Marshall MG250DFX Dummy Load Hook Up

[Hawk]

Hello Everyone,

New to this forum. New to some aspects of guitar amps,or perhaps many!. Bought a Marshall MG250 and wanted to hook it up to my Weber Dummy Load Box, but I soon realized that there are two speaker outputs on back of amp, left and right, min 8ohms, 2 X 50w rms into 8 ohms. My first guess is that I need a second dummy load. I understand how parallel/series speaker hook ups work but not quite sure if I can hook up the speakers together and run a line into a quarter inch speaker cable into the dummy load. The speakers are 8 ohms each. Would they be hooked up internally in series, 16 ohms? If they were parallel they would be  4 ohms, yet the caution on the back is 8 min ohms. The two outputs have thrown me off! Can I somehow hook this amp into one dummy load? Thanks very much.

[g1]

  No you can't run both outputs into the dummy load as there are 2 separate power amps (stereo).
But what are you trying to accomplish?  Dummy loads are only required for tube amps.
You can run a solid state amp happily with no load.

[Roly]

there are two speaker outputs on back of amp, left and right

The speakers are not connected in series or parallel, they aren't connected to each other at all.  This is a stereo amplifier, i.e. it has two output stages and hooking their outputs together is a short path to disaster.  Yes, on the face of it you need two dummy loads.

Here is the circuit;
http://www.amparchives.com/Amp%20Archives/Marshall/Schematics%20&%20Layouts/MG%20Schematics/Marshall%20MG250DFX.pdf


Yes you can connect your dummy load to one side, left or right, and simply disconnect the other side (say by inserting a blank plug in that speaker socket) but this will defeat some of the Fx with depend on a stereo output, and make them sound lifeless or ineffective.

As g1 asks, what are you trying to achieve here?

[Enzo]

yes, what are you trying to accomplish?

In a tube amp, a dummy load is used so you can crank the amp up to power tube overdrive, resulting in a different distortion sound compared to preamp overdrive.  The dummy soaks up the excess power so you can hear the distortion  without it being deafening.

But in a solid state amp, the power stage doesn't overdrive like that, all it does is clip or go into limiting.   So really all you have to do is turn down the master.  A dummy load doesn't do anything for you.

And as others mentioned, this is a stereo amp - two separate amps and speakers, left and right - so even if you wanted to do it, you'd need two loads.

Now if you are planning to use the amp as a preamp for recording or something, you can simply disconnect the speakers.

[Hawk]

Thanks for all the input everyone!. I want to open up the amp and test voltages etc.to help with my knowledge of transistor amps. I want to attach a scope to the ouput as well as a voltmeter. Basically, I thought it would be safer to remove the speakers and replace with dummy loads while I take measurement in and out of the amp.  Hopefully that makes sense.

[Hawk]

So, basically, it's safe to assume that I can run the amp without any load at all, therefore disconnect the speakers.

[Roly]

Hopefully that makes sense.

Not really.

It's a solid state amp, not a valve/tube one, so you can simply disconnect the speakers.  It's valve/tube amps that must never be driven without a load (or you'll fry the output transformer).  On the other hand s.s. amps must never be driven with a shorted output.


You really only need a dummy load if you want to measure/verify the power output, but you can do that individually on each or the stereo outputs, you don't normally need to do both at once.

If you are planning to mess around inside then you would be wise to make and use a Limiting Lamp.

Before you plug it in identify all the mains wiring inside and cover it up with cardboard or plastic to avoid accidental contact.   xP

[Hawk]

Thanks Everyone, that was a huge help! Enjoy your day!