Jumpering an internal (chassis) ouput jack to an external (add-on) jack & jack plate?

[Mango Moon]

Hello All,

I don't know if this thread belongs under MODS & TWEEKS or THEORY & DESIGN but I'll try it here first. I'm hoping to get some expert guidance on the best (most practical) way to modify (jumper) a single 8-ohm output jack to an external jack plate, thereby enabling safe/easy switching between an internal speaker 'or' a comparable external speaker (never both).

Unfortunately, this will require a bit of explanation, but basically, I've got a small combo amp with a single 8-ohm output jack (on the chassis) and a single 8-ohm (internal) speaker, and on occasion, I would like to switch speakers by disconnecting the internal speaker altogether in favor of another external cab with different speaker type installed. However, the lone output jack on the amp's chassis is literally 'buried' (very difficult to reach) deep on the backside of the chassis, with very little spatial clearance, making it impractical to connect/disconnect the internal or external speakers.

Consequently, I'm considering the installation of a single Switchcraft 1/4" female jack and jack plate (to be located somewhere on the wooden back panel of the amp) and then permanently jumpering the internal (stock) output jack (the one on the chassis) directly to the external (added) jack using a 12-gauge speaker cable with 1/4" plug on one end (internal jack) and soldered ends on the other (external jack). In this way, the internal speaker could be easily connected or disconnected, and similarly, an external cab could be easily substituted in favor of the internal speaker when desired.

However, the question is this . . . would this modification compromise the integrity of the amp or its tone in any way? Would doing this sort of arrangement cause any undo noise or sonic interference from the multiple connection points? In other words, is this a ridiculous idea or is it a perfectly safe and logical approach that others have often done?

Many thanx!

[Steve Conner]

It sounds like a perfectly safe and logical approach. The only downside I can imagine is if drilling a hole in the back panel would hurt the resale value of the amp.

Are you going to hang the internal speaker's plug out of the back and plug it into your new jackplate, or use a jack socket with a shorting contact and solder the speaker wires to it permanently? Either would work fine, but the second option might look neater.

[Enzo]

First, no need to wire anything to the amp. Mount your jack and switch on a plate, and just make a wired plug from it to plug into the hidden amp jack. The amp is left 100% stock, and your access is presented as desired. Basically we made an extension cord. You can plug the internal speaker into that, or unplug that speaker and use an external cab. I think this is what you proposed already.

SImpler still, disconnect the internal speaker + lead from the speaker, and run it instead to a jack on a plate to its tip contact, then run a new wire from the cutout tip contact down to the empty + terminal on the speaker. Now yet another piece of wire from the speaker - terminal to the sleeve of the jack. Now the jack normally passes the amp output along to the speaker like normal. When you plug an external speaker into the jack, the cutout contact disconnects the internal speaker automatically. You don;t even need to get at the hidden jack at all to do that.

On a jack, the cutout contact is the one normally used to ground the tip of an input jack.

12ga wire is excessive, you certainly don;t need that large a wire, and it is a lot more clumsy to deal with than #20. Getting it into a plug casing might be tough. #12 wire is in the walls of your home to carry 20 amps of 120 volts. A 50 watt amp at full bore sends 2.5 amps into 8 ohms.

Will it hurt anything? It is just a wire and a connector. Nothing more than the amp sees on any day by plugging in a speaker.

[Enzo]

Aha, simul-post with Steve.

I would envision an L bracket screwed to the inside side wall of an open backed combo, jack mounted to that. That leaves a couple screw holes in the side wall. No drilled holes needd.

[Steve Conner]

Yup, what Enzo said Except he called it a "cutout contact", which is probably the more correct name.

[Enzo]

Perhaps it is one of those things like "preamp tube" where the application determines the name.

I think the real term is shunt contact, but who knows...

The one of three that never touches the plug, how's that?

[Mango Moon]

Hello Steve and Enzo!

Many thanks for the quality feedback that you've provided me here guys. As usual, you guys are insightful and very-very helpful. However, you've covered a lot of ground here (for a newb!), much of which I'm not sure that I completely follow, but let's see . . .

Between the two of you, it sounds like you've recommended as many as three different approaches, with one (the so-called "cutout contact") appearing to be the preferred option, and if this device (the cutout contact) is what I think it is, I would tend to agree with this approach. NOTE: I'll come back to this subject of preferred style or type of jack momentarily. I also like the idea of not drilling any holes in the tolex-covered wooden back panel in favor of an 'L'-shaped metallic bracket of some sort, mounted to the inside wall somewhere. That is a far cleaner, more aesthetic approach in my view, and a great suggestion!

Also, regarding the 12-gauge wire issue, in my original post, I suggested the use of 12-gauge wire for this proposition solely because the speaker cables that I have traditionally used here to connect output jacks to external cabs and/or attenuators have all been 12-gauge 1/4" to 1/4" Live Wire Elite brand cables - please see the following link . . . http://accessories.musiciansfriend.com/product/Live-Wire-Elite-1414-Speaker-Cable?sku=331177. However, I am certainly open to using something of lesser weight at your suggestion.

Now, back to this business of the jack itself, and its design and installation. As you both know from previous posts of mine, I'm no trade-school educated amp tech, but I work with pride on all of my own amps here (with expert guidance from pro's like yourselves), and I'm certainly capable of conducting this modification with little difficulty. However, I'm going to need to 'clearly' identify the correct parts and the proper connections needed. In my preliminary research on this subject, I have seen mention of what I will call a "switching" or "bypass" jack (suspected of being the same as your "cutout contact") which allows for permanent connection (soldering) of the internal speaker but effectively disables that (internal) speaker connection whenever an 'external' cabinet is connected. Something like this Neutrix brand jack pictured here . . .



ENZO: Is this the sort of thing that you had in mind? If so, and if you are suggesting that I mount this sort of "switching" jack on an 'L' bracket mounted on the inside cabinet wall, I follow you just fine. Apparently, this type of jack allows for a permanent connection of the two (internal) speaker wires and two or more wires from the internal (chassis) output jack (i.e., from the output transformer). However, it is not clear to me just how I would make that connection between the internal (chassis) output jack (or transformer) and this added "switching" jack? In other words, the existing output jack is extremely inaccessible, so the question becomes . . . do I somehow 'hard-wire' (solder wires) between the new "switching" jack to the internal connections of the existing chassis jack, essentially disabling the existing chassis jack from future use? Or wouldn't it be easier and less invasive if I simply used a 12-gauge speaker cable with 1/4" elbow plug on one end (connected to the existing chassis jack) and soldered connections on the other end (soldered to the "switching" jack)?

As always, I greatly appreciate your assistance guys and your continued patience with my novice level skill-set!

Many thanx

[Mango Moon]

It sounds like a perfectly safe and logical approach. The only downside I can imagine is if drilling a hole in the back panel would hurt the resale value of the amp.

Are you going to hang the internal speaker's plug out of the back and plug it into your new jackplate, or use a jack socket with a shorting contact and solder the speaker wires to it permanently? Either would work fine, but the second option might look neater.

STEVE: By the way, just to clarify, my hope is to use something that will entail a permanent (soldered) connection of the internal speaker to a switching jack that disables the internal speaker when an external cab is connected. I hope that helps clarify. Please see my lengthier post below which explains it in somewhat more detail including photo of switching jack.

Thanks again.

[Enzo]

You don;t have to go anywhere near that hidden plug on the chassis, just reroute the wires at the speaker end.


Look at your jack photo, there are three sets of contacts. The upper contact blade on each is the main contact, it bears against the plug to complete electrical circuit to whatever. Now when the jack is empty, see how that blade drops down and touches the small lower contact in front? When you push a plug into the jack, it moves those long blades upwards as they slide along the plug surface. This pushes them away from the lower contact, thus cutting out whatever is connected there.

The contacts on the left are the ones for the tip of the plug - the hot lead if you like.

Now visualize this wiring. Disconnect the wire from the hot or + terminal of the speaker. COnnect it instead to the long blade on the left of the jack.. That would be the solder terminal on the far side where we cannot now see. Now run a new wire from the cutout contact under that leftmost blade. The solder terminal on the lower left as we look at it. That wire now goes down to the vacant + terminal of the internal speaker

Now with nothiing plugged in the jack those contacts come together completeing the circuit. SO current from the amp output flows through the jack and on to the speaker. The amp works as it always did. When you plug another speaker into the jack, then the amp output is on the tip of the plug, and the cutout contact is now open because the blade is pushed upwards away from it. Look at the jack closely as you plug something in and out, the contact operation is easy to follow visually.

The only thing left is to get the ground lead out to the jack as well. The ground is the sleeve of the plug, which is the far right blade. Run a new wire from the ground or - terminal of the old speaker up to the rightmost blade - the solder terminal on the far side out of sight for the right blade.

The jack only has to switch the hot leads, it doesn;t matter if the ground stays connected to both speakers.

But if you wanted to, the ground contact on the jack has its own cutout, which could be wired similarly for the grounds. But why waste the effort?

I actually has a Switchcraft jack in my mental image, but it makes no difference at all what brand jack you use. The physical payout is different between those brands, but electrically they work the same.

[Mango Moon]

ENZO:

Thanks for the detailed explanation on this. I'm afraid I'm not familiar with the Switchcraft brand equivalent of this Neutrix switching jack (never seen a reference to one anywhere), and frankly, I'd much prefer to use Switchcraft if such a jack is available, but either way, I think I'm good-to-go at this point with the connections needed for this mod.

I'm going to secure a suitable piece of angled aluminum for the inside mounting platform and the proper switching jack, and I'll try to follow-up with my results soon. Until then, thanks again to you and Steve for everything.

[Enzo]

Absolutely nothing wrong with those Neutrix jacks. Marshall has been using them for the last 50 years after all. Well Marshall may have used different brands, but the same type. Informally I call those "Cliff" style jacks, since as far as I know, the Cliff company was the original brand of them.

Switchcraft doesn;t make this style of jack. The SWitchcraft ones I had in mind are the exact same ones Fender has used as input and speaker jacks on amps like the Twin Reverb for those same 50 years. Different physically, but electrically they work the same.

I have been working with the Switchcraft type jacks for over 50 years myself, so I don;t even have to think about it when I use them, but I think the plastic jacks like you have selected are probably easier to visualize the operation with. Some folks get confused as to which solder tab is doing what with the Switchcrafts, I think everyone understands the plastic ones though.

Don;t worry about it, the brand of jack doesn;t matter. All that matters is there is that cutout contact to do the job for us.