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Old 01-01-2018, 08:34 PM   #1
Fletcher_DD-537
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Default Weird amp question.

My Mustang III is having power amp issues- the volume cuts in and out. I;ve fiddled around with it, an isolated the problem to the master volume- so it's got to be the power amp. They were dodgy from the start on these V1 amps, which is why V2 came out so quickly.

Anyhow, my questions are thus:

1) is it SAFE, and would it work- if I plugged the fx send of this amp into the fx return of my Bassbreaker combo? Basically connecting the preamp section to the power amp of another amp. I would have both amps plugged into the same outlet, so no voltage issues/ phase problems.

I like some of the effects on my Musty, esp the vintage tremolo, and have some patches set up that I like.

2) is it worthwhile to hunt down a new power amp for the Mustang, and to get it installed?

3) If not, how difficult would it be to install a speaker plug into a Mustang? So I can use it as a 1-12" cab for my Super Champ X2. Other than the power amp problem, there's nothing wrong with it and I don't want to just chuck it. It's got a Celestion G12T-100 speaker in it that sounds very good, and might just make a good extension cab for both of my other amps.

3A) btw, anyone know how many ohms that speaker is?
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Old 01-01-2018, 08:41 PM   #2
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I had a similar problem on an amp and it turned out to be a faulty input jack. I replaced the jack and the problem disappeared. The tone improved.

Are you certain the master volume potentiometer isn't dirty? Did you spray contact cleaner into the pot?
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Old 01-01-2018, 08:59 PM   #3
Fletcher_DD-537
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Quote:
Originally Posted by Hi Hat View Post
I had a similar problem on an amp and it turned out to be a faulty input jack. I replaced the jack and the problem disappeared. The tone improved.

Are you certain the master volume potentiometer isn't dirty? Did you spray contact cleaner into the pot?
The volume gets softer and louder at random when the pot is untouched. If you adjust the MV, it'll often come back up- but it eventually starts wavering again. Does NOT happen with headphones plugged in. (which is why I didn't consider an input jack problem)
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Old 01-01-2018, 11:15 PM   #4
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I had the same problem with a Line 6 Spyder 2, 75 watt amp. I ordered a new board and installed it. CRAP, the problem was still there. That only left the output power amp. I can plug the guitar in it and pick up the signal from the headphone jack and run it into any other amp I want. I didn't have to do any Send/Receive loop things in order to use the whole preamp section. I have a second Line 6 Spyder amp so that Spyder 2 is out in the shed. I'm considering using it as an external speaker cabinet. Can I fix the amp? Yep, but I'm too damn lazy to hunt down the correct output power amp parts and relace them. Line 6 can send me what I need but they charged me $75 for the brains(preamp board) and I ain't paying their prices for $5 worth of parts. If your amp has a headphone jack, its probably stereo, like mine, and you can split the signal and run it into two amps or a large Stereo amp. If you run it into two amps or a large stereo, the stereo chorus is KILLER because you can hear it moving back and forth.
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Old 01-02-2018, 02:29 AM   #5
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It does have a headphone jack. The amp is not stereo, but several effects are, so what you described would likely work.

However, neither of my other amps has an auxiliary input, so I've got nothing to plug it into... Only way I can think of is via the fx loops.
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Old 01-02-2018, 05:17 AM   #6
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Quote:
Originally Posted by Fletcher_DD-537 View Post
It does have a headphone jack. The amp is not stereo, but several effects are, so what you described would likely work.

However, neither of my other amps has an auxiliary input, so I've got nothing to plug it into... Only way I can think of is via the fx loops.
When you split the left and right signals, just put a 1/4" guitar plug on each and plug into the guitar jack of each amp. The signal is plenty strong enough to drive the amps.
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Old 01-03-2018, 03:14 AM   #7
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An alternative to PK's suggestion is to connect from the effects loop send on the Mustang to the effects loop return on the Bassbreaker, which you did ask about in your first post.

There is no danger/safety issue doing this.

There is one potential problem when you use more than one amp at the same time, a "ground/earth loop".

The signal from the guitar needs a connection to the electrical ground/earth. When you are only using one amp, that connection is through the amp's ground/earth connection at the power outlet.

When you are using one guitar to send a signal to more than one amp at the same time, each amp will have it's own separate ground/earth connection at the power outlet it's plugged into, and that's where you could have a problem. It's not a safety problem, it's an audio problem. One guitar signal with multiple ground/earth connections can create a signal loop through those ground/earth connections, causing excessive signal hum through the amps.

The solution is to use an "isolation transformer" in the signal path for every amp you DON"T want a ground/earth connection to the guitar. The guitar should have only ONE ground/earth connection through one of the amps, but not through any of the other amps.

So how would you set this up? In the case where the guitar is plugged into the Mustang and then the signal from the Mustang's effects send is connected to the Bassbreaker's effects return, IF you have a ground loop problem between the two amps, you should connect the Mustang's effects send to an isolation transformer input and then connect the transformer's output to the Bassbreaker's effects return. This will isolate any ground/earth connection from the guitar to the Bassbreaker but leave the ground/earth connection intact between the guitar and the Mustang. The Bassbreaker will still have a safe ground/earth connection through the power outlet it's plugged into, but the transformer will isolate the Bassbreaker's ground/earth connection to the guitar.

You might not need an isolating transformer when you connect two amps together through their effects loops, but if you try that connection and you are getting a lot of unwanted hum, the most likely cause will be a ground/earth loop between the two amps and the guitar. An isolation transformer that is in the signal path between one of the amps and the guitar is the way to get rid of that hum.

NEVER, NEVER, NEVER, disconnect the ground connection of an amp by cutting off the ground pin of it's electrical plug or disconnecting the amp's ground wire to the power outlet. The amp NEEDS that ground/earth connection to operate safely. People have DIED from LETHAL electric shocks from ungrounded amps.

When you are using one guitar with more than one amp at the same time, you need one clear connection from the guitar to the ground/earth through one of the amps. Any other ground/earth connection from the guitar to any other amp may need to be isolated to eliminate excessive hum formed by a signal loop between the guitar and multiple connections to the ground/earth. Do not isolate every amp, leave one amp un-isolated (to give the guitar one connection to ground/earth) and then isolate every extra amp you are trying to use at the same time.

So, assuming you think you do need an isolating transformer, how much do they cost? Many of them are advertised as "hum eliminators" and passive versions which do not need a power connection can be as cheap as approx. US$20 for a basic unit Many signal DI units can do the same thing if they have a "ground lift" capability. Price will vary based on features built into the DI unit.
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Old 01-03-2018, 01:49 PM   #8
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I have both a Mustang amp and a Super Champ X2. I've never found a schematic for the Mustangs, but I have the schematics for the Super Champ. These amps are quite similar, in that they use a digital processor for the pre-amp and effects, followed by a solid state (Mustang) or tube (SC) output stage. The volume pot on the Super Champ is simply another input to the digital processor. It's not part of an analog attenuator between the pre and output amps, as your original post would imply. Try cleaning the pot. If that doesn't work, try changing the pot. The problem could be the power amp, but it's more likely a dirty or volume bad pot. Good luck, and let us know how you make out.
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Old 01-03-2018, 02:59 PM   #9
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Quote:
Originally Posted by pdawg0 View Post
I've never found a schematic for the Mustangs...
Links to Mustang schematics and board layouts are here:

https://forums.fender.com/viewtopic.php?f=27&t=100594
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Old 01-03-2018, 04:03 PM   #10
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Thanks, Big John. Looking at the Mustang III schematic, the Master Volume is not an input to the digital processor, like I thought. It's actually a double pot that controls the gain of an op amp circuit, one op amp for each stereo channel (left and right). The master Volume also effects the headphone out, and I would not expect both sections of the pot to be faulty in the same way, at the same time. After seeing that, I'd also lean toward an issue with the power amp. But spraying some cleaner in the pot is cheap and worth a try.
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