JCM 800 as a poweramp -- Power Amp IN VS. FX Loop?

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michiganmusicalcraze

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Hi guys, I'm new here. I need the experts advice. I have a JCM 800 2204 and a very nice high gain external rackmount PREAMP. I want to run the preamp into the marshall using the marshall as a power amp. What would be better for that purpose only? I want the best tone. A power amp in? Or an FX loop? Would the metro zero loss fx loop be good for that in a line level version? also, does the head's master volume still work? Or should it be not active so that my preamps master volume controls all the volume? What is better? Thanks guys I really appreciate it!
 
I think Page Hamilton used to go direct into a Marshall's input, FWIW
 
Some loops can sound horrible.
An unbuffered power amp insert directly into the phase inverter and bypassing the master volume would give you the purest sound.
That said, that doesn't necessarily guarantee it will be better sounding, sometimes buffering circuitry can color the sound in a good way, etc.

I run an extremely high end preamp through different power amps with different types of inserts and they all sound different, I wouldn't say one is better than the other, but the amps with buffered fx loops are pretty high quality.
 
I did this many years ago….just put in a power amp input in place of the low gain input. Worked great for me. THe presence control will still work.
 
Back in my rack days, a long time ago, I used two JCM 800 heads for power and a rack full of effects and a rack-mounted Boogie Preamp.

I bypassed the preamp sections of the Marshall heads with a early version of the Kasha Quickmod-Mini. I knew John Kasha and lived close to his shop. It basically houses a 12AX7 tube and has a cable with a male connector that plugs into the V3 socket. You just plug in a line level signal in to the Quickmod-Mini.

I had a rack-mounted mixer that my effects and preamp ran through before going to each head L & R stereo. Good thing I had friends that liked beer and didn't mind carrying my rack and 4x12 Marshall cabs.
 
Thanks to all so far! Very helpful.

How easy would it be to have a local tech (not genius) make a power amp input on the back? What should I specifically ask him to do so he get's it right? Can you break it down for me a bit? AND would this sound just as good or better than an FX loop with my preamp just running into the return? (Metro Zero Loss Loop for example?)

I just want to make sure I make the right choice before I go to the tech Saturday. Thanks guys, VERY informative!
 
So would a metro effects loop be a waste for my application? Simple Power amp input equally good? And can anyone tell me exactly what to request from the tech as far as doing the Power Amp Input so I get a good tone and it doesn't end up a tone sucker?

Thanks again guys, you've been very helpful.
 
I asked around and was told the metro loop in line level (my preamp is line level of course) might be a better route? But you guys here said great results with a "power amp input"? Just elaborate here and help me out a bit guys. Thanks for bearing with me. Just want to reiterate, i'll only use for the external preamp never any effects.
 
If you want to achieve the purest connection between the preamp and the power amp, then you want to run the preamp into an unbuffered direct power amp insert directly before the phase inverter (PI). The PI is located after the master volume on a stock JCM 800, so if you have this installed, there will be no circuitry between the preamp and the power amp except for the patch cable and output/input jacks.
Keep in mind, a preamp through a separate power amp sharing different power transformers vs. a preamp and power amp housed in the same head sharing the same power transformer will react differently, and by saying that I am not saying that one is better than the other, just that they will react differently.
 
michiganmusicalcraze":2zu608g7 said:
Hi guys, I'm new here. I need the experts advice. I have a JCM 800 2204 and a very nice high gain external rackmount PREAMP. I want to run the preamp into the marshall using the marshall as a power amp. What would be better for that purpose only? I want the best tone. A power amp in? Or an FX loop? Would the metro zero loss fx loop be good for that in a line level version? also, does the head's master volume still work? Or should it be not active so that my preamps master volume controls all the volume? What is better? Thanks guys I really appreciate it!
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You've got a killer guitar amplifier with great tone right there. Don't desecrate it by running a rack mount preamp into it!

If you really want to use the preamp, then get a dedicated power amp and speaker cab for it, and run it adjacent to the JCM 800.
 
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