Footswitching the channels?
barry (Administrator) 03-05-2015 at 01:36 AM.
Post: #1
Standard arrangement
The two inputs of the P1800 let you choose whether to use one or two preamp stages. So the Normal input provides the classic 18W/Dominator circuit and the Hot input adds an extra preamp stage in front of that.

This is exactly like the Marshall 2204/2204 amplifiers from the 1970s and 1980s. Most people will use the Hot input for more distortion.

Inevitably, one of the first questions I get asked is: "Can I footswitch the two inputs?" Well, like those 2204/2203 amplifiers, you can't footswitch them as-standard. The sockets are used to pass the signal through, and it's not possible to use an A/B footswitch as you would on a JTM45 / Tweed Deluxe / etc.

But there is a way...
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barry (Administrator) 03-05-2015 at 02:28 AM.
Post: #2
Inspiration from a Park
One approach is to look at what Marshall did on the very rare Park 1210 Rock Head amplifier from the late 1970s. This followed the idea of the 2203/2204 amplifiers but added a form of footswitching.

Have a look at the left side of this schematic: http://www.prowessamplifiers.com/schemat...20Head.jpg

The quality of the copy is poor, but here's an outline of the basics and how it relates to the P1800:
1) The upper input (labelled 2) is like the P1800's Normal input
2) The lower input (labelled 1) is like the P1800's Hot input
3) If you plug into the upper input, the lower input is out of circuit and does nothing except idle (just like the Hot stage on the P1800)
4) If you plug into the lower input (with nothing in the upper input), the output of the lower stage is fed into the upper input (as with the P1800).

But where it gets interesting, is when you use a 3-conductor (stereo) socket for the lower input, a stereo cable and a dedicated footswitch. In the attached image, the red circle shows the stereo socket and the blue rectangle shows the footswitch schematic.


Attached File(s) Image(s)
   
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barry (Administrator) 03-05-2015 at 03:54 AM.
Post: #3
Can I add this to the P1800?
The Park 1210 arrangement is an electronically elegant solution. In some ways, it's surprising that Marshall didn't copy the idea on the very popular 2203/2204 amplifiers.

There may be a little noise as the inputs to both valve stages are sent out of the amp to the remote footswitch. I haven't had time to try it yet, but it should work just fine in the P1800. You would need:
* a DPDT (latching) footswitch in a suitable box
* a stereo 6.35mm jack-to-jack cable
* a stereo jack socket to replace the P1800's standard Hot input

I will look into creating a kit and some instructions for this modification as soon as time allows.
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