AVT 275 FX Loop Mod

[beammeupwelshy]

This might have already came up before, but thought I would address this as I reckon it makes my 275 a way more versatile amp.

The FX loop in these tube preamp, stereo solid state power amps is parallel.

Meaning it splits the preamp output and sends some to the power amp and some to the FX unit your using. This is great if you want to use Reverb or Delay etc, not so great if you want to stick a Boss Multi-FX unit in the FX loop for example. Do you really want your chorus or pitch shift to only be half there when it gets mixed with the preamp signal before heading into the power amp? Not me. Really don't see the point of a parallel FX loop on a guitar amp.

I wanted a serial FX loop so I could decide on how much of an FX/signal ratio I want. Don't even get me started on the 4 Cable method.

So anyway I asked Marshall to send me the amps schematics, which they did, (didn't think they actually would but they are cool guys). Me and a friend of mine who is electrically more clued up than me took a look. We figured Resistors R91 and R92 could be disengaged to completely separate the preamp from the power amp. Its a pretty easy mod, I cut out the two 15Kohm resistors and put a switch on the back panel, I ran shielded audio cable to the switch from the board resistor points and hooked this up to the switch (you put the two resistors back in at the switch end).

The resistors are located behind the front panel. You have to take all the knobs off, pop the board off its tags and carefully pull it out of from the insides of your amp. I didn't have to disconnect any other cables or plugs to get it into a position where I could work on it. Solder your two pairs of wires into the resistor contact points and put a switch on the back panel. There are looms of cables you can cable tie these new wires to and then just stuff it all back together. Be careful! plus stay away from any thing that looks like big capacitors. I'm no electronics expert but if you know what a soldering iron is and can figure out how a switch works then your sweet.

Now you have a serial FX loop, or you can flick the switch and have it back to its filthy parallel state.

One thing you should note is that in Serial mode, you need the FX loop Level turn all the way up and the preamp out has to be "physically" connected to the power amp either by an FX Pedal in the loop or just a short patch cable.

One little quirk is of this is the Digital onboard FX are still conected to the power amp at all times. This means you can select the serial mode on the newly installed switch and have your preamp disconnected to the power amp, play your guitar, and just have the completely wet FX sound of the on board FX's playing. Kind of cool, but Ive never used it other than just mucking around.

Anyway, this really helped me out, now I use a Digitech RP1000 in a 5 cable method (Stereo FX RTN), and the whole thing works really great, I can also unplug everything flick the switch and its back to being stock. Oh, and its stereo, only thing i wish is that Marshall would make a simple (no modes) four channel stereo amp. Then I could have power amp valve tones. I know i could go rack stuff but I just want a half stack that stereo and has four simple "as me" channels. Maybe somthing like a DSL with selectable modes and a master volume section. Come on marshal, I'll buy one.

 

[thrawn86]

This is cool. Man, you should post the schem up......they are very hard to find!

 

[beammeupwelshy]

I would but they are at my old's house 700Kms away. Though I'm pretty sure I saw .pdf on the net. I have one but it says its too large to load. I can't remember where I got it from now either.

The file is "Marshall AVT150 Complete" It has the extra bits for the 275 in it as well.

 

[beammeupwelshy]

Maybe I can compress this bad boy now i think about it.

That's if i can figure out how to work the forum tools.

 

[beammeupwelshy]

Wont let me add an attachment anymore, that's weird. Maybe the admin has block this feature for me or something.

 

[beammeupwelshy]

No, I'm just retarded. I figured out how to attach stuff. But even compressed its too big for a .pdf file.

But some kind person has already stuck it up on another forum. Page three on the pdf, Resistors R91 and R92.

Marshall AVT50 Valvestate 200 Combo blowing main fuse

FYI: The AVT 150 and the 275 are basically the same but of course anything to do with the stereo power amp section has two things instead on one thing.

 

[bullseyeuk]

I've been looking for this mod for a couple of years now. Is it possible that you could be more specific with the instructions? like what type of switch and how to wire it. if you could post or message me a quick sketch it would make my life a lot more complete.

 

[beammeupwelshy]

Yeah cool man, its fairly straight forward.

1 - Pull the amp out of the box. Take a quick second to note where the filter caps are, I don't think these are as dangerous as the ones on valve amps but just don't touch anything near where the IEC cord goes in.

2 - Pull all the knobs off, because you need to get at that board. I think from memory once all the knobs are off you just pop the board off the little plastic dogs. I didn't need to disconnect any other plugs (I'm fairly sure), its a bit tight but you can work on it like this.

3 - Find R91 and R92 they are labelled with little white writing. Desolder these and pull them off the board, save them for later. If you have a solder sucker, suck the solder out of the holes, this will help a bit to solder the wires to the board.

4 - Using some kind of Low level audio cable probably a single core with a shield (pretty sure that's what I used), solder the wires to the board terminals where the resistor used to be. You then run the wires via some other wires to the back of the amp where you will drill a hole and put the switch in. I think I cabled tied the wires to a bunch of thin grey wires that were on the same board. I don't think it matters too much, but try to make it neat and keep them away from anything that looks either high voltage and the valve of course, just because you don't want any noise to get into your signal. Plus you want to keep it safe.

5 - Solder the wires to the switch, you put the resistors you saved here, just because its easier to do.

Now you have it so with the switch on or off (depending on which way round you want the switch) you can have the FX loop as it is factory, or fully serial.

Note: When in moded mode you physically break the connection between the pre and power amp, so you have to patch the FX send to the FX rtn for it to work. So you either use an FX unit in the loop or just a little patch cable. Or you just flick the switch and its back to normal with a parallel FX loop.

I think that's about it from memory, if you are used to soldering and stuff like that then its really easy, if you aren't, I'd say give it go but be really careful and don't rush it.

I will put up a diagram of the wiring soon.

 

[beammeupwelshy]

Hope this helps.

 

[bullseyeuk]

Much appreciated. i'll try he mod this weekend and let you know how it goes. i'm excited now i have a new project

 

[bullseyeuk]

Well it works. excellent mod keep up the good work

 

[beammeupwelshy]

Good stuff.

I really don't know why they didn't just make this amp so you can have a serial FX Loop in the first place. I can't imagine a situation where I'd want a parallel FX loop on my amp at all. Every dedicated guitar FX unit has a wet/dry mix and your supposed to send the whole signal through them and tweak to suit, and every studio style FX unit has a wet/dry mix so you can control things to suit your situation, because in a studio you might want a serial or parallel FX loop depending on what your doing. A parallel loop (on a guitar amp) basically limits your options big time.

I guess they did it because its stereo and they wanted to be like "run your stereo reverb through this bad boy" but what if I want to use my stereo ring modulator or stereo whammy pedal or a de tune FX or anything that requires full signal processing, oh well you can always "run your stereo reverb through this bad boy". Oh, cheers.

Anyways rant over hope the mod continues to work well for you.

 

[iseverynamegone]

Nice thread to find as i searched 'why does my voodoo lab chorus sound weak in the effects loop of my AVT275'. Could I not just take off the 2 resistors and leave them open and then i will have a permanent serial effects loop ?

I dont imagine that, having run my Chorus through it as is, that I will ever want it back parallel?

 

[beammeupwelshy]

Yep, but just remember that it separates the power section from the preamp section, so if you cut them out permanently from now on you will have to always patch the send to the return either with your pedals or just a cable if not using pedals, and also turn the FX loop knobs all the way up. But yeah, you'll have what you want.

I put a switch in so if I wasn't running pedals I didn't have to think about patch leads, it just goes back to normal function. But I totally agree, I don't think I ever used the parallel ability on mine since I modded it, so realistically, just cut em' out and have a more usable amp

 

[BTdave]

Hello,

So does this mod stop the signal from being split for the parallel loop all together, or does it just disconnect the dry path from passing through? I'm concerned about volume/signal loss. Thanks for the info by the way. Just bought the foot controller only to find out the loop is useless in its current state.

Thanks

 

[beammeupwelshy]

Hey.

Yeah, it disconnects the pre amp and power amp sections, so you physically break that link. This means that you either patch the pre out to FX return or you stick an FX pedal in the loop, either way you have to patch it, as it's now fully serial. However there is an internal signal path to and from the onboard Digital FX that is still working, bascially you can either just turn those FXs off on the pedal or use them if you want, they don't caue a problem as they are purle wet signals (100% FX), but because of the mod the FX RTN knob for the digital onboard FX becomes more like a volume knob for the wet signals as aposed to a mix knob. No real big deal, plus you can have fun with the pure 100% wet FX rtns if you want to.

You won't have any trouble with signal loss as long as you turn the FX loop level all the way up on the front panel, and have an FX pedal that has a volume control to make up any gain loss by using it, which probably won't be any anyway.

Bear in mind the only reason you'd do this mod is to use an FX that requires a fully serial loop, such as a Boss multi FX or whatevers.

Hope that helps.

 

[BTdave]

The issue we have is using boss type effect pedals in the loop. Phaser is the worst problem because even with the phase maxed you still hear the dry. I'm not talking about volume loss with specific pedals really, just overall max volume of the amp being affected by this mod for those of us who dont use all in one effects units with a level control.

 

[beammeupwelshy]

Ok, yeah this mod should sort you out. Floor pedals like those have enough gain to make up any loss (if any) since they pretty much work at the same audio level as the preamp does. Then of course after the mod you can choose the amount of wet and dry from the pedal, like it should be.

Since I did it Ive never used the loop in its parallel state since most FX units and pedals for guitar aren't designed to work that way, most of them just expect to see a serial loop, so I say go for it. But if you mod it with a switch you can put it back to factory if you for some reason ever want a parallel loop again.

Cheers.

 

[BTdave]

Okay, I'm using the head version so there was only 1 resistor to remove (r91) because it is mono. Haven't had a chance to test it yet, but should be easy to reverse if there's a problem.

 

[beammeupwelshy]

Nice, let me know how it goes.

Cheers.