# Electronics Question: Wire from outside of the chassis to the speaker basket



## greco (Jul 15, 2007)

I was vacuuming the dust out of the bottom of my (new to me) 80's Fender Stage Lead SS amp and noticed a wire attached externally from the amp chassis to the speaker basket via a speaker mounting bolt.

This thread appears to be discussing the same topic but is inconclusive re: the reason.
https://www.thegearpage.net/board/index.php?threads/ground-from-speaker-to-chassis.268102/

I'm assuming (and that is never good) that the sole purpose of this wire to provide some type of additional safety precaution from a grounding perspective?

I have never seen one or heard anyone mention this before.

Thanks

Cheers

Dave


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## Granny Gremlin (Jun 3, 2016)

Not sure I understand - pic(s) plz? If I do understand, it should not be required, as the dudes in the other thread say. You could check the ground connection on the speaker out jack, (continuity from sleeve to power jack ground pin) and if that's fine lose the extra wire next time you change the speaker.


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## greco (Jul 15, 2007)

Granny Gremlin said:


> Not sure I understand - pic(s) plz? If I do understand, it should not be required, as the dudes in the other thread say. You could check the ground connection on the speaker out jack, (continuity from sleeve to power jack ground pin) and if that's fine lose the extra wire next time you change the speaker.



















There is no speaker jack. The speaker wires come directly out the chassis and goes to the speaker terminals.


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## jbealsmusic (Feb 12, 2014)

As suspected, that third wire links the grounded chassis to the basket for grounding purposes. The amp's design isolates the chassis ground from the negative terminal on the speaker output.

Fender has done this on a few of their designs. The most recent being the Fender Mustang combo amps. According to Fender, it is important that the 3rd wire NOT be omitted from any speaker replacement and needs to maintain good electrical contact with the speaker basket. You may also want to heed their warning regarding changing the speaker on amps with this curious 3rd wire:
_"There are speakers on the market that have the negative terminal common with the frame of the speaker – these speakers CANNOT be used in a Mustang amplifier, as this will result in damage to the power amp."_​


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## Granny Gremlin (Jun 3, 2016)

OMG that's bloody stupid.

Also means that if the basket is painted/powder coated (rather common!) you have to sand off the area around one of the mount holes to get a ground connection. And oh yeah, you can't lose that 3rd wire. Stupidest thing I have seen in an amp design just about ever. Like how do you even know which speaker has basket connected to -ve terminal before you buy it - that's not a spec that is commonly listed.


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## greco (Jul 15, 2007)

jbealsmusic said:


> You may also want to heed their warning regarding changing the speaker on amps with this curious 3rd wire:
> _"There are speakers on the market that have the negative terminal common with the frame of the speaker – these speakers CANNOT be used in a Mustang amplifier, as this will result in damage to the power amp."_


Thanks for all of the information and especially for telling me about possible damage if I use another "unsuitable" speaker. 

I now wonder how I can determine which speakers ARE suitable. Possibly a whole new branch of electronics is about to unfold. 

I had been trying an AlNiCo speaker that I had wired to the amp's output spade/quick disconnect connectors with jumper wires (with no ground wire form the chassis to the basket). Maybe I should just leave everything and carry on. Unfortunately, as you know very well, the stock speakers in these amps are, shall we say, limited in there sonic/tonal palette. Oh well.


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## greco (Jul 15, 2007)

Granny Gremlin said:


> OMG that's bloody stupid.
> 
> Also means that if the basket is painted/powder coated (rather common!) you have to sand off the area around one of the mount holes to get a ground connection. And oh yeah, you can't lose that 3rd wire. Stupidest thing I have seen in an amp design just about ever. Like how do you even know which speaker has basket connected to -ve terminal before you buy it - that's not a spec that is commonly listed.


I was typing and didn't see your response until now. I guess I won't be thinking about upgrading the speaker until I am somehow sure that all will be OK.


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## greco (Jul 15, 2007)

This is from the schematic for this amp. 

Note the "Note:"

I assume that is what this discussion is all about??


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## knight_yyz (Mar 14, 2015)

If i understand correctly, to by a new speaker you would have to check for continuity between the negative termainal and the basket. Continuity will cause a short. no continuity should be fine. It looks like the amp has a ground, the speaker has a ground(negative terminal) and the basket has a ground. All 3 needing to be separate


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## greco (Jul 15, 2007)

I am now officially electronically dazed and confused. 
Please don't worry ...that is quite normal for me.


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## jb welder (Sep 14, 2010)

greco said:


> I am now officially electronically dazed and confused.


So am I. There are several different conversations going on here. As far as I know we are not talking about a Mustang, or a speaker that has (-) terminal connected to frame, which is very rare. 
Nothing stupid about this wire, not safety related, just helps with certain interference issues as far as I know.
Greco, can you post the part of the schematic that shows where that speaker minus lug goes?


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## greco (Jul 15, 2007)

jb welder said:


> Greco, can you post the part of the schematic that shows where that speaker minus lug goes?


This is a .pdf of the manual and it contains the entire schematic:
OM_leg_gtramp_Stage_Lead_1982 (2).PDF The link didn't work!!...LOL
As usual , you will need to remove the "s" from "https"
Here is part of the schem:
(Hopefully it contains what you are looking for)


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## jb welder (Sep 14, 2010)

Ok. You can see the minus lug does not go to ground, it goes to a .22R7W resistor, then to ground. This is a 'current feedback' arrangement. (Guitar Amplifiers)
If the negative lug was also connected to ground, the .22 resistor would be bypassed. Messing with the feedback loop will make the amp unstable and it could even go into oscillation and blow.
This is all independent of the speaker frame. I have a hunch you have a reverb tank in there and the ground strap for the speaker frame may prevent issues with the reverb.


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## Granny Gremlin (Jun 3, 2016)

I still think that was a stupid design choice.


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## greco (Jul 15, 2007)

jb welder said:


> Ok. You can see the minus lug does not go to ground, it goes to a .22R7W resistor, then to ground. This is a 'current feedback' arrangement. (Guitar Amplifiers)
> If the negative lug was also connected to ground, the .22 resistor would be bypassed. Messing with the feedback loop will make the amp unstable and it could even go into oscillation and blow.
> This is all independent of the speaker frame. I have a hunch you have a reverb tank in there and the ground strap for the speaker frame may prevent issues with the reverb.


Thanks for explaining and for the link re: _"current feedback" arrangement_.

Yes, there is a large reverb tank (about 17 inches long and about 4 inches wide) in the bottom of the cabinet. 

@jb welder Just to clarify, do I need to look for specific spec'd speakers (as mentioned earlier in the thread in @jbealsmusic 's post ....#4) if I wish to try a new speaker or do I ONLY need to be sure that I maintain the wire from the chassis to the speaker. Thanks again.


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## jb welder (Sep 14, 2010)

greco said:


> @jb welder Just to clarify, do I need to look for specific spec'd speakers (as mentioned earlier in the thread in @jbealsmusic 's post ....#4) if I wish to try a new speaker or do I ONLY need to be sure that I maintain the wire from the chassis to the speaker. Thanks again.


You only need to avoid speakers that have the minus lug connected to the frame. I was unaware you could still get them. It's a really bad idea. If @jbealsmusic says they are available, I won't disagree. But I would have thought you would have to seek them out, and I don't know why anyone would build them that way.



Granny Gremlin said:


> I still think that was a stupid design choice.


I have no idea what you are talking about. If you mean a speaker manufacturer connecting the minus lug to the frame, I agree. The frame needs to be floating until grounded if desired.


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## jbealsmusic (Feb 12, 2014)

jb welder said:


> You only need to avoid speakers that have the minus lug connected to the frame. I was unaware you could still get them. It's a really bad idea. If @jbealsmusic says they are available, I won't disagree. But I would have thought you would have to seek them out, and I don't know why anyone would build them that way.


To be clear, I was only quoting Fender on that matter. I am not familiar with any guitar speakers currently on the market that do that.


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## greco (Jul 15, 2007)

jb welder said:


> You only need to avoid speakers that have the minus lug connected to the frame.





jbealsmusic said:


> I am not familiar with any guitar speakers currently on the market that do that.


Thanks very, very much for clarifying. Finally some "good news" in this thread.


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## High/Deaf (Aug 19, 2009)

I suspect that amp has a bridged output section. That's an old trick where neither leg (+ve or -ve) gets tied to ground, both are driven out of phase to each other by the output transistors. By driving them out of phase, you double the voltage (since they are each doing the same wrt to ground, just in opposite directions) and because P=Vsquared/R, you get 4X the power. 

There are some limitations. You can't tie either output to ground or you will short out one active side of the amp. Speakers are find because they are passive and floating. You need to be very careful not to plug these into active equipment like a reamper (i.e. Fryette Powerstation or Badcat Unleash). My powered Kemper with the Class D amp has the same configuration/limitation.

Also, going below the rated output impedance can possibly destroy the output section in no time, since it is already producing as much power as it can into the rated impedance. This configuration is usually done to maximize power output (very little component tolerance above the rated level) so trying to get more out of it usually leads to smoke and tears. I fixed quite a few power amps in the 80's that RnR guys bridged and then ran into two bass bins (4 ohm load). Funny enough, it did work for a few months before the amp finally said "screw this, I quit on you - and right at the start of the second set, you barstard!".


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## greco (Jul 15, 2007)

An update...

I tested the continuity from the speaker terminals to the basket:

*With the "grounding wire" in place* (between the basket and amp chassis) both the negative and positive terminals have continuity to the basket.
*With the "grounding wire" NOT CONNECTED *there is no continuity.


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## jb welder (Sep 14, 2010)

Please specify actual ohms reading rather than 'continuity'. Some meters give a continuity 'beep' for anything under 200ohms. Big difference between 175ohms and .2 ohms.


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## Dorian2 (Jun 9, 2015)

Interesting thread.


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## jb welder (Sep 14, 2010)

greco said:


> *With the "grounding wire" in place* (between the basket and amp chassis) both the negative and positive terminals have continuity to the basket.
> *With the "grounding wire" NOT CONNECTED *there is no continuity.


Ok, you are good. With the speaker disconnected from anything, neither lug of the speaker is connected to the basket.
With speaker all connected, & ground wire from chassis to basket: neg.spkr. lug will read approx. .1ohm to chassis ground. This is R110 on the schematic (follow spkr.neg. all the way left). Spkr. pos. will measure approx. 6.1ohm to ground (assuming spkr DC resistance is 6 ohms). This represents the speaker resistance in series with R110.

On a side note re: why is the ground wire on the Mustang series so important they note it must be connected:
Mustang is a modelling amp with a lot of digital (computer) stuff going on. I imagine the ground wire to the speaker basket may be a FCC requirement to keep from interfering with communications equipment.
Kind of like those resistor spark plugs that engines will run just fine without, but are legally required so you don't cause radio interference.
Or maybe the ground wire on the mustang speaker helps keep outside devices from interfering with the mustang digital circuits.


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## greco (Jul 15, 2007)

@jb welder Thanks for your ongoing supportive responses and the information re: the expected resistances and the reasoning behind the measurements. 

Sorry for my late response. I hope to measure the resistances today. 

I have been dealing with trying to keep the melt water from snow combined with a fairly significant rainfall from entering my basement.


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## High/Deaf (Aug 19, 2009)

As an aside, some meters struggle with accurate low R measurements (say < 1 ohm). Some meters have a conductance setting (which is the inverse of resistance, or 1/R). That may give you a more accurate reading (once converted) for the low R measurement you are expecting on the -ve lug.


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## jbealsmusic (Feb 12, 2014)

Threads like this are why forums like this are awesome! Now this discussion can benefit everyone on the internet with the same conundrum. So cool!


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## jb welder (Sep 14, 2010)

Meter probes will often account for fraction of an ohm readings. I'm not really expecting you to be able to accurately read the .1 ohm, just want to account for the low reading.
In the modern era, we now see references to milli ohms, and special instruments like LCR meters will give you a more accurate reading, but they are usually not required. Especially for prehistoric electronics like tube amps.


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## greco (Jul 15, 2007)

I took the reading with my inexpensive and likely inaccurate meter, especially for these low resistances. The meter is auto ranging.

The DCR of the speaker is 7.2 ohms

Positive speaker terminal to the speaker basket is 7.9 ohms

Negative speaker terminal to the speaker basket is 1.3 ohms

Close enough for rock and roll?


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## jb welder (Sep 14, 2010)

greco said:


> Close enough for rock and roll?


Yes, even jazz in a pinch. 
The critical issue with regard to the OP question is that when it was all disconnected, there was no resistance measured (open circuit) between either speaker lug and the speaker basket (frame).

I wish I had a picture of a speaker with the minus lug connected to the basket. It's usually pretty obvious. The lug may be directly riveted to the basket with no fiber insulator, or there may be a visible metal strap from the minus lug to the basket. Those are the type of speakers you want to avoid. But as mentioned before, I don't think you would find them in modern production.


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## greco (Jul 15, 2007)

jb welder said:


> Yes, even jazz in a pinch.


OK...Now I feel totally fine!

Once again, thanks for everything!

I appreciate all the ongoing help you provide to me (and many, many others) here.


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