Separate channel inputs for the M15Mk1

I've built the boards and will start wiring my M15Mk1 soon. I'm thinking about skipping the channel switch and use 2 separate inputs instead, using an A/B/Y box to select channels or use both at the same time.

To get both channels in phase, I would like to use inputs on both sides of the PI, just like on the Marshall 18W, i.e. connecting one channel to the cap that is now connected to ground.

Any comments on how this would work? I suppose it should, since it works on the 18W, but then again, perhaps there's some other design consideration with the grounded PI input that I'm not aware of.

Re: Separate channel inputs for the M15Mk1

The basic amp is finished, but I'm still working with various mods. I haven't done the separate channel inputs yet. I have two input jacks, but at the moment it's just a standard hi/low impedance setup.

I'm thinking about doing a paralell effect loop instead (with solid state buffers), where the return signal would go to the "unused" phase inverter side. This would prevent me from doing the dual channel input mod.

At the moment I'm working with Power Scaling. When I'm done with that, I'll decide if I want the separate channel inputs or the effect loop.

Re: Separate channel inputs for the M15Mk1

I did the separate channel input mod today. Lifted C17 frmo ground and connected it to the output of the crunch channel instead. Works quite well. Now I need to build myself an A/B/Y box though...

You have to make sure the unused input is grounded though, otherwise there will be hum from the unused channel when only one input is used. You can do this by connecting the switch tab on the input jacks to ground.

Re: Separate channel inputs for the M15Mk1

I actually modded to separate channel inputs, but after some more modding, I went back to a hi/lo input.

Yes, it's basically a standard Marshall/Fender hi/lo setup. I.e. using the switch tab of the jacks to connect one of the 68k resistors to ground when the lo input. This results in a voltage divider that takes the volume down by 6dB.

I mounted the resistors directly on the jack tabs. Now, all the tube amp gurus say that those resistors should preferably be mounted as close as possible to the tube in order to suppress RF intererence, prevent oscillation etc.

To make up for the resistors being far away from the tube, I put two ferrite beads (small inductors) between the input and the tube. I'm not sure if this really helps, but at least it didn't hurt...

So now it looks like this:

Standard Marshall/Fender input with two 68k resistors and an 1M to ground. From the resistors, a shielded cable to the center pin of the V1 socket. From the center pin, two ferrite beads go to the grids of V1.

I really like having a low input. It makes it easier to tame this beast when you want a cleaner sound with humbuckers. Or, if you want to setup for a basic clean sound and then use a booster to make it distort.

Hope this makes sense!