Jcm900 4500 hum

[lowell]

The cap is fine. Does anyone know what this is called? (the resistor, cap, and 2 diodes from audio ground/common to earth)

[g1]

Not sure what that network is called, sorry.

It seems to me there may be a missing or 'iffy' ground somewhere, and you are providing a 'band-aid' by shorting out that network.
Two other places that also should go to chassis are the AC cord receptacle, and the CT of the heater winding. Make sure they are good connections.
Otherwise check all the other grounds, especially ones such as screws grounding pc board traces to chassis, and even pot nuts. They all need to be clean and tight.

[lowell]

Right on, will do

[uneumann]

Not sure what that network is called, sorry.

It seems to me there may be a missing or 'iffy' ground somewhere, and you are providing a 'band-aid' by shorting out that network.
Two other places that also should go to chassis are the AC cord receptacle, and the CT of the heater winding. Make sure they are good connections.
Otherwise check all the other grounds, especially ones such as screws grounding pc board traces to chassis, and even pot nuts. They all need to be clean and tight.

AFAIK this is a form of "ground-lift" circuit. It's safer (and probably required by electrical codes) than simply breaking the connection from circuit ground to chassis (and earth).
Chassis is always earth ground so it's safe to touch. This network just elevates and isolates circuit grounds a bit from chassis. The diodes limit the absolute separation to ~.6v. The cap is an ac short so signals have a low-z connection to earth ground. The resistor is there to provide a DC path for voltages below the diode threshold.

In theory the network reduces ground-loop hum when the amp is connected to other grounded equipment (effects, etc). If you're having hum with the stand-alone amp, the problem likely lies within the amp ground connections. If all is working as intended, there is no current through the network so shorting it should have no effect. Since you are getting an effect, something is causing current to flow through that network. Without more info, it's a guess, but something could be touching chassis ground that should not. All circuit grounds should be to the "star point" side of the network - not to chassis. If something is accidentally touching chassis, it could cause the problem since a chassis connection could cause current through the lift circuit. Did anyone mod the amp? Is there a jack shorting to chassis? As g1 mentioned, there may be a loose or broken connection in the ground circuit. Check the ground connections around the presence control (VR6) since that's affecting the problem. It's not a standard presence control in the output stage. It uses a preamp ground that must tie back to the "star point".

[Helmholtz]

What about "ground separation" circuit/network ?

[lowell]

I will look into it... so I am noticing now, that the reverb sounds wonky. The reverb isn't super strong, and the decay is "wobbly." It may just be a bad tank IDK... but I wonder if that's is part of the issue. It's an odd tank... 310ohm/2575ohm. I don't have a sub so will have to order it to see how a tank sub sounds.

[minim]

I have this amp and tore my hair out trying to get to the source of hum which seemed to center around the reverb.

I know you mentioned changing V2. I found this position to be very sensitive to noisy tubes, and after trying several I found the hum either greatly reduced or disappeared altogether. Maybe you want to try a couple more. Worth a try at least.

Also, your description of the reverb sounds pretty accurate of a properly-functioning Dual Reverb. Quite frankly, it's weak and metallic-sounding. I have owned a couple of Dual Reverbs and they've all been the same. You have to crank it past 3pm before getting any real depth. And by then, as you say, the decay sounds "wonky". Metallic, harsh, artificial.

[SoulFetish]

okay, I'm jumping in here at the end at the risk of this already being addressed, but one thing with some 900's and this kind of problem:

So, the ground to chassis connection is made only in one location via the back panel PCB. Unfortunately, the connection is made through the earth terminal on the board mounted IEC jack.
This connector receives a lot of mechanical stress, and (as we know) can cause the solder joints to break. I suspect if you pull the back board and look at that solder joint (the earth conductor), you'll find the solder joint broken and that is the source of the noise.

(now I'm going to go back and read the thread. sorry, I know it's ass backwards )

edit: so, I'm now pretty certain this is your problem, which is why when you use a jumper from the ground to chassis, the noise goes away.

[lowell]

okay, I'm jumping in here at the end at the risk of this already being addressed, but one thing with some 900's and this kind of problem:

So, the ground to chassis connection is made only in one location via the back panel PCB. Unfortunately, the connection is made through the earth terminal on the board mounted IEC jack.
This connector receives a lot of mechanical stress, and (as we know) can cause the solder joints to break. I suspect if you pull the back board and look at that solder joint (the earth conductor), you'll find the solder joint broken and that is the source of the noise.

(now I'm going to go back and read the thread. sorry, I know it's ass backwards )

edit: so, I'm now pretty certain this is your problem, which is why when you use a jumper from the ground to chassis, the noise goes away.

Ya know, I had already read your other thread on this. That unfortunately isn't the issue here. Thanks for the input though.

[lowell]

I have this amp and tore my hair out trying to get to the source of hum which seemed to center around the reverb.

I know you mentioned changing V2. I found this position to be very sensitive to noisy tubes, and after trying several I found the hum either greatly reduced or disappeared altogether. Maybe you want to try a couple more. Worth a try at least.

Also, your description of the reverb sounds pretty accurate of a properly-functioning Dual Reverb. Quite frankly, it's weak and metallic-sounding. I have owned a couple of Dual Reverbs and they've all been the same. You have to crank it past 3pm before getting any real depth. And by then, as you say, the decay sounds "wonky". Metallic, harsh, artificial.

Thanks I'll try it.

[Jazz P Bass]

I had one of these with the same hum issue.
It was centered around V2 .
It took a pile of 12AX7's to find 1 that did not hum.

[SoulFetish]

Ya know, I had already read your other thread on this. That unfortunately isn't the issue here. Thanks for the input though.

Well sh*t... I should probably temper my enthusiasm when posting suggestions, as my "certainty" is certainly subject to change when it's wrong.

So, where did you make the connections when you jumped the circuit ground to the chassis to get the noise to stop?
and if you run a clip lead from the IEC ground to the chassis, it doesn't have the same effect?

I would agree with the others about V2 being very sensitive to particular tubes, but I'm not sure how that would jive with the grounding circuit.