Overloading Twin Input Amplifiers

[boedonaldson]

I have a 50watt Marshall JCM 800 Head. It is the two input (high and low sens) and was wondering if i would be able to combine both the channels using a ABY pedal (A, B or both at once) much like a 1959 style marshall (4 input bright/normal channel) is daisy chained. Would this work or would i just blow the amp up? I know people use A/B switches to use the 2 input as a clean/dirty between the High and Low input. But im thinking of combining both to kinda cascade and get some interesting tonal and distortion variations. Any tips, advice or info from people who have done this before would be greatley appreciated. Cheers
Boe

[brett]

Hi
If you are talking about paralleling the inputs, then I suspect that you won't get much extra gain than the high gain channel on its own.  Paralleling 2 equal channels only gives an extra 3dB of gain.

I don't know much about cascading, so take this with a grain of salt....
Ccascading should give a substantial increase, because the gain is cumulative.  In this case, your main problem will be too much gain.  To achieve the cascade, you'll have to access the output of the low gain channel and feed it into the high.  My guess is that a simple way of achieving this is to dedicate the effects loop output to the low gain channel.

Other will know more...

Good luck

[AL]

I'm no expert... at all but I don't see where that would hurt the amp. I'm not sure how usefull it will be. From what I remember of doing that when I was younger and we only had one amp all it did was lower the volume.

The reason you get some differences in 4-hole Marshalls is you're using different tube stages. If you're doing what I think you are you're just running two signals into the same stage (same 12AX7).

AL

[RedHouse]

If it's a JCM800 you can't run both inputs at the same time. The JCM800 "Hi - Low" inputs are high/low gain not high/low sensitivity as in the traditional Fender/Marshall/Vox type inputs.   

In the JCM800 the preamps are cascaded (in series) until you plug into the lower (or right jack on later JCM800's) then you bypass the 1st triode.

That jack actually interrupts the signal chain at the input to the 2nd triode.

You would need some re-wiring to run both at the same time (like a Plexi/Ali type)

[Dr Ron]

Besides the rewiring issue mentioned above, the two inputs may be out of phase.

Here's a pedal that does what you need.
I'd call to see if it can be used on a marshall amp.

http://www.barberelectronics.com/LaunchPad.htm

The Barber Launch Pad,
Ultimate high headroom uncolored CLEAN BOOST.
Two outputs that represent both + and - phase. This feature allows phase correcting Blackface and Silverface Fender amps, also you will be able to combine the channels for ultra flexibility and combined harmonics of parallel preamps.

BTW
This would be awesome for DIY --- does anyone know of a schematic that does this?

[John Lyons]

It was mentioned/explained above but in my words:

The low input only uses one triode of the first Preamp tube. (bypassing the second stage)
The High input uses both the tridoes in the first preamp tube. When you are using the high input , that's the most gain available.

You can switch from low to high with an A,B box but adding high and low will not do anything more than using  the high input .


John