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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Thu Nov 15, 2018 9:31 pm
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https://music-electronics-forum.com/sho ... hp?t=23380

From this discussion, the conclusion appears to be that
solid state amps which lack output transformers, won't be damaged
by running without speakers connected. It is mentioned that
very rare exceptions among the ranks of very old solid state amps,
may exist. But not in the cheap new amps.
Cheers


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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Fri Nov 16, 2018 2:25 am
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Location: England
It is true that some solid state amps can run without a speaker load but I have seen (Marshall) solid state amps where you are warned not to switch on the amplifier without a speaker attached. Headphone jacks vary. Some take the speaker output, but through a dropper resisitor as the full power would ruin the headphones. Some amps have a separate low wattage amplifier for the headphones. This is inserted between the preamp and the power amp and disconnects the circuit into the power amp so that it produces no output. I haven't seen the schematic for a GT40 so I can't provide a definitive answer other than a 'safe' one.. Perhaps you should ask Fender tech support about this?


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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Fri Nov 16, 2018 12:03 pm
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Thanks for the many detailed replies! And sage wisdom.
I'll track down a support ticket area.
edit: support ticket submitted, with reference to this topic

(and if a Fender rep with the knowledge of GT40 safe-usage status
when running it without speakers connected, happens to read this,
sharing that official knowledge
would be great. Extending an out-of-warranty product's
useful state of being is a good thing.

The dummy resistors aren't expensive, or large,
so they are definitely a good option.
Cheers


Last edited by guildorf on Fri Nov 16, 2018 12:23 pm, edited 1 time in total.

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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Fri Nov 16, 2018 12:23 pm
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stratocaster1983 wrote:
Quote:
I think GT 40 speakers are 8 ohms total resistance,
4 ohms for each 6.5" speaker. If you keep your new speakers
in that ohms range, there shouldn't be any problem.


Ahem, in a stereo amp, the speakers are not connected in parallel. There are two separate power amp each with a 4 ohm speaker.


I only looked at this thread because it was the only one which looked recently active but couldn't help my reading flow being tripped up.

If the impedance of two 4Ω speakers is 8Ω then they would have be in series, not parallel.

But in congruence with your reply, if they were connected in any way in which their impedance could be calculated in combination, they would not be stereo.

Incidentally, impedance is the correct term for measurement of resistance to an alternating current such as an audio signal and is quite different as as impedance varies with frequency.


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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Fri Nov 16, 2018 4:56 pm
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guildorf wrote:
Does using the headphone jack bypass that need for load?
Thats the only way i use the Mustangs. How does the Mustang V
deal with that, does it have some resistor wired to the cab jack
that gets bypassed when a cab cable pin has been inserteded?



When you plug in headphones, the input to the power amp will be muted, so the power amp is doing nothing. However you can't use the headphone jack for an external speaker - you will need an amplifier to raise it to useful levels.


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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Sat Nov 17, 2018 2:48 pm
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I'm going to give you some tips and info you won't want to hear. If this is your first dive into Guitar amp electronics don't attempt this. Get a GT100. If you need more speaker area, adding an extension speaker to that amp isn't as risky. The stereo power section of the GT40 is comprised of 1 large scale integrated power amp circuit package that contains 4 amplifier sections that are usually not worth paying to repair if any one of them blows. The newbie possibility of blowing out the unrepairable power section is high. Why risk losing the amp? That being said, here's how it's done. Each stereo channel uses two of the 4 amps in a bridge circuit to drive a speaker impedance which should be indicated somewhere on the 6" speaker. In the GT40 case you must match the original speaker impedance whatever it is. Lower ohms may cause damage and higher will lose volume. With a bridge circuit there's no Ground and Hot like years ago. Both wires of each channel are hot and none of the 4 can touch any of the others or amp ground (or it can be over in a flash). Both Bridge circuits channels are OK if unconnected to a speaker but a miswire/short might fry the power IC even though there is circuitry in the LSI to help protect against that. You must use 2 separate Plastic body speaker Jacks ( 3 pole circuit shorting jack) that do not share any wiring (no old all metal jacks with common grounds). Each channel would have the speaker jack wired to definitely disconnect the internal speaker and run your external speaker since any Parallel connection would lower the impedance. It's certainly doable but this is a tall order with a high fatally ruin the amp risk if you haven't done this before. Get the 100.


Last edited by MYREL Tech on Sun Dec 02, 2018 7:54 am, edited 2 times in total.

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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Sat Nov 17, 2018 4:16 pm
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Thanks for the excellent info! I notice on my Mustang I V 1 that the
removable speaker cable connectors, have no exposed metal,
perhaps to deal with the inadvertant shorting possibility you mention
with the GT40 componants exposed to an unsuspecting world.
Perhaps that's industry standard these days.
Cheers


Last edited by guildorf on Sat Nov 17, 2018 8:11 pm, edited 1 time in total.

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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Sat Nov 17, 2018 5:25 pm
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MYREL Tech wrote:
[color=#000080]I'm going to give you some tips and info you won't want to hear…


Thanks. I might still do the dual external out… but I'm giving up on bridging to mono. In any case, I totally appreciate the info!


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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Sun Nov 18, 2018 7:34 am
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Just so it's clear, the GT40 has 1 LSI with 4 10watt amplifier sections inside it. Each of the two power sections uses 2 of the four 10 watt amps in bridge mode giving a 20watt output per each stereo channel into one (Guessing 4 ohm based on the Spec sheet for the main Power amp LSI chip) 6" speaker. Add both channel power ratings together and you have the 40watt GT rating. A single external channel would still be just be 20 watts at best. If you're already thinking about a 12" speaker in an external cabinet then get a powered 12" speaker and plug into the headphone jack. That's as reversable as it gets and keeps your GT40 ready to sell when the next great amp comes along in a couple of years. I'm kind of sensitive to hacking up equipment since I've seen a huge amount of well intentioned mods to amps and guitars go wrong and then I hear the owner's stories that always begin with a what seemed like a great idea at the start. "Measure twice..."


Last edited by MYREL Tech on Wed Dec 05, 2018 12:46 pm, edited 2 times in total.

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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Sun Nov 18, 2018 8:27 am
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Don't do it.
The Mustang amps use pwm outputs. That is, a 1-bit digital stream that is put into a harsh filter circuit to make it ac again.
The speaker is an integral part of the filter. Changing the speaker will produce undesirable effects to tone response.
There is also a feedback loop on the speaker circuit that adjusts for speaker distortion. A different speaker with a different specs will affect the tone in an unpredictable way.
Plugging in the headphone jack also lowers the gain of the amp so you don't blow your ears out. Hooking it up to another amp will give lower gain than running it through the Mustang speakers.
For all the effort to adapt the amp to extension speakers, you won't get what you wanted.


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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Mon Nov 19, 2018 9:45 am
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I don't have a GT40 schematic but this is my best guess as to the Power amp section. Learned some new facts about it. Fender has used higher power versions for the Mustang II series (like TDA 8905) so this should get the attention of you Jr. Engineers out there.

https://www.nxp.com/products/media-and- ... ts:TDA8566



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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Mon Mar 18, 2019 12:30 pm
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Were you able to open up your GT 40. If so, any advice for doing it?

I don't plan on replacing the speakers. My GT 40 is making a loud vibration/rattle type noise on certain notes. I'm hoping it is just something loose inside the amp that I could correct if I was able to access it.


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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Mon Mar 18, 2019 3:14 pm
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mminyard wrote:
Were you able to open up your GT 40. If so, any advice for doing it?


No, never got around to cracking that sucker open. BUT YOU SHOULD! And take lots of pics and post them all here!


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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Tue Mar 19, 2019 11:13 am
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Since my amp was still under warranty, Fender agreed to let me return it to them for service. So for now anyway, no taking apart by me. I'm hoping they can resolve whatever is causing the vibration/rattle type noise.


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Post subject: Re: Disassembling Mustang GT 40 / Adding an External Speaker
Posted: Sun Apr 28, 2019 6:59 am
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I successfully added an external speaker jack on GT40 today. the speaker grill was attached by 4 of very strong velcro tapes and it wasn't hard to access the speaker wiring.

the pictures are here
https://slowbean.net/thread/fender-mustang-gt40-external-speaker-mod/


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