Extensive amp modding (need some help)

[thebigcheese]

Alright, I'm trading my Blues Jr. for a Vox Night Train, which I like much better, but it still doesn't have more than one channel or an FX loop. Neither of those seem particularly difficult to add, since all I really want is to have the same channel twice--I dig the distortion I get from cranking the gain, but it would be impractical to try to switch from bright to thick and crank the gain in the middle of a song. So it seems like basically all I'd have to do is put together a copy of the EQ/gain section and preferably also the master volume knob section so that I can have two different sets of settings, then link those to a switch that could also be footswitchable. I'm not really sure how to do the footswitch, but the rest I'm pretty solid on, except for exactly where to insert it on the PCB. Here's a link to the schematic: http://www.prowessamplifiers.com/schematics/Vox/Night_Train_Updated.html

It looks to me like I would cut the trace before C29 and run that to the switch, which would then run back to that spot on the PCB and also to my copy. The output would then go to where #2 is on V3B on the schematic. The thing I'm not sure about that is that lug 6 of the bright/thick switch goes to a separate section of the circuit, so how would I do that with channel 2? Do they both just connect there, or am I going to need diodes or to also wire that to the switch? Then the Volume section would also need to be wired to the switch so the gain channel isn't louder than the clean channel, but that knob seems to be connected two places on the schematic. Do I just need to splice in at lug 1 of the volume knob, or am I going to need to splice in farther back?

Beyond that, I also need to know how to make the channels work with a footswitch. I really have no idea how to wire that to work in addition to the amp switch. Mesas have a middle position on the switch for the footswitch, which could be nice, but if it's going to make it more complicated, I'll pass. Also, when I switch channels, am I going to have to worry about "pop" when I switch?

Finally, I really don't know where to stick the FX loop. It seems that, when adding an FX loop to an existing amp, you also want to add some level knobs. Where do I splice that in, and how would I wire it up?

Thanks. I'll definitely make sure to post pictures and stuff when it's done

 

[Minion]

Well this seems pretty ambitous , I think you are just asking for trouble .....

If you don"t know what you are doing You could Die !!! Right were you wanted to cut a trace at C29 is a couple Hundred Volts that could kill you ....

But if you are going to go through with this I would Duplicate the Channel up to before the Phaze splitter and use a relay to switch between the channels .... You will need to switch the inputs and the outputs at the Same time so a Dual relay would be good

An effects loop is problematic because the Output impedance of a Tube stage is very high and Most effect processors are fairly low impedance so they won"t interface well unless the signal is buffered , Effects pedals would work better as they have a high input impedance .....

Pluss I don"t see how you are going to fit a whole new Tube section with all the Knobs and Power supply into the same chassis as the Vox ..... Pluss it is going to be expensive to build ....

I suggest you get a good Booster pedal that you can engage to overdrive the tubes and to disengage to go clean .......


Good luck

 

[thebigcheese]

Maybe you're misunderstanding me. I don't actually want a second, separate channel. What I want, essentially, is a way to quickly switch the knobs to different positions so that I can go from clean (gain knob turned down) to dirty (gain knob up, maybe some EQ changes) by stomping on a switch instead of having to turn the knobs. Basically, imagine that stepping a switch would move the knobs and flick a switch. That's really all I want. I can't think of any reason I should need another tube section for that. I'm not sure why I would need a relay, either.

And I already have a good booster pedal, but I'm sure I will often want to also switch from bright to thick (bright for clean, thick for heavy distortion).

Don't worry, I'm well aware of the risks. I want to do the mods because it's a relatively inexpensive amp, but it gives me all the tones I want, so it seems silly to spend $1000 more just to add a couple features that wouldn't cost me more than $50 to add myself.

 

[mshilarious]

If you want knobs to move, you need motorized knobs and some sort of control mechanism. That could be anything from a sample & hold circuit to a microcontroller. It would be far easier to add a couple of knobs and use a switching mechanism (relays are a good idea due to the impedance of the circuit at those points, you don't want to add a cable run to a footswitch there).

You gotta wonder why more amps don't incorporate the "automated" channel approach, but then guitarists are either way too purist or really want 700 knobs on their amps

 

[thebigcheese]

Yeah, adding knobs is what I'm going for. Seems a whole lot less complicated than motorizing things, though it will take up a bit more space on the panel (still gotta figure out where to put everything). I certainly wouldn't mind motorized knobs, but I imagine it would be a good deal more expensive. And I'd have to figure out how to program it... Maybe that will be the "improved" version for later--give myself a little LCD and some of those knobs that rotate forever, then give myself a couple programmable options... Hmmm, that would be fun.

But for now, maybe I should stick with the basics. Am I at least on the right track with regards to how I would wire it in? How does the relay function in this context, and would it be possible to have both a flip switch on the amp and a footswitch to switch between the knob settings?

Also, what kind of relay am I going to need? I happen to have a 5VDC DPDT relay and a 12VDC DPDT relay on hand right now. I know the 12V is non-latching, but the 5V might be.

 

[thebigcheese]

So I did some asking around on other forums, but mostly just got a bunch of pompous a-holes telling me I don't know anything and should leave it to more experienced people or give up. But I did occasionally get some useful information, and it sounds like I can't get by just adding a switch for two sets of knobs because that would introduce noise and capacitance issues, which the relay would help mitigate, but overall the tone would be altered, so it wouldn't be that ideal. So I guess I have to give up on that idea. However, I'm now thinking that replacing the knobs with digital ones and programming a microprocessor with a little LCD could be a fun alternative if it would eliminate some of the issues of the switch without introducing too many new issues. Something like how the MOTU Traveler works--create two "presets" with the now digital faders at different levels and a way to switch between them. It'll probably be a lot of work, but... could be a good learning experience.

Can you think of any new issues that might introduce? Ideally, the pots in the amp now would just be replaced by digital signals from the chip, but I'm not really sure how to set that up. I'd also have to power the chip, but I imagine I can't just take power from the existing circuit, so I might have to introduce some other transformer or something to make it work. Maybe not? I dunno.

Edit: Now that I think about it, motorized pots would be a much easier solution than trying to do it all digital. The only problem would be that bright/thick switch...

 

[mshilarious]

I would ignore all guitarists who slavishly worship 'tone'. Intangible concepts that cannot be quantified are useless in electronics. There is a big difference between the statements "I can't quantify tone" and "no one can quantify tone". Those making the latter statement have probably never tried. We are not dealing with a group of people that think in especially innovative ways . . .

Anyway, back to relays. Without staring at a datasheet, how bad can the capacitance of a relay be? A few pF? With a source impedance of 100K and a stray capacitance of 100pF (which I'm pretty sure can be improved substantially), that's still a corner frequency of 16kHz. It's not too hard to do that badly with the guitar cable itself.

If you were super-fastidious you could insert a low-impedance stage to avoid any problems with capacitive loading. But I can't see any problem with the footswitch itself; it only has to carry a low DC voltage.

Why don't you post a link to the datasheet of the devices you are looking at?

 

[pathdoc]

I've got a solution that's simple but expensive. Buy a second identical amp and an A/B pedal. Simply switch between your 2 amps with the use of your new a/b footswitch. No moding required.

 

[thebigcheese]

I've got a solution that's simple but expensive. Buy a second identical amp and an A/B pedal. Simply switch between your 2 amps with the use of your new a/b footswitch. No moding required.

I had considered that initially, but was hoping for a simpler solution that could be done with just one amp--I had thought that it would be easy enough if I just had to add a couple knobs, but it seems to be getting more complicated than that.

I haven't looked at any specific relays yet... I'll be honest, I don't even know what to look for. I'm starting to think that the simplest solution would just be to put together a little volume pedal or see if I can get what I want out of my overdrive pedal. If I'm going to stick an FX loop in there anyway, that should be plenty sufficient. Might give me some more control over the tone, too, which would be nice.