Microfet 100

[Harley Hexxe]

Hey Richard,
   I know it depends on the amp. Generally when it comes to guitar power amps though, they don't usually like to see each other.

[rnolan]

Hey Harley, I suspect you are correct, and you don't see allot of models capable of bridging, seems Carvin have decided it's a good feature, I think they have been offering it on many of their amps over the years.  I was having a discussion the other day with an Audio File tube nut friend, he's bridging his stereo leak tube amps, needs some circuit tweaks but the result is the 2 output transformers purr as they don't have to work as hard.

[Harley Hexxe]

Hey Richard,

     While that may look good on paper, and sound good in theory, it may be a disappointment in the end. After all, the character of an amplifier is the result of the output of the preamp and the poweramp working together. If the preamp output is strong enough, and feeds the input of the poweramp hard enough, then that will drive the output transformer harder. The sum of all these things is what gives the amp it's great overdrive tone.
    At least that is how vintage tube amps work. That over-driven tone is what every amp maker in the world is trying to reproduce, including ADA. That's the problem. They've taken the part of the equation I mentioned above, out of the picture, and replaced it with pristine amplification, and compensate for this by putting all the gain in the preamp.
    Your 50 watt Marshall is a perfect example. Disconnect the poweramp section of the amp, and in it's place, use your Carvin. Will it sound as good? Better? Not as good? Or maybe not good at all?

        Harley 8)

[rnolan]

For what he wants to do it's a good idea as he's chasing supper clean HiFi.  I see it as 2 different approaches and to some extent objectives.  If you want the preamp/poweramp sound then what MJMP does is probably the closest using ADA MP1/2. And this works well when you can play loud (a constraint we all find ourselves with).  I use a power soak with the Marshall and run it really hard, the soak allows others to share the room  >:D , and me to maintain a sound I like.  The other approach is, as you say, put it all in the preamp, and use clean transparent poweramps.  Although some look for some more warmth in their poweramp, whatever gets the sound you're chasing.  And some use the MP2 cab sims and use PA style cabs.

I must say though, there is nothing quite like the Marshall fully cranked, no soak, nobody to tell you to turn it down, it's a wild journey full of energy (I'm sure you know..)  My current record is it could be heard 13kms away ::)

[MarshallJMP]

That's the problem with tube heads,you have to cranck them,especially the older ones.And usually it's just too loud.And yes you can use a load to tame down the volume or like they are doing now lowering the supply voltages but it just doesn't sound the same.Also in a head the preamp and poweramp share the same power supply which also has an influence on the sound and the louder you get the more influence it will have.

Now as far as bridging amps goes, I never liked the idea.Maybe a brief explanation on how bridging works. So you have 2 poweramps and what they do is feed one amp the straight input signal and the other amp the same input signal but with a 180° phase shift. So one amp will do the positive signal while the other one does the negative signal. This will double the output voltage and current. That's why with a amp that can handle 4 ohm/channel you can use only an 8 ohm in bridge mode or else you would blow up the amps. Now on the output side one output is connected to the + connection of the speaker and the other amp on the - of the speaker.So you have to keep that signal isolated from any ground connections.So plugging a bridge ouput into let's say a microcab will blow the amp that drives the - of the speaker.And in case of the MT amps the B channel. Maybe this has got something to do why the B channel is mostly busted with these amps.You will see on the MT200 the bridge out jack is isolated from the chassis.

[Dante]

Now on the output side one output is connected to the + connection of the speaker and the other amp on the - of the speaker.So you have to keep that signal isolated from any ground connections.So plugging a bridge ouput into let's say a microcab will blow the amp that drives the - of the speaker.And in case of the MT amps the B channel. Maybe this has got something to do why the B channel is mostly busted with these amps.You will see on the MT200 the bridge out jack is isolated from the chassis.

If I remember correctly, I blew the channel in my MT100 (from Richard) by connecting the cables while the amp was on. Don't do what I did 

[MarshallJMP]

That might not be a good idea with these amps and actually any amp.

Hey Dante, did you check the fuse inside the MT100? There are 2 glass fuses on the PCB board,it could be that one of these is blown. These are 3A fast blow. Now if you are lucky the fuse blew before the mosfet's blew. So you could check them out (only one will be blown) and replace and see if it works. If it blows again then the power mosfet's are gone.Now if this happends you can always the 2 mosfets and 3 driver transistors.It will cost you less then 10$ in parts.

[Harley Hexxe]

@MJMP:
   You finally answered the question I started asking 9 years ago when I first joined this Forum! That would help to explain why the B channel of the MT 100 is always blowing. There has to be a way to isolate the ground in them to stop them from blowing the B channel. With that information, maybe I can come up with a way to correct that.

@ Dante: That is what I was doing with my other Classic when it blew, I was hot-plugging in and out of the effects loop. So, the moral of the story is: Always turn off your ADA gear when changing connections!

Harley 8)

[MarshallJMP]

Oeps sorry it took so long 

Best thing to do if you use the bridge out is using plastic jacks ,this way the B channel stays isolated.If you use metal jacks the metal is connected to the out of B channel and if it touches a ground it will blow the B channel.

Now I find it strange that you blew up your classic, normally hot plugging an effectloop should not damage anything.With an amp it's a different thing but an effectloop... ?

[Harley Hexxe]

MJMP,

    Well that is what I was doing when that Classic blew. I had the idea of trying to run a parallel channel in my effects loop, so I had the ADA effects in one channel, and was just going to plug a short patch cord into the other channel to even out the balance between the loops. That was when the preamp died.

     Harley 8)

[MarshallJMP]

And it's completely dead or ... ?

[Harley Hexxe]

MJMP,

    It's dead in the sense of any sound coming out. It turns on and goes through it's diagnostics.

[MarshallJMP]

So nothing at all, soundwise?

[Harley Hexxe]

It's been a long time since I tried it, but I believe no sound at all. IIRC, this is the one I tried to change a cap and maybe a resistor in the back corner of it, but my soldering iron wasn't hot enough.

[MarshallJMP]

Ah you had problems with the tube channels?