use of a driver transformer has on the overall rejection of hum.
It is the Epiphone that floors me.  When I got it, it was working, sort of, but there was no real appreciable hum.
The whole front panel and chassis seemed to be the ground plane.  Every ground was haphazardly connected to the nearest point, i.e. Cathode resistors to the back of a volume control, Pre-amp B+ capacitor and cathode resistor grounded together, etc.  Just about every "bad" layout connection you could name.
Added the 3-wire grounded AC line and there is now hum in the system.

It is next on my schedule so I will take another look at the wiring.
Michael

Enzo

I don't offhand know what is happening here, but be aware that the death cap and switch and related stuff ONLY work to suppress hum from that ONE source.  So if there is a list of twenty sources of hum in your amp, that only works on one of them.

mandu

If you reverse the phase and neutral, does the hum goes away? If it works, you still have to rewire the on/off switch correctly that it disconnects the live end for continued safety.
Regards.

phatt

Quote from: Saransk on December 19, 2021, 02:07:29 PM
I understand about the idea that a lot of things can cause hum but I just was wondering sing the whole idea of the polarity switch and the "death cap" was to ground the side that produced the least amount of hum.
Wiring an older amp with a new 3-wire cord, and using the standard wiring code, it might not connect the amp hot & neutral up to the side that hummed the least.

I've never tried swapping the black & white wires to see if that changes anything.
I also can't tell what, if any, effect that the use of a driver transformer has on the overall rejection of hum.
It is the Epiphone that floors me.  When I got it, it was working, sort of, but there was no real appreciable hum.
The whole front panel and chassis seemed to be the ground plane.  Every ground was haphazardly connected to the nearest point, i.e. Cathode resistors to the back of a volume control, Pre-amp B+ capacitor and cathode resistor grounded together, etc.  Just about every "bad" layout connection you could name.
Added the 3-wire grounded AC line and there is now hum in the system.

It is next on my schedule so I will take another look at the wiring.
Michael

So far you have mentioned;
A Gibson 100 head, a Vox, a fender,,,and now it's an Epiphone.
Can you please just focus on one Amplifier as it's just not feasible for the members to keep up with constantly changing units.

If you have a hum in many many Amplifiers then it's highly likely something else is causing issues. :-X

Working on amps *Outside of the Chassis* can lead to induced hum from Fluro lights, Computer Equipment and many other sources of RFI/EMI.

So do you have ANY amplifier that does not hum?
Phil.

joecool85

Quote from: phatt on December 23, 2021, 09:21:42 AM
Quote from: Saransk on December 19, 2021, 02:07:29 PM
I understand about the idea that a lot of things can cause hum but I just was wondering sing the whole idea of the polarity switch and the "death cap" was to ground the side that produced the least amount of hum.
Wiring an older amp with a new 3-wire cord, and using the standard wiring code, it might not connect the amp hot & neutral up to the side that hummed the least.

I've never tried swapping the black & white wires to see if that changes anything.
I also can't tell what, if any, effect that the use of a driver transformer has on the overall rejection of hum.
It is the Epiphone that floors me.  When I got it, it was working, sort of, but there was no real appreciable hum.
The whole front panel and chassis seemed to be the ground plane.  Every ground was haphazardly connected to the nearest point, i.e. Cathode resistors to the back of a volume control, Pre-amp B+ capacitor and cathode resistor grounded together, etc.  Just about every "bad" layout connection you could name.
Added the 3-wire grounded AC line and there is now hum in the system.

It is next on my schedule so I will take another look at the wiring.
Michael

So far you have mentioned;
A Gibson 100 head, a Vox, a fender,,,and now it's an Epiphone.
Can you please just focus on one Amplifier as it's just not feasible for the members to keep up with constantly changing units.

If you have a hum in many many Amplifiers then it's highly likely something else is causing issues. :-X

Working on amps *Outside of the Chassis* can lead to induced hum from Fluro lights, Computer Equipment and many other sources of RFI/EMI.

So do you have ANY amplifier that does not hum?
Phil.

Agreed 100%.  My brother used to live in a house that no matter what I did my amp would make a ton of noise.  Never did get to find out if it was interference from something in the house or if the power was "dirty" - he moved.  Fluorescent lights can cause issue and inexpensive LED lights can as well, especially high power ones like the outdoor flood style lamps.
Life is what you make it.
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