Ground issue with power brake

[Nicolas S]

Hello,
yesterday evening i was playing on my jmp style build with a power brake that i bought recently .
I touched the power brake and was i have my hand on it the sound fade away a little bit and there's a hiss which appear ...

If someone have an idea on what could be wrong with my build, i think it's a grouding issue, i have a rail behind the pots and only one grounding point neat the filter caps .

 

[Pete Farrington]

Stuff like that generally doesn't happen when the open face of the chassis gets screened off.
I guess the amp chassis wasn't in a cab, or if it was, that the cab doesn't have proper electrical screening? eg foil lined floor.
And that you weren't also touching anything grounded (eg guitar strings) when you touched the power brake?
And that you, the power brake and the amp chassis were all in close proximity?

 

[playloud]

Could also be an issue with a jack? If you're touching it, perhaps the connection is coming a bit loose.

Edit: if you're concerned about the ground path specifically, you should check continuity between the sleeve of the input jack of the guitar cable and the sleeve of the output jack of the PB.

 

[paul-e-mann]

Hello,
yesterday evening i was playing on my jmp style build with a power brake that i bought recently .
I touched the power brake and was i have my hand on it the sound fade away a little bit and there's a hiss which appear ...

If someone have an idea on what could be wrong with my build, i think it's a grouding issue, i have a rail behind the pots and only one grounding point neat the filter caps .


View attachment 154678

How's it sound when you're standing 10 feet away? If using an attenuator you probably have your amp cranked, standing near any amp cranked is gonna cause interference, stay back at least a few feet.

 

[dtier]

My John H attenuator had a similar problem when I first used it. I grounded the box to the input jack and it corrected it. Can't explain why a low impedance circuit would be affected in such a way.

 

[2L man]

Strange phenomenons can come when Chassis is used "actively" for HV return current because 0V is also Signal Reference. Also Instrument cable standard is poor when its shield is used for SR.

Bias resistors seem to be connect to Chassis so this amp has at least two places where HV return current leak to chassis and return to PS thru the lug you mention. There is also wire in potentiometer cases which make amp more prone to EM noise.

Nowadays mains Safety Earth can have lots of EM noise when it run long length with Live and Neutral and for the safety SE must be wired to Chassis when it is metal. Thats why using Chassis for current flow has been forbid in electric regulations about 50 years.

If normal 6.3mm mono jack cable is used most likely Power Brake box has connection with SR. Your body bring "an antenna" there. Body can be receiving antenna and audio noise increase. Or additional mass which impedance lowers the audio noise.

 

[Nicolas S]

Stuff like that generally doesn't happen when the open face of the chassis gets screened off.
I guess the amp chassis wasn't in a cab, or if it was, that the cab doesn't have proper electrical screening? eg foil lined floor.
And that you weren't also touching anything grounded (eg guitar strings) when you touched the power brake?
And that you, the power brake and the amp chassis were all in close proximity?

Hi, the amp was in the cab but yes i don't have the metal sheet on the cover of the cab, i think i was just touching the Power brake at that moment . The power brake was just on top of the amp near the control panel . I tried the same thing with an other amp (Class 5) and the phenomenon didn't occured .

 

[Nicolas S]

Could also be an issue with a jack? If you're touching it, perhaps the connection is coming a bit loose.

Edit: if you're concerned about the ground path specifically, you should check continuity between the sleeve of the input jack of the guitar cable and the sleeve of the output jack of the PB.

I will check that

 

[Nicolas S]

How's it sound when you're standing 10 feet away? If using an attenuator you probably have your amp cranked, standing near any amp cranked is gonna cause interference, stay back at least a few feet.

I was just in front

 

[Nicolas S]

Strange phenomenons can come when Chassis is used "actively" for HV return current because 0V is also Signal Reference. Also Instrument cable standard is poor when its shield is used for SR.

Bias resistors seem to be connect to Chassis so this amp has at least two places where HV return current leak to chassis and return to PS thru the lug you mention. There is also wire in potentiometer cases which make amp more prone to EM noise.

Nowadays mains Safety Earth can have lots of EM noise when it run long length with Live and Neutral and for the safety SE must be wired to Chassis when it is metal. Thats why using Chassis for current flow has been forbid in electric regulations about 50 years.

If normal 6.3mm mono jack cable is used most likely Power Brake box has connection with SR. Your body bring "an antenna" there. Body can be receiving antenna and audio noise increase. Or additional mass which impedance lowers the audio noise.

the grounding is the same as old Marshall, i think it's the chassis the main conductor, maybe my pots are not doing a good contact with it , i think i will try an other grounding scheme ..

 

[paul-e-mann]

I was just in front

Back up Jack!