How a Footswitch Toggles a Guitar Amp's Channel (AmpMechanic  

Glad you liked it. Thanks for commenting!

LOL. Longer? That has to be your tongue I see in your cheek. Thank you.

Glad you're enjoying it. Was the same for me.

I'll be honest in that I still struggle with the physics behind that. All I know is what I saw when I took the measurements and there was undeniably 14 volts. The first time I saw that, I scratched my chin. Then I checked again. Then I pinched myself to make sure I wasn't dreaming. I kept coming back to that. It never changed. I'd welcome an explanation if anyone has one.

@@AmpMechanic I need to do some more research, that scetch of the circuit you made of the "clean channel" will be fun to breadboard and lay a study on it. I believe all your measurements are spot on. I could be wrong but the voltage can be 14 volts thru the coil without activating the relay as long as the amps are low. Potential but not enough flow (amps) but just enough amps to "push" the dropped down voltage through the base of TR4. All I can say is that Marshall circuit design is a thing of beauty and somthing to behold.

@@fullwaverecked Later, after recording the segments, I came across something (I think it was a video) that made it clear it was the amps on base of the BJT that turned it on or off. I tried to square this with other explanations. As you point out, there appears to be 14v there regardless. So, the "amps" explanation supports what I observed. On the other hand, the spec sheets for the transistors (which are very non-standard in their presentation and thus confusing) seem to imply that voltage controls the flow from zero to saturation. I suspect that it's both. In other words, the presence of voltage alone is not enough. You need a certain current. I just wish the spec sheets were clear. "Here's the minimum turn-on voltage" and "Here's the minimum turn-on current." In the end, I ended up in the exact same place you are and resolve to get a breadboard out and do some experiiments. To do this, I had to finally fix my broken power supply that's been in my queue forever. I almost finished with that (just waited for a replacement trimpot that's on back-order) and there will be a video for that as well.

@@AmpMechanic I have a vintage eico 6v/30v .5 amp power supply. Do you think that will run my breadboard? 500 millivolts is enough?

@@fullwaverecked Did you mean 500 milliamps? I think that's more than enough. If you look at my redraw of the circuit, the mA required appears to be well below that. But take a look at some BJT spec sheets first. I think it would depend on which one you decide to work with. I am currently sorting through a giant stash of loose parts that I picked up last year and I'm pretty sure there's a ton of different BJTs in there. So, I need figure out what I've got and I'll experiment with those. Also, as you probably know, most ICs are just a bunch of BJTs packed into one chip. So, you can always find one of those ICs, look at the spec, and play around with that on your breadboard. My stash included a lot of those as well and I've actually sorted through many of them already. I give them (and pretty much everything else) away to whoever wants the stuff from my inventory. I just need a SASE for shipping it back.

I can't say for sure because I haven't gone into a spec by spec comparison of the BC184 vs. MPSA13, but I suspect that it has something to do with some of the key differences between the two such as Base-Emitter On Voltage (0-2V for the MPSA vs. .55-.7V for the BC184) and the saturation voltage (the MPSA which is a darlington, has almost 2x the saturation of the BC184). The MPSA is really intended as an amplifier, but clearly not used as such in this case.

Hi @Ross Tough. This really depends on the amp and the sort of foot switch it expects. Generally though, since the channel selector is typically toggling between two states (clean and dirty/overdrive) which is usually represented by a circuit being on or off, you need a latching footswitch (which toggles between two states) and a cable with quarter-inch TS (tip/sleeve) jacks on the end(s). Sometimes though, the factory foot switches for certain amps have multiple foot-buttons on them but work through a single cable. Essentially, this involves multiple circuits in the same cable and the jacks could be TRS or even TRRS. So you need to double check what the amp expects.

Very fair (that the video was longer than it should have been) and thanks for the criticism. Yes, the work is only 10 minutes worth of work to diagnose and fix. But explaining how the different components (relays, transistors, etc.) work in unison and animating the circuit takes a bit longer. Especially if it's being explained in way that beginners to intermediates will understand. But you are definitely right. As I got closer to the finished video, I realized it could have been shorter. It's just that it would require reshooting everything. Also, I don't do this for money. I realize that some people might be disappointed by the length and others might appreciate the level of detail. I'm just paying it forward.

@@AmpMechanic you're absolutely right. For me as a total beginner in amp mods (or electronics in general) it was VERY helpful to figure out how I could add a footswitch to my MG15CDR. Although I still have to figure out what c31/c30 and d1 do exactly, trying to add such a circuit to my amp :) Nevertheless please keep the good work up!