JCM800 as preamp

eoinrussell · Post by **eoinrussell** » 25 Apr 2010, 23:34

I am playing with a band and my 50 watt JCM800 isn't loud enough, go figure.
I have a solid state amp that is 400 watts and is louder than I could ever need. I would like to use the JCM800 as a pre-amp for the solid state amp.

Can I remove the power tubes in the JCM800 and drive the solid state amp from the DI output or the send of the loop?
Do I need a dummy load if I remove the power tubes?

Is this Atlas Wire Wound 16 Ohm dummy load acceptable for this application if I do need a dummy load?
Is it safe unboxed like in the photo or should I box it up?

Thanks.

defaced · Post by **defaced** » 26 Apr 2010, 00:23

If you pull the power tubes, you do not need a load on the OT. However, all of the voltages in the amp will rise because the power tubes aren't pulling current and loading down the PT - this could be negligible, or it could be bad, depends on part voltage ratings. As for where to do the hooking up, I don't know, I'm not familiar with the 800.

eoinrussell · Post by **eoinrussell** » 26 Apr 2010, 00:54

Thanks!

Here's a link to the schematic. http://www.drtube.com/schematics/marshall/4210.gif
It looks like the DI comes from after the power tubes and the effects send comes from before. I imagine that will affect the tone, I'll have to try both.

defaced · Post by **defaced** » 26 Apr 2010, 16:40

It looks like it'll work. I can't say how well, but it should make some noise for you.

Aharon · Post by **Aharon** » 26 Apr 2010, 16:56

I imagine that you will use the DI out to go into the 400W SS power amp and then you will have to hook up that amp to a set of speakers.
Why not just hook up the DI to the power amp and leave the MArshall running as is,can't you do that?
Aharon.

JHS · Post by **JHS** » 26 Apr 2010, 17:15

I recommend to run Marshall on 11 with your fav. 4x12" and to use a H&K Red Box 4 or similar device for sending the output signal to the mixing console and/or another poweramp.

JHS

Aharon · Post by **Aharon** » 26 Apr 2010, 19:45

And yes you should box the resistor if you are going to use that,they get hot,make sure its rated for more than the power of your amp by a healthy percentage.

bancika · Post by **bancika** » 27 Apr 2010, 12:15

I'm running my 2W trainwreck into a 9W resistor dummy load and it works fine. I want to try adding a 150uF air core inductor in series, same as Palmer PDI-03 to see if it makes any improvements....

topbrent · Post by **topbrent** » 28 Apr 2010, 06:59

Doug Hoffman of Hoffmanamps.com - EL34world.com has a great stereo Marshall preamp.

Two tweakable Marshall pre-amps that you can run into what ever you wish.

Here are a few links.

http://el34world.com/projects/StereoPreAmp1.htm
http://el34world.com/projects/images/st ... layout.gif
http://el34world.com/projects/images/st ... ematic.gif

-

eoinrussell · Post by **eoinrussell** » 04 May 2010, 18:34

I am running the JCM800 with the power tubes in and the dummy boxed up and set at 8 Ohms. I am taking the output of the DI on the JCM800, and running that into the input of the 400w pre-amp. This way I can get 3 sounds, clean from the 400w amp, crunch from the JCM800 clean channel (it doesn't really do a real clean sound) and dirty from the JCM800 dirty channel - plus I get a much more versatile EQ section from the 400w amp than the JCM800. It sounds fantastic and is incredibly loud. I ran the rig for over 4 hours at rehearsal, no smoke and no noticeable heat from the dummy load so all seems to be going well.

Thanks for the guidance.

Ronsonic · Post by **Ronsonic** » 10 May 2010, 05:34

Protect your hearing.

Sounds like a fun rig.

Ed G. · Post by **Ed G.** » 22 May 2010, 13:44

Wow, a Marshall not loud enough?

eoinrussell · Post by **eoinrussell** » 24 May 2010, 00:57

Yes, the JCM800 alone isn't loud enough. I'm not sure if there is something wrong with it. It's 50 watts, so I only have so much expectation. After the master volume is half way up, it doesn't get any louder. It does sound fantastic though...

I'm going to try a mesa Triaxis to see if it gives me the preamp sounds I need without the bulk/weight of the JCM800. It's a different animal but I'm pretty flexible as long as the sound is good.