# 6G2 build - Hum starts when volume pot turned down below 2



## Lincoln (Jun 2, 2008)

Another day, another learning opportunity. 

Fender 6G2 build, working really well. except when you turn the volume pot all the way down to zero. Turning the volume below about #2 makes a hum in the speaker.
Happens whether a guitar is plugged in or not. Starts at about 150 ohms resistance on the pot and goes to zero, not changing tone or volume.

However, if I leave the test lead of my multi-meter on the wiper connection, there is no hum. It vanishes. Yup, another ghost story.

I'm using a grounding plate, everything is grounded to the plate, nothing is grounded to the pots. The plate then has a single ground connection to the PT.
V1 is 7025 and has shielded wire on both grids.

Ideas???


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## nonreverb (Sep 19, 2006)

Sounds like a ground loop somewhere. What happens if you ground out the back of the pot?


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Sounds like a ground loop somewhere. What happens if you ground out the back of the pot?


when I touch a ground to the back of the pot, it makes a tick, but the hum doesn't change. 

I turned the tremolo off (grounded it out) and that didn't help either. And I'm reading zero volts between PT ground at any point on the pots or grounding plate.

It's more than a hum. It's the same sound you get when you short out an input jack with your finger.


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## nonreverb (Sep 19, 2006)

Is your input jack isolated?
Pull the first preamp tube out and tell me if it still hums.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Is your input jack isolated?
> Pull the first preamp tube out and tell me if it still hums.


no, the inputs are on the grounding plate too.

with the first tube pulled, no sound at all, nothing.


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## nonreverb (Sep 19, 2006)

Lincoln said:


> no, the inputs are on the grounding plate too.
> 
> with the first tube pulled, no sound at all, nothing.


No hum though when you adjust the volume...


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## Lincoln (Jun 2, 2008)

and it still makes the noise when both input jacks are empty and shorted.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> No hum though when you adjust the volume...


no sound at all. dead quiet from zero to ten


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## nonreverb (Sep 19, 2006)

nonreverb said:


> No hum though when you adjust the volume...


First preamp tube's cathode resistor grounded to the same plate?


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## nonreverb (Sep 19, 2006)

You have a ground wire running from the input jacks to the ground plate?


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## Lincoln (Jun 2, 2008)

nonreverb said:


> First preamp tube's cathode resistor grounded to the same plate?


yes


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## Lincoln (Jun 2, 2008)

nonreverb said:


> You have a ground wire running from the input jacks to the ground plate?


no soldered connection from the input jacks to the ground plate. Just physical contact. 
Shielded wire from the inputs to the grid on V1 is grounded to the plate, not the input jacks.


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## nonreverb (Sep 19, 2006)

I wonder if the loop could be the input jacks.


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## nonreverb (Sep 19, 2006)

Easiest way to tell is unscrew them from the chassis and place them somewhere away from contact with the chassis then alligator clip a test lead from the ground to the plate ground.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Easiest way to tell is unscrew them from the chassis and place them somewhere away from contact with the chassis then alligator clip a test lead from the ground to the plate ground.


here's the way it was.









with the inputs floating free, huge hum, no guitar sound

with the inputs still floating but grounded with a test lead, same as before.









A new twist! If I crank the tone pot to 10, the amp doesn't do the hum at zero volume anymore, but sweeping the volume sounds like tuning a radio, and guitar sounds are cut out.


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## jb welder (Sep 14, 2010)

You've tried other tubes in that V1 slot ?
Any other symptoms, amp is otherwise functioning well?


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## oldjoat (Apr 4, 2019)

two thoughts 
1 wiper arm on the pot is intermittent
2 crack in the pot resistor board 

turn amp off 
clip one VOM lead on the pot wiper arm 2[ middle] ( adjust VOM to resistance scale of pot )
clip the other lead to either end of the pot (lead 1 or 3)
sweep the pot clock to clock slowly
if the meter jumps around ... bad wiper / dirty pot
if meter jumps and stays at infinite (either end of sweep) ... bad resistive track in pot .... 
if infinite, connect the leads to leads 1 and 3 
repeat the sweep ... if it jumps from reading to infinite , bad pot .... replace.


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## Lincoln (Jun 2, 2008)

jb welder said:


> You've tried other tubes in that V1 slot ?
> Any other symptoms, amp is otherwise functioning well?


no, I have not tried other tubes in V1. I will now though. 

Yes, the amp is working perfectly, sounds good, loud, no background tones or noises. Trem works good, although it could be a little slower. 
Only thing wrong is it cuts out and hums when the volume in turned right down, or the tone is turned right up.


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## Lincoln (Jun 2, 2008)

Tube wasn't the answer. I'll sweep test the pots next.


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## nonreverb (Sep 19, 2006)

I've seen this exact condition before...it's a ground loop from what has been described. It's finding where the loop is.


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## nonreverb (Sep 19, 2006)

Can you take a pic of the entire inside chassis Lincoln?


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## Lincoln (Jun 2, 2008)

nonreverb said:


> I've seen this exact condition before...it's a ground loop from what has been described. It's finding where the loop is.


Picture coming. I've got a choke installed in it right now, thought maybe dropping the B+ a bit might help, but it didn't.


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## Lincoln (Jun 2, 2008)




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## nonreverb (Sep 19, 2006)

How's the ground connected to the volume pot? I see the shielded wire comming off it but not the ground strap.


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## Lincoln (Jun 2, 2008)




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## Lincoln (Jun 2, 2008)

nonreverb said:


> How's the ground connected to the volume pot? I see the shielded wire comming off it but not the ground strap.


Yes, it's there. A bare wire from the terminal the shielded wire is attached to, to the ground plate









the brown wire wound around the blue wire is abandoned on both ends. I replaced it with a shield wire.


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## nonreverb (Sep 19, 2006)

Well, the input jacks can go back since it makes no difference. Perhaps try a different ground point for the volume pot. Also, check to make sure it's well attached to the chassis. I once went through a similar exercise with similar symptoms and it turned out to be a loose volume pot that didn't make a solid ground connection.


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## nonreverb (Sep 19, 2006)

Perhaps try grounding to the pot itself.


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## oldjoat (Apr 4, 2019)

Lincoln said:


> brown wire wound around the blue wire is abandoned on both ends.


try grounding only 1 end of the brown wire to the chassis ... 
the twisted blue and brown cancel out any stray noise.
the blue may become an antenna (without the brown )


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## nonreverb (Sep 19, 2006)

It's interesting to note that the original circuit board grounds were all to the chassis and the pot grounds were all to the pots themselves.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Perhaps try grounding to the pot itself.


I will try that.



oldjoat said:


> try grounding only 1 end of the brown wire to the chassis ...
> the twisted blue and brown cancel out any stray noise.
> the blue may become an antenna (without the brown )


I'll try that too.

I've been measuring 1 meg pots. I've got new CTS pots in there, and they measure "funny" when compared to Fender NOS CTS pots. The new pots go right down to zero ohms on both ends. The NOS pots do not. One stops at 25 ohms, the other at 30 ohms. Also the new CTS pots get almost full scale by 1/2 rotation. 85%, maybe 90% of the value at 1/2 scale. The old Fender pots are constant. I'm going to try switching the volume & tone pots for these Fender ones.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> It's interesting to note that the original circuit board grounds were all to the chassis and the pot grounds were all to the pots themselves.


Yeah, I know. But somewhere I heard grounding to pots is not a good idea, even though it's been done forever. Hmmmmm


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## Lincoln (Jun 2, 2008)

Well, I replaced the volume and tone 1 meg CTS pots with NOS CTS made for Fender pots. Problem gone. Totally gone. 
The cutting out/hum problem I was having was right where the new CTS pots dropped to zero ohms. The older CTS pots I put in don't drop to zero, and they had better taper to them. The Amp sounds great.

To confuse matters, I grounded both pots with a wire soldered on the back of the pot, to the grounding strip. At the same time I changed pots. So we'll never really know. 😝


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## Lincoln (Jun 2, 2008)

oldjoat said:


> try grounding only 1 end of the brown wire to the chassis ...
> the twisted blue and brown cancel out any stray noise.
> the blue may become an antenna (without the brown )


while I was in there mucking about, I loosened the board and pulled the old brown wire right out of there. And ran a new blue wire. 
No more possible antenna


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## nonreverb (Sep 19, 2006)

Awesome work Lincoln!


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Awesome work Lincoln!


A big *THANK YOU* to everyone who helped me get through it.


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## Paul Running (Apr 12, 2020)

Some info on grounding:


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## Lincoln (Jun 2, 2008)

Paul Running said:


> Some info on grounding:


my biggest take-away on that was "keep your ground wires as short as possible". Good reading for sure.


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## Lincoln (Jun 2, 2008)

What I'd like to know is why there is such a difference in the taper of new CTS pots compared to old CTS pots of the same value.


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## jb welder (Sep 14, 2010)

My understanding is the new CTS pots have a few tapers 'spliced' together to make the pots. So the track is no longer a continuous piece, and does cause various issues.



Lincoln said:


> To confuse matters, I grounded both pots with a wire soldered on the back of the pot, to the grounding strip. At the same time I changed pots. So we'll never really know. 😝


If you had time, access, and desire, you could revert to the previous ground scheme there and see if the issue returns.


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## Lincoln (Jun 2, 2008)

jb welder said:


> My understanding is the new CTS pots have a few tapers 'spliced' together to make the pots. So the track is no longer a continuous piece, and does cause various issues.


The new pots, and I measured about 6 of them, (1 meg) were all at 850K by 1/2 sweep. The other half of the sweep was to go from 850K to 950K.

The older pots were pretty much linier. 1/4 sweep was 250K, 1/2 was 500K etc. And they didn't shut right off either. 



jb welder said:


> If you had time, access, and desire, you could revert to the previous ground scheme there and see if the issue returns.


As much as I'd love to do that, the chassis is already in the cabinet, and I spent enough time on the thing. Between the ghost voltage, ground loop issue, slowing the trem, taking intensity out of the trem, I wrote off most of a weekend. Me and brownface just don't get a long. I struggle with them. Gotta be something I'm doing that they don't like.


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## nonreverb (Sep 19, 2006)

Lincoln said:


> The new pots, and I measured about 6 of them, (1 meg) were all at 850K by 1/2 sweep. The other half of the sweep was to go from 850K to 950K.
> 
> The older pots were pretty much linier. 1/4 sweep was 250K, 1/2 was 500K etc. And they didn't shut right off either.
> 
> ...


Sounds like you're replaced LOG pots with linear pots. You might find they don't give much more volume after about 2.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Sounds like you're replaced LOG pots with linear pots. You might find they don't give much more volume after about 2.


Yeah, I need to figure out how to order the right ones. The ones I like have a Fender part number on the back. Wonder if they are a "special" taper?


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## Lincoln (Jun 2, 2008)

Fruits of my labour








no slit in the grill cloth, just a funny reflection









yes, we have no speaker.


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## gtrguy (Jul 6, 2006)

Lincoln said:


> Yeah, I need to figure out how to order the right ones. The ones I like have a Fender part number on the back. Wonder if they are a "special" taper?


They aren't special. You'll see the pot values listed as *A*250K or *B*250k, *C*250k, etc. The letter denotes the taper.


Potentiometer Taper Charts | Amplified Parts


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## Lincoln (Jun 2, 2008)

gtrguy said:


> They aren't special. You'll see the pot values listed as *A*250K or *B*250k, *C*250k, etc. The letter denotes the taper.
> 
> 
> Potentiometer Taper Charts | Amplified Parts


My problem is that CTS doesn't "sell" a 1 meg linear pot with solid shaft. Everything is audio taper.

a little more learning done & knowledge gained.

Here's the correct part: $10 each instead of $3 each
*Fender CTS 1 Meg Linear Solid Shaft Pot 0095704049*


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## nonreverb (Sep 19, 2006)

Lincoln said:


> My problem is that CTS doesn't make a 1 meg linier pot with solid shaft. Everything is audio taper.
> 
> a little more learning done & knowledge gained.


You'd want audio taper for the volume Lincoln.


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## Paul Running (Apr 12, 2020)

Pot taper (commercial-log is segmented):


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## gtrguy (Jul 6, 2006)

Lincoln said:


> My problem is that CTS doesn't "sell" a 1 meg linear pot with solid shaft. Everything is audio taper.
> 
> a little more learning done & knowledge gained.
> 
> ...


The only Linear taper pot in the 6G2 is the 250k for the trem intensity. The 1Meg tone and volume pots are both supposed to be 1Meg Logarithmic (A) taper not Linear.

CTS - A1M SOLID Shaft Pot


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