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Post subject: Mustang III v.1 - input circuit question
Posted: Mon May 11, 2015 8:53 am
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A long time a ago I found that in order to get really nice clean sounds out of my Mustang III v.1, I had to turn my guitar volume down. I set my volume pedal pot so that the max volume was about 90% and have always used it with my Mustang.

Last night I plugged in a guitar without the volume pedal and was reminded how terrible it sounds without it. I'm pretty sure that there's a biasing problem at the input stage...most likely an op amp/ buffer. It sounds like there's a tiny bit of clipping.

So while I understand that Fender does not want to release the schematics, could you tell me if there is a biasing pot at the input stage that can be easily adjusted or is it all surface mount stuff that can't be easily modified?

I really don't want to pull it apart unless I know that there's a good chance I'll be successful.

Thanks!


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Mon May 11, 2015 10:44 am
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I have about 15 electric guitars (solid, hollow and semi hollow), which all sound pretty fantastic in my MII and MIV V1.1

can you tell us what kind of guitar you are using? perhaps active pickups?


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Mon May 11, 2015 11:55 am
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I'm using a Fender Strat and a Washburn HB-32 with Seymour Duncan 'Hot-Rodded Humbuckers'...no active pickups.


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Mon May 11, 2015 2:12 pm
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Adjusting bias is a function of output tubes, not solid state preamps like the Mustang. If your guitar is overloading the input stage of the amp, try lowering the pickup height. There is no sensitivity adjustment of the input stage on these amps. Some amps, like most traditional Fenders, have two inputs, normal and -6 so they can handle a wide range of input levels, but most solid state amps can handle plenty of level.


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Mon May 11, 2015 3:29 pm
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Apparently, your not familiar with IC circuit design. Yes, there are usually biasing resistors in the input buffer stage of any transistor amp. Usually there's no reason to change these. However, in manufacturing there are always some variations from one unit and the next. Op amps have different gain levels, resistors can be on the edge of tolerance...etc.

I'll open this up and have a look. I just hope it's not surface mounts - my eyes aren't that good anymore...

Also, my pickups are quite a bit further from the strings than spec.


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Mon May 11, 2015 4:17 pm
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I too have noticed often having to set the gain level of essentially 'clean' amps (65 deluxe, twin etc) to 1.0 (and set the bias quite low) in order to get a clean sound. Especially when using a heavy dose of the BigSky. I would've thought there'd be alot more headroom before breaking up.


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Tue May 12, 2015 12:56 am
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Seems like there's two different things being discussed in this thread.

The OP is concerned that the initial analogue stage of the amp, before the signal is digitised for DSP processing (amp modelling etc) may be faulty, and is clipping in an unwanted / undesirable way.

The post above ("set the gain level of essentially 'clean' amps (65 deluxe, twin etc) to 1.0 (and set the bias quite low) in order to get a clean sound") is talking about something different: deliberately-created overdrive distortion within the DSP amp models. Turning down the gain on the modelled amps wouldn't have any effect if the problem were the OP's assertion of clipping in the pre-DSP input stage of the amp.

Ref the second problem above, I just tried the '65 Twin model on my Mustang, using a Gibson Les Paul with standard humbuckers - so reasonably high-output passive pickups but admittedly not the hottest available. I can turn the gain up to 6 and it still sounds clean (remember that the Mustang's "gain" control for the '65 Twin amp model corresponds to the "volume" control on the real amp, as explained in other topics on this forum). Do you have any other effects active (either within the Mustang or external) that could be contributing to the unwanted distortion?

Ref the OP's problem: it's always possible you have a faulty amp, along the lines you suggest. More likely you are just overdriving the input. Can you measure the AC voltage your guitar is sending into the amp? I note the specs for your SD 'hot-rodded' pickups say they have a higher (than usual for humbuckers) output, but don't give specifics. For comparison, my LP as above generates a few hundred millivolts RMS if I play it really hard and I have no problems (that I can hear) with clipping at the amp input. Do you have any other equipment at all connected to the guitar or amp, or in the signal chain anywhere?


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Tue May 12, 2015 2:11 am
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My point was I find the input sometimes seems too "hot", sometimes even dirty-ing up the "clean" amps even on low "gain" settings. Turning down my guitar volume would correct this as it is lowering the input signal to the amp so I thought it was a similar issue.


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Wed May 13, 2015 8:14 am
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What pot value in the guitar? In a amp theres a resistor that goes from hot to ground essentially paralleling your guitars pot. Usually a 1M in a tube amp. You can try a 250k if the pot is 500 or 1m. But I would use a meter and plug a cable into the amp, then measure the resistance from the tip to ground of the other end. Whatever it is add another one depending on what the amp's input measures in the guitar from the output lug of the pot to ground. Or pop the cable end if it's not shrink wrapped and add it from hot to ground of the plug. If that works you can make a box with 2 jacks and that resistor value whatever works. Or open the amp up and put it from the in jack's hot to ground. Of as i mentioned add it to the cable and you can have one cable for the mustang and use another for any other amps you have.

You said when you don't use the vol pedal is when it sounds bad. When you use it you are paralleling resistance with the input so thats why i suggested this. It's the same as using the pedal but set at full volume. In fact, you might want to measure the pedal's pot and use a resistor of that value. However, then when you use the pedal it will drop the signal probably much too low if you have that resistor in circuit, so it might be best to make a cable with resistance like i said because that isn't permanently in circuit as it would be in the guitar or amp and when u use the pedal you can just use a different cable. I really can;'t imagine having the issue you do tho ! Unless your pickups are uber hot. The fender models in mine with gain way down couldn't be cleaner. But as i've said many times, the mustang's fender models are not near as clear and pristine as the real thing, but thats not because of any input level.


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Wed May 13, 2015 4:50 pm
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Thanks all for the suggestions. I should point out that when I use the volume pedal or turn my guitar volume down, the sound I get is extraordinary. I'm kind of freak about clean, full tone.

This problem happens using my '75 Strat single coil neck pickup, so I don't think that it's a problem with high pickup output. This doesn't happen on my other amps. For now I'll continue to use the volume pedal. Someday when I'm in the mood I'll pull the Mustang apart and have a look at the input stage. I just wish I could get an answer about whether this is all surface mount electronics or not. If it is I won't be able to do anything with it one way or another.

BTW, I really do think that there's some slight clipping going on. Even when I use any of the high gain/high distortion setting & effect, it sounds an awful lot better with the volume turned down or using the volume pedal.


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Wed May 13, 2015 5:14 pm
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One other thing....

I'm PISSED OFF that Fender has stopped producing the 3250H super bullets! I used to use the pure Nichol 3150Hs, then they stopped making them, so I switched to 3250Hs. Now they're not making them either anymore.

I guess I'm gonna have to either switch to 3150Ms (I need a wound G Stirng!) or go with D'Addarios and give up the bullet ends. Either way it's a total bummer!


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Thu May 14, 2015 3:41 am
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Quote:
I just wish I could get an answer about whether this is all surface mount electronics

There are some topics in this forum with pictures of the amp's innards and circuit boards, for example:
viewtopic.php?f=27&t=86283

It looks like it's mostly custom ICs and surface-mounted stuff.

An alternative suggestion: if you really think it's just a case of the input stages being overdriven (for whatever reason, possibly an amp problem rather than your guitar/pickups/etc), then why not just connect a simple potentiometer (ie volume attenuator) inside the amp, between the input jack and the circuit board. It looks like the input jack is a separate component, conventionally wired, so this would be a straightforward modification. You could then just set the potentiometer to 90% or whatever value works for you.


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Post subject: Re: Mustang III v.1 - input circuit question
Posted: Fri May 15, 2015 4:47 pm
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Thanks! Looks like that answered my question. I'll stick with the volume pedal - that's working pretty well for me.


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