Hum on one channel of hybrid amp

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Breezio

Well-known member
Joined
Jan 20, 2019
Messages
50
Location
Berkeley, CA
Dear all,

After the succesful rebuild of a B+ regulator (https://groupdiy.com/index.php?topic=71117.msg906020#msg906020) I am now facing a hum on the right channel of my AMC CVT3030 amp.

There is moderate 120Hz hum (see pic below) for which I cannot  identify the source.
The hum is always present, is not affected by the volume level and the audio signal works fine.

The hum is present only when a signal cable from the preamp is connected to the right tube stage.
However, I cannot measure anything on the tube input (CN501) and switching L/R channels from the preamp does not change anything either (meaning that the hum is present as soon as a signal cable connects to the tube module)

Any suggestion?

Thanks again

Fabrizio
 

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Breezio said:
switching L/R channels from the preamp does not change anything either (meaning that the hum is present as soon as a signal cable connects to the tube module)

This is a little confusing to me. You are saying that the exact same source works in one channel without issues, but in the other it has the hum?

Meaning it's that particular amp channel for certain?
what is the source anyhow???
Those tubes are not in sockets?? :eek:

 
Hi Scott,

Yes, as strange as it might sound, If I exchange the signal channels from the preamp or if I swap the tube modules, the problem persists on one side of the amp (the right channel).

On the preamp I checked all the electrolytics on the signal path, resolderd them, but nothing changes.

I am starting to think that there might be something related to the position/wiring of the right tube power module with respect to the preamp and power supply modules.

And yes no sockets, this is how the amp was originally sold. They added them later on and that's on my list of improvements, as soon as I get rid of this crazy thing...

Any ideas?

Thanks

Fabrizio

 
Breezio said:
Yes, as strange as it might sound, If I exchange the signal channels from the preamp

How exactly are you doing this?

Breezio said:
or if I swap the tube modules, the problem persists on one side of the amp (the right channel).

and how are you doing this? Pulling the entire board out and swapping?? Are those fans under the tubes?? ???

Breezio said:
On the preamp I checked all the electrolytics on the signal path, resolderd them, but nothing changes.

sounds like a lot of work.... what do you mean checked them? for tolerance value?

Breezio said:
I am starting to think that there might be something related to the position/wiring of the right tube power module with respect to the preamp and power supply modules.

The original layout???

Breezio said:
And yes no sockets, this is how the amp was originally sold. They added them later on and that's on my list of improvements, as soon as I get rid of this crazy thing...

Probably a good mod....Tubes can go hokey .....

I still don't follow this progress enough to have any ideas but there are some bias adjustments too.... ?? Where are they at....? I need to look at the manual at some point ....hopefully someone will have a better idea or picture of what's going on.....

Not much room in there for sure...
 
Hi Scott,

Thanks for taking the time to help.
Here is a recap of what is happening and what I have tried so far:
  • When the power module on the right is connected to the preamp, through either left or right signal cables, 120Hz hum is present. The tube module is connected to the preamp through the cable indicated in the pictures below. I can't measure anything relevant with the oscilloscope at the signal connector (either GND or R/L pins).
  • When the signal cable (left or right) is disconnected from tube module on the right, the hum is gone
  • If I connect only the GND of the signal cable between preamp and power module, hum IS NOT present (see picture)
  • If I connect only the R pin of the signal cable between preamp and power module, hum IS present (see picture)
  • If I disconnect the signal cable from the left tube module (the right being connected) the hum is still present
  • If I physically swap the right and left tube modules (boards with tubes attached) the problem persists on the module to the right. Under each module there is a fan.
  • On the preamp I checked the signal electrolytics for leakage (none) and resoldered them too.
  • I checked tube bias and everythig is fine
  • I have been trying to check with the oscilloscope probe where the hum interjects on the tube module but it's not easy since everything is so crammed and I am afraid of short circuiting.
I am attaching a manual/schematics of the amp and post more pics below.I have no layout docs but I can take more pictures.

Thanks again!
Fabrizio

 

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You have one cable and connect it to one side there is no hum but that same cable connected to the other side produces hum?

It could be a layout issue, it could be a power supply issue, it could be a tube issue. Hard to swap tubes when they are soldered in place vs on sockets.  you may have a bad cap or two.  Shotgunning parts may or may not get you closer to a solution, the same goes for re-soldering. 

Hum at 120 is an octave of 60 cycle.   
 
pucho812 said:
You have one cable and connect it to one side there is no hum but that same cable connected to the other side produces hum?

No, whichever cable I use to connect the right tube module, whether from L or R channel on the preamp, is still associated to hum.
 
right so  you are getting  hum regardless on the right channel, so it is isolated to the right channel.  could be a layout issue, a wiring issue, a parts issue.  We can make some assumptions like did it have shielding and does it need it? Is there a faulty part? is there a wire out of place?
 
So I am slowly getting sort of desperate...

I have additionally tried the following, without any kind of success/change:

- Put lid on
- Exchanged heaters cables between L and R tube modules
- Exchanged B+/Bias/GND connectors between L and R tube modules
- Inspected and gently poked with a fine wood stab almost all components on all boards

What I still find unexplicable is the relationship between signal cables and hum on the right module, without being able to measure anything on the cables themselves but only at the tube module output.
 
Could be that the power amp ground is not super great.

Connect a gator clip to the power amp in (with the preamp disconnected) and try touching it to the power amp ground and other ground points. If the hum is gone with the input grounded locally (to the power amp ground) and present when connected elsewhere (which based on what you've said should be the case), then that would suggest that the ground is not great.

Can you measure any resistance between the power amp ground and the supply ground. Check the solder joints along the way.

Can you measure any voltage between the preamp out and power amp in (meaning with the cable disconnected so that it's an open circuit)? Compared to the other side?

 
squarewave said:
Could be that the power amp ground is not super great.

Connect a gator clip to the power amp in (with the preamp disconnected) and try touching it to the power amp ground and other ground points. If the hum is gone with the input grounded locally (to the power amp ground) and present when connected elsewhere (which based on what you've said should be the case), then that would suggest that the ground is not great.

Can you measure any resistance between the power amp ground and the supply ground. Check the solder joints along the way.

Can you measure any voltage between the preamp out and power amp in (meaning with the cable disconnected so that it's an open circuit)? Compared to the other side?

Hi squarewave, thanks for the suggestions. Here is what I found

The power supply board distributes supply to the preamp and from the preamp to the L and R tube modules.
So there is a central connector and one each on the sides

  • If I connect the right tube module signal in to the local ground the hum is gone. If it's connected to the ground on the supply connector to the right, the hum is sensibly reduced (I can still see it on the oscilloscope).
  • If I connect the tube module signal in to the ground coming from the PS board on the central connector the hum is present.
  • If I connect the right tube module signal in to the the ground on the supply connector to the left, the hum is present

Between power amp ground and supply ground I measure about 13ohm, however that is the same for both left and right tube modules

Between preamp out and power amp in I measure values on average between -3mV and + 3mV, on both sides, however the right (bad side) tends to wander more, even up to +10mV or -7mV.

I am attaching a picture of the amp. Outside of it is the B+ regulator circuit (works very nicely).

So based on the above, what do you suggest I do?
I tried for instance adding a direct connection from the power supply ground to the right module ground but nothing changed

Thanks again,

Fabrizio
 

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Breezio said:
Between power amp ground and supply ground I measure about 13ohm, however that is the same for both left and right tube modules
The resistance between grounds should be basically zero (maybe a few tenths of an ohm just because of oxidation on metal surfaces and probes). Check resistance along the way. Does it increase across a particular solder joint?

Could be it got hot and one of the solder joints is breaking. Look carefully for little cracks. Or just skip to re-heating the solder joints that connect grounds. Use a little new lead solder and / or add flux with a flux pen if you have one. If there joint has ground plane thermals you might have to increase the iron temp and hold it longer for the solder to reflow properly.
 
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