Please help me figure out wiring for low/high input jacks

[thehoj]

I know this seems ridiculous, but I'm having difficulty understanding the schematics for the low and high input jacks, and how they will physically wire up with the 2 shorting jacks I'm using.

Here's the preamp schematic I'm working with
http://vwtweaked.ca/images/jcmpre.gif

Here's how I think it gets wired up
http://vwtweaked.ca/images/inputsdiag.jpg

First of all, does that look right?

Secondly, for some reason I'm having difficulty with this and just want to clear it up completely in my mind.. So I get how the Low input works, once you plug into low you disconnect the shorting pin from the signal and basically just bypass v1a altogether.

How exactly is the High input working? If you plug into that you're no longer shorting the signal to ground, and I see how that signal would then go into the 68K resistor and hit v1a. How does the signal travel to the next stage?
Does it go via cathode through C2 up through c3 and through the shorting pin which would be connected to the signal and take the same path after that?
Wouldn't the fact that that shorting pin is connected to ground AND the signal connector of the low jack be an issue?

On the last build I did I just had a single input and so for some reason I'm finding this confusing.

I appreciate any help on the matter.

[Enzo]

No, that is not it. I THINK you might be confusing the model schematic you provided and the classic two jack input as on many fender models. Your drawing appears you are using the common Switchcraft-type open frame jacks.

Look at the schematic, both jacks have a cutout on the tip contact - you refer to the tip as signal. But the high gain jack in the schemaqtic also has a cutout on the sleeve contact - the ground contact. The metal bushing open frame Switchcraft jacks don;t have that, but it is common and usual in the Cliff-type jacks that Marshall uses. So plugging into high lifts the ground cutout, which in turn ungrounds the low cutout.

You can;t readily use those jacks to make this arrangement. About all you can do it leave out that ground. Or use Cliff jacks.

So I get how the Low input works, once you plug into low you disconnect the shorting pin from the signal and basically just bypass v1a altogether.

Correct, the low input is shunted to ground when the jacks are empty.

How exactly is the High input working?

I hope you now get the jack I described. Plug into high, it ungrounds the tip and signal enters V1a through R3, the 68k. But the high jack ALSO lifts the ground or sleeve cutout. That is the one carrying ground up to the low jack. See the arrowpointon top of the high jack? That indicates a moving contact, not a solder joint.

If that ground cutout had NOT opened, then the low jack tip would continue to be grounded, and no signal would travel on to V1b.

So with that ground out of the way, signal leaves V1a by way of C3, then through the close but not grounded cutout of the low jack, and on to R5 and glory...


Make sense?


The point of this seemingly mysterious grounding is that when you plug into low, the high input remains grounded, thus reducing noise in V1a. But V1a is still able to generate noise on its own, so the ground cutout shunts the OUTPUT of V1a to ground by shorting C3 to ground through that extra ground cutout. And since in that case the low jack already lifted its tip cutout, the signal path is not grounded.

So in either case, the signal path leaving the low jack for R5 is ungrounded. The jack you are using is the one to open that grounding.

If you must use the Switchcraft jacks, you would eliminate the wire from the left end of C3 down to the ground cutout of the high jack. In that arrangement then, the high input would still ground off when not in use, but C3 would still have the POTENTIAL for noise, being no longer grounded off when not used. MAy not be a problem at all, or may be noisy.

Is that clear or do we need to work on it some more?

[thehoj]

Thanks for the detailed reply. I'm using the switchcraft open frame style, so that explains that.

I guess my best option is to get ahold of some of the cliff style ones then, like this https://taweber.powweb.com/store/cliff_ns.jpg?

[Bruce / Mission Amps]

Thanks for the detailed reply. I'm using the switchcraft open frame style, so that explains that.

I guess my best option is to get ahold of some of the cliff style ones then, like this https://taweber.powweb.com/store/cliff_ns.jpg?

Using two simple Switchcraft jacks, here's a quick and dirty scratch drawing of how I do it on my Crusader amp.
The little circle at the ends of the wires means to use shielded cable and ground the the shield at the end from where the signal comes from... leave the other end ungrounded.
Sorry for the slop, I just got home from a long gig night and little tired....

[Enzo]

Cliff jacks will let you wire it EXACTLY like the Marshall example. But Bruce's plan works fine too, using your Switchcraft. It is pretty much the circuit without the grounding of the high gain stage output.

[tubeswell]

There's also a handy pic on the Hoffman site here:

Common hookup info

[Enzo]

That is a nice page, but it doesn;t cover his amp. He doesn;t have the common hi/low as in every Fender in the world. His is the extra tube stage between them type.

[tubeswell]

Duh the 2203! (should've read the OP more thoroughly sorry ) Without the grounded output from the high-gain stage, extra-aneous noise would creep back into the B+ from random plate resistor noise in the high-gain stage.

But I wonder if you could overcome that tendency for extra noise to some extent by making the high-gain stage input parallel with the Low-gain stage when the Low-gain stage was in use? That way the high-gain stage plate resistor would be doing the same sort of thing (approximately) as the low gain plate resistor, which would at least mask the randominity in the B+ to some extent right? Could that work? (Sorry for the diversion)

Or - See the attached - which has a (random) voltage divider after the V1a coupling cap, that becomes unswitched when the Low-gain input is plugged in. (edit - actually I can see its a similar sort of concept as Bruce's pic)

[Hardtailed]

But I wonder if you could overcome that tendency for extra noise to some extent by making the high-gain stage input parallel with the Low-gain stage when the Low-gain stage was in use? That way the high-gain stage plate resistor would be doing the same sort of thing (approximately) as the low gain plate resistor, which would at least mask the randominity in the B+ to some extent right? Could that work? (Sorry for the diversion)

In my 2204 clone, I added a switch that parallels the two triode (I call it "Plexi mode" cause it makes it similar to a jumpered Plexi). Had to use a dual-ganged pot for gain so I could control both stages.
I find this MUCH more interesting than the Low input, which I basically just never use, cause the Plexi mode is just so much better (both triode stage are biased differently, so the mix of the two makes an interesting texture)

[tubeswell]

In my 2204 clone, I added a switch that parallels the two triode (I call it "Plexi mode" cause it makes it similar to a jumpered Plexi). Had to use a dual-ganged pot for gain so I could control both stages.
I find this MUCH more interesting than the Low input, which I basically just never use, cause the Plexi mode is just so much better (both triode stage are biased differently, so the mix of the two makes an interesting texture)

So I thought about what you said and combined it with my last idea. Switchable parallel/series with either a marshal style stage V1a or a Fender stage (V1b) on one flick of the switch, or two stages in series off V1a, or 2 stages in parallel off V1b on the other flick of the switch. How'zat?