Hum and hiss Marshall JCM 800

[rbotn]

Hi everybody.
I hope that somebody can help me thinking. I have looked at this amp for a while, and I have problem thinking rationally. I have uploaded a video who explain the problems with the amp. I think it is a grounding problem, I have checked all point of soldering, twisted the filament cables, tried different tubes and change the input jacks. Still with noise. So if somebody can tell me what I have overlooked I will be very thankfull.

 

[neikeel]

Video unavailable -this video is private.

 

[rbotn]

Thanx. Now it is public

Video unavailable -this video is private.

 

[neikeel]

Couple of questions - is the low input jack new/cleaned/lubed and making good contact to ground?
Has the amp always done this, or is it a recent problem, if so what happened to it preceding the noise issue?
The key is probably that the noise is quieter when jack plugged into that input.
The noise is not abolished by absent V1 and V2, so the noise is coming from down stream of the preamp but being amplified when V1 and V2 inserted. I presume the master pot is clean and tidy and has good ground.
Does the amp do the same in the head cab (ie the aluminium plate completes the Faraday cage?
Have you tried the amp with different power cable and different circuit in the building?
Do the output jacks have good chassis ground?

It maybe that if the above are all affirmative that you may need to rework the grounding scheme of the amp (ie several discrete nodes, separating power amp from preamp, moving heater ground to the input jack ground point etc).

 

[Ray Baker]

Hi everybody.
I hope that somebody can help me thinking. I have looked at this amp for a while, and I have problem thinking rationally. I have uploaded a video who explain the problems with the amp. I think it is a grounding problem, I have checked all point of soldering, twisted the filament cables, tried different tubes and change the input jacks. Still with noise. So if somebody can tell me what I have overlooked I will be very thankfull.

rbotn, Couple of things from my own past experience with similar hum:
1. Fluorescent lighting can cause it.
1.a Can you try turning out the lamp while doing this?
2. This weekend I received my new 77' JMP 2203, plugged it in and it had the same exact hum my 2555 had which I was going to have the capacitors changed because of it. Picking up on that I simplified the power and went directly into the outlet and skipped the power supply in my pedalboard and the one my head was plugged into. Hum went away, and I didn't need to change the capacitors after all. It was a similar hum to yours.
2.a Are you using a powerstrip? If so try going directly to your outlet.
2.b Can you check to assure the grounding in your outlet power is correct? There is a small plugin tool that will light up and provide an indication of proper outlet wiring.
3. Capacitors:
3.a how old are they?
3.b Have you inspected them for obvious signs of degradation? (leakage)
4. Power Tubes:
4.a Can you flip it over an observe the power tubes while turning the volume knob?
5. High Voltage:
5.a Do you happen to have any other high voltage users plugged-in in the vicinity? No more than a five foot radius to your work area?

I have heard that same hum come from different sources I listed them above as, at least in my experience, the likely culprit.

I hope this may be helpful and Good Luck. Let us know what you ultimately find. neikeel has some good points too about the Faraday Cage and the power cord.

I don't know him personally but he is a member here, but hopefully David Hopkins see's this as well, Given the amazing mods he has done I bet he might even know.

 

[rbotn]

Thanx for lot of tips. i will try some of them this week-end and report back the result.
@neikeel asked for what happened . The owner told me that he tried to adjust the bias, then he short-circuited two pins on one of the EL34.
@Ray Baker. I am using a variac with ground between the wall and the amp. I have also tested the amp with lights and other equipment turned off.
This amp have two trimpots, one for bias and one for filament hum I think. There is a variation in the noise when I adjust the trimpot for filament. But I also tried a 6,3DCV for filament to the preamp tubes V1 and V2. No difference.

 

[neikeel]

well if he flashed 480v plus through the hum bias pot you must consider this component compromised. Try removing the pot and using the 100R virtual centre tap method - you never know!

 

[CROWEMAG]

Thanx for lot of tips. i will try some of them this week-end and report back the result.
@neikeel asked for what happened . The owner told me that he tried to adjust the bias, then he short-circuited two pins on one of the EL34.
@Ray Baker. I am using a variac with ground between the wall and the amp. I have also tested the amp with lights and other equipment turned off.
This amp have two trimpots, one for bias and one for filament hum I think. There is a variation in the noise when I adjust the trimpot for filament. But I also tried a 6,3DCV for filament to the preamp tubes V1 and V2. No difference.

Maybe carbon from burn on the tube sockets causing conductivity? Check those. If they look charred....change them.

 

[J5684]

Have you scoped the hum or done something to see what frequency that is? I'd go that route then go from there once the hum is identified, it would help narrow it down.

I listened to it again and sounds like 120 to me. 120 hz hum originates from the rectifier. The difference from filament hum (60 cycle) is that the frequency is doubled by the full wave rectifier ala 120hz. It's a give away when using a scope to troubleshoot more or less.