Marshall MG100HDFX. super help needed.

[ajkStrungtheoretic]

Hi, I'm the new guy around here so hopefully you'll forgive me if I break any forum taboos. That being said, I need some help before I do something stupid. Many moons ago I purchased a Marshall MG100CDR head, which of course was just Marshalls standard, 100watt, solid state head. I never used the direct out on the front panel of the receiver until I had the amp for several years. I really never had a reason for it since I would record tracks by simply mic-ing my cab. Well, long story short, I was lead to believe by an individual that if I used the direct-out on the CDR head, I did not need be hooked up to a speaker cab. They said that when you jacked out of the line-out, the head no longer sent a load to the speaker outputs. Low and behold, I later found this to be untrue because when I did just that, it fried my CDR head. I'm aware that the CDR head wasn't exactly top of the line, but I was extremely happy with the OD/2 channel and came to very much love its tone over the years. I've now purchased the upgrade, that being MG100HDFX head, which of course is basically the same thing with the churched up effects, emulated direct line out and the DDS feature. My question now is this: Can I use the direct line out on the HDFX head without sending a load to the speaker cab? Several knowledgeable people have assured me that I can do so with no risk of burning another transformer, but I'd really like to know for sure before I do something stupid and fry another head. Please help.

Thanks,
ajk

[ajkStrungtheoretic]

Really?

No one? I'm not sure how to bump posts on here so I'll just do this, bump.

ajk

[CharlieP]

I have the MG250DFX which is 50W each speaker. My manual says never operate with out speakers. If you are using the emulated line out you can cut the sound to the speakers by using the master volume control. This does nothing to the emulated sound from the headphone jack. It is controlled by the volume at clean section. Never operate without speakers plugged in. You can record from the emulated/headphone jack if that is what you are trying to accomplish.
Basically I have the same features as your head and can plug to larger speaker cabs.

[oc disorder]

Hi ajkStrungtheoretic and welcome to the forum.

Usually the rule of thumb is that a transistor amp can be run without a load
but a tube amp cannot be run without a load (speaker).
Having said that strange things do happen e.g. plugging the speaker in
with the amp on while holding your guitar may cause an unwanted oscillation
which pushes the amp. Well maybe a bad example...
When techs test an amp at full power they usually use a "dummy load" instead of a speaker.
This is basically a large wire wound resistor the same ohmage and wattage as the speaker .
So you could mount one in a box and wire a socket to it and plug it in instead of a speaker.
This is another alternative.
If you are going to do a lot of recording that way - you could take it to a competent tech and have him wire in a mute switch.
The TDA 7293 output chip that I think yours uses has facilities for muting and standby.
Its already being used to some extent with the Marshall circuit to mute it briefly on turn on.
Mounting a low profile slider switch on the back would enable to mute the speaker output with the speaker still connected.
If that is the chip in your amp this is the pdf document with the specifications and example circuits. Print it out and take it to the tech with your amp.
TDA 7293 120V - 100W DMOS AUDIO AMPLIFIER WITH MUTE/ST-BY
http://www.st.com/stonline/products/...re/ds/6744.pdf
and Twist has posted the amp schematic here:-
http://music-electronics-forum.com/t14816-post117358/

[CharlieP]

Hi ajkStrungtheoretic and welcome to the forum.

Usually the rule of thumb is that a transistor amp can be run without a load
but a tube amp cannot be run without a load (speaker).
Having said that strange things do happen e.g. plugging the speaker in
with the amp on while holding your guitar may cause an unwanted oscillation
which pushes the amp. Well maybe a bad example...
When techs test an amp at full power they usually use a "dummy load" instead of a speaker.
This is basically a large wire wound resistor the same ohmage and wattage as the speaker .
So you could mount one in a box and wire a socket to it and plug it in instead of a speaker.
This is another alternative.
If you are going to do a lot of recording that way - you could take it to a competent tech and have him wire in a mute switch.
The TDA 7293 output chip that I think yours uses has facilities for muting and standby.
Its already being used to some extent with the Marshall circuit to mute it briefly on turn on.
Mounting a low profile slider switch on the back would enable to mute the speaker output with the speaker still connected.
If that is the chip in your amp this is the pdf document with the specifications and example circuits. Print it out and take it to the tech with your amp.
TDA 7293 120V - 100W DMOS AUDIO AMPLIFIER WITH MUTE/ST-BY
http://www.st.com/stonline/products/...re/ds/6744.pdf

First Marshall explicitly says not to power this model amp without speakers. There is a reason. To do so on someones say-so is not the way to go. You already have an emulated/headphone jack. Control the line out with clean volume and turn the speaker volume off with master or at least turn it down some. Do not disconnect the speaker period. It ain't worth it! I have the manual and specifically says you can damage the OT by turning on without speakers..

[oc disorder]

You're right Charlie P.
The master vol to the power amp is post the headphone send.
Should be in the manual.
But it has no OT !

[CharlieP]

You're right Charlie P.
The master vol to the power amp is post the headphone send.
Should be in the manual.
But it has no OT !

Whatever it has it says do not power without speakers. I just read it. I assumed it had some kind of OT or circuitry to do the same thing.

[ajkStrungtheoretic]

thanks guys.

I do appreciate all of the clear thought replies. I did see that bit in the instruction manual about NOT powering on the head without being hooked up to speakers, and believe me I did take heed the warning. On the other hand on no less than three occasions I've witnessed guys in the studio using solid state heads without a cab running direct out of the emulated or equivilent there of, into the board via standard direct out box. Two of the guys I've seen do it were using solid state marshall heads and neither of them ever had any burn out problems. After reading all of your replies perhaps its just a matter of luck that they haven't had any issues yet. I am aware that for silent recording one can simply bring a cab, run a line out of the speaker output into the cab and then easily turn down the master volume to facilitate silent recording. My only issue with this is the dragging of my bloody oversized cabs back and forth from the studios. I guess I'll just have to endure the heavy lifting, lol.

Thanks guys,
ajk

[CharlieP]

I do appreciate all of the clear thought replies. I did see that bit in the instruction manual about NOT powering on the head without being hooked up to speakers, and believe me I did take heed the warning. On the other hand on no less than three occasions I've witnessed guys in the studio using solid state heads without a cab running direct out of the emulated or equivilent there of, into the board via standard direct out box. Two of the guys I've seen do it were using solid state marshall heads and neither of them ever had any burn out problems. After reading all of your replies perhaps its just a matter of luck that they haven't had any issues yet. I am aware that for silent recording one can simply bring a cab, run a line out of the speaker output into the cab and then easily turn down the master volume to facilitate silent recording. My only issue with this is the dragging of my bloody oversized cabs back and forth from the studios. I guess I'll just have to endure the heavy lifting, lol.

Thanks guys,
ajk

Or they had a dummy load on the outputs.

[Enzo]

The amp doesn;t need a load. What the caution is about is never to plug in speakers with the power on. Assuming you will be playing through speakers, that means don;t turn it on before connecting speakers.

And likewise never pull teh cord out while the amp is powered.

I swear to god you don;t need dummies on solid state amps.

[CharlieP]

The amp doesn;t need a load. What the caution is about is never to plug in speakers with the power on. Assuming you will be playing through speakers, that means don;t turn it on before connecting speakers.

And likewise never pull teh cord out while the amp is powered.

I swear to god you don;t need dummies on solid state amps.

It isn't stated that way and a lot of us dummies have them.
I am one dummy that wants to keep mine working right!

[ajkStrungtheoretic]

The amp doesn;t need a load. What the caution is about is never to plug in speakers with the power on. Assuming you will be playing through speakers, that means don;t turn it on before connecting speakers.

And likewise never pull teh cord out while the amp is powered.

I swear to god you don;t need dummies on solid state amps.

Enzo, I've read through some of your posts and replies and have come to conclusion that if you're not Jim Marshall himself, then you're closest damn thing that I'll come to corresponding with him.... joking of course, but bottom line, I can tell that you know what the hell you're talking about, at least in the case of Marshall amps, apparently. So, basically what you're saying is that in case of the MG100HDFX head, one is not required to send a load to a speaker cab? Not that I have just cause to disbelieve you, but when you think about it, it does stand to reason that when powered on, the amplifier has to send a load somewhere, and when it doesn't, bad things could happen.. That is, unless Marshall has some sort of feature that, when jacking out of the emulated line out, the amp ceases to send power to the power outputs on the rear of the amp itself. If this actually IS the case, then...well, damn, thats kind of cool, but at the same time I know that several people, myself included, sometimes enjoy recording via emulated line out AND micing a cab speaker at the same time as it gives a bit more to play with when it comes time to mix everything down. And, as knowledgeable as everyone seems to be around here, there still doesn't seem to be any sort of consensus one way or the other.

ajk

[Enzo]

You will never go wrong being cautious. Charlie clearly doesn't accept my view. And he will never blow up an amp doing it his way. I have no doubt in my mind that I can run SS amps flat out without a load, I do it everyday on my bench. Ther is nothing special about a Marshall SS output stage, they are the same as any other. MF350 excluded. I am not going to guarantee your amp will never fail. I will imagine any failures that do occur would not be from lack of load. But that is my opinion, nothing more.

SS amps do not push power anywhere, they make it available. If you have a SS amp putting out 100 watts into 4 ohms of speaker load, then without changing ANYTHING else, connect it to an 8 ohm load, you will find the amp now producing 50 watts. Change again to 16 ohms and the amp produces 25 watts. 32 ohms? 12.5 watts. 64 ohms? 6.25 watts. Not much stress on that amp now. 128 ohms? 3.12 watts.

No one has 128 ohm speakers of course, but you can plot output power for that same amplifier output signal all the way out to infinity. It gets lower and lower. And into an open load, infinite impedance, the amp produces zero watts.

[CharlieP]

Not that I don't believe you. You know what you are doing. I don't and I would be the unlucky one.

[Enzo]

And if you get good reliable results, then I am not about to tell you to change anything.