5e3 hum problem

[tubelovin]

Just built another 5e3 with all the best goodies. The problem is a low frequency hum that never goes away. I think I've isolated it to before the .1 mfd caps going into the vol. pots by shorting the cold side of the cap. When I do that the hum stops. I've changed the caps checked everything like solderjoints, changed tubes etc. Also if I crank it up it sometimes it starts to make this plop plop plop noise. Any ideas?

[Bill Durham]

Just built another 5e3 with all the best goodies. The problem is a low frequency hum that never goes away. I think I've isolated it to before the .1 mfd caps going into the vol. pots by shorting the cold side of the cap. When I do that the hum stops. I've changed the caps checked everything like solderjoints, changed tubes etc. Also if I crank it up it sometimes it starts to make this plop plop plop noise. Any ideas?

If you are an experienced builder this is probably not helpful, but are your filament leads twisted really tight? Also, if any of your signal leads are parallel with the filaments string you might get some coupling from that.

BD

[Enzo]

Here is one tip. Determine if the hum is 60Hz or 120Hz. They sound really similar. 120Hz is power supply ripple from wherever. 60Hz hum comes from radiated fields or other mains related sources.

[tubelovin]

my leads are twisted, maybe not tight enough. I haven't had that problem before with the heaters. I think its ac hum because it's a lower freq than If I were to induce hum by touching a grid with my finger.

[Bill Durham]

hum

Just re-read your original post.. does it go up and down with the volume controls? Also does it go away when you plug in a guitar?

just trying to help
BD

[tubelovin]

Yes it does get louder when I turn up the volume and no there isn't any change when I plug in my guitar. That tells me that it's in the pre stage. I don't have a lot of experienve, so it could be something simple. Is it possible that it's a ground loop? How would I diagnose?

[Bill Durham]

Yes it does get louder when I turn up the volume and no there isn't any change when I plug in my guitar. That tells me that it's in the pre stage. I don't have a lot of experienve, so it could be something simple. Is it possible that it's a ground loop? How would I diagnose?

TL,

I just took a look at the schematic for the 5E3. I would suggest that you float the input jacks above chassis ground and also connect the ground side of the 1 meg resistors back to where the cathode bypass cap and resistor are grounded.

this might help
BD

[tubelovin]

I was actually using the ground teminal of the input jack as the ground for the cathodes and pots. I was thinking of getting those plastic washers to isolate the jacks from the chassis, just haven't ordered those yet. Whats your opinion on grounding pots to there case. I read that it can contribute to ground loops, but I see many people doing it.

[Bill Durham]

I was actually using the ground teminal of the input jack as the ground for the cathodes and pots. I was thinking of getting those plastic washers to isolate the jacks from the chassis, just haven't ordered those yet. Whats your opinion on grounding pots to there case. I read that it can contribute to ground loops, but I see many people doing it.

I need to email you a paper done by some guys on the 5E3 and its various problems, at least in their opinon, one of which is grounding. send me an email to billdurham at attglobal dot net.

The ground buss in a pcb, non-mv Marshall IS the buss wire that is run between the cases of all of the pots on the front panel. Everything from the board ties to that point. Since that is the point that was chosen in that case, I think it works good for them, but if you are thinking about doing it in addition to what you have done already, I don't think it probably make things worse before making them better. You need to have a plan for your grounding scheme and most folks agree that you should have one and only one connection to the chassis. If you connect to the chassis in multiple places, eddy currents can develop between those connections through the chassis. Net result is hum and or noise. I'm not saying that is your problem now, but it could be if you have multiple connections of your ground buss to the chassis..as in the input jacks, the output jacks, a wire from the board ground point to the chassis, the center tap of the PT high voltage winding....etc.

[tubelovin]

Just changed the 22K resistor in the B+ string and the hum went away.....sort of. It was a metal oxide now it's carbon film.

Now I'm getting grid hum on V2. It's the lone one from the control to pin #2. I noticed that when I touched the amp the hum went away. Then I figured I would tape some aluminum foil to the back panel as a shield and the hum is almost gone to the point where you have to have your ear to the speaker to hear it at all.

Why would one 5E3 that I built be so quiet and the other so noisy? The build is exactly the same, but this one hisses and hums.

[TD_Madden]

Haunted.