Ampeg rB115 combo mod

[nosaj]

i have a guy who has a ampeg RB115 combo and he wants to add a speaker jack to the amplifier so he can use the cabinet as just a speaker(without the amplifier) but still retain abilitity to use the combo amp. I can't find a schematic on this amp. Does anyone forsee an issue with this? Would we need a switch to separate the speaker from the amp when used as just a speaker?
Thanks,
nosaj

[glebert]

Yes, you need to separate the amp from the speaker when you use it just as a speaker. You could probably use a TRS switchjack that will let the amp connect to the speaker when nothing plugged in but disconnect them when a plug is inserted. I've done this with multispeaker cabinets that I wanted to use as either mono or stereo. I would say that this could potentially confuse someone in the future since usually adding a jack to a combo amp is so that you can use the amp with an external speaker.

[nosaj]

What about this? This look ok? Now the cable can be a standard Instrument cable correct?
Thanks,
Nosaj

[glebert]

I don't like that way because the S line is connected to amp, speaker, and jack always. It's totally possible for the external amp or cable to have signal on that line, either because it is wired wrong or something like a bridged amp. I can try to find or make a better diagram later tonight if necessary. Might be a way to do it with a TS jack, can't remember off the top of my head.

[g1]

Now the cable can be a standard Instrument cable correct?

No. Speaker cable.

[glebert]

This is how I would do it, someone can correct me if I'm wrong. Note the colors of the lines don't necessarily mean anything. "Audio Source" is the amp in the combo amplifier. With no plug in the jack the signal goes from the audio source to the speaker. With plug inserted in jack the internal amp is completely disconnected. Even though it is a TRS jack you would just use a regular TS speaker plug. If you can find a TS switchjack that has a switch on the shield as well as on the tip you could probably use that instead, but those seem hard to find.

[nosaj]

No. Speaker cable.

Yea i know that Don't know what i was typing. My question really was speaker cable used can be a standard Instrument phono plug( mono) to activate this switch?
nosaj

[nosaj]

Ok another question. i know in the past there are some amps out there where the speaker can't be grounded? (Like a fender mustang or something similar)
Not having a service manual or a schematic for this amp how would one tell by looking at the amp? I imagine I could float the jack with some fiber or plastic washers if needed.
nosaj

[The Dude]

Yes. You want to use a mono plug. On the ground thing, take your meter and measure resistance from speaker ground wire to chassis ground and see if it reads a dead short.

[glebert]

Ok another question. i know in the past there are some amps out there where the speaker can't be grounded? (Like a fender mustang or something similar)
Not having a service manual or a schematic for this amp how would one tell by looking at the amp? I imagine I could float the jack with some fiber or plastic washers if needed.
nosaj

I would just assume that neither amp (internal or external) wants to see a ground on either line unless it supplies it. That's why I suggested using a switchjack that isolates both lines of both amps. I also wanted to add that if there is any concern that someone might plug the external amp into the speaker when either of the amps is on I would probably use a toggle switch instead of a switchjack. There is a possibility of the amps being connected to each other at the moment a plug is inserted into the jack.

[The Dude]

IMO, a switch is a worse option. I think it's far more likely that someone would have the switch in the wrong position using an external amp causing more damage. I think a switching jack is far more safe than giving a customer the switch option. Using a jack instead of switch is a more idiot proof option.

[nosaj]

Will a cliff jack work(what i have on hand. not finding and switchcrafts in the mix.
nosaj

[The Dude]

Which cliff jack? Got a part number, picture, number of contacts, etc?

[nosaj]

Here is what I have.
nosaj

[The Dude]

Yes, that should work. Just wire the speaker to the tip and sleeve contacts and the amp to the corresponding switch contacts. When an external plug is inserted, connection would be broken to the internal amp. When nothing is plugged in, connection will be made from speaker to amp via the jack.

Edit: Here's a pic (which is similar to what glebert drew up in post #6)