I recommend making sure that the input jack is clean and connected well to both line and ground.  If not, it could be only letting some signal through causing these issues.

Quote from: g1 on April 11, 2024, 12:24:42 PM

Quote from: RookieRecurve on April 11, 2024, 10:09:19 AMMy main concern is with the ground.  I am unsure if sharing a ground with an external amp and an internal amp would cause any issues? 

It can cause issues with ground loops or worse.  Some amps require their output (-) terminal to be isolated from ground for things like current feedback schemes.
If you want it to be safe as possible, use a Cliff S2 style jack, which will be insulated from chassis, and also will allow switching of both the positive and negative wires.

Thank you!  This is what I had suspected, but your answer confirms this.  Much appreciated.

Quote from: RookieRecurve on April 11, 2024, 10:09:19 AMMy main concern is with the ground.  I am unsure if sharing a ground with an external amp and an internal amp would cause any issues? 

It can cause issues with ground loops or worse.  Some amps require their output (-) terminal to be isolated from ground for things like current feedback schemes.
If you want it to be safe as possible, use a Cliff S2 style jack, which will be insulated from chassis, and also will allow switching of both the positive and negative wires.

Quote from: Tassieviking on April 10, 2024, 03:33:55 PMThe safes way to do this is to add a socket behind the speaker that the speaker goes to directly.
Also a speaker out socket on the back of the amp chassis, then you need a short speaker jumper cable with a plug on both sides to attach the combo amp to the combo speaker.
Edit: I have done this to some of my combo amps as it turns them into a separate head and separate speaker cab.
Sometimes I want to play the Marshall through the Fender speaker or the Sunn speaker, I just un-plug and plug into another combo for the sound I want.

Thanks for the reply!  Yes, I have considered doing what you have done, and adding two jacks with a short jumper.  To me, this makes things more simple.  The reason I am considering an alternative approach is that it is even more idiot-proof if it works in the same way that a headphone jack does, where plugging in disconnects one, and connects another speaker.  My main concern is with the ground.  I am unsure if sharing a ground with an external amp and an internal amp would cause any issues?  I am comfortable that I would not have both heads on at the same time, but if I ever sell this, or someone else is using this amp, that it is safe for them to use it as well.  I am pretty sure the 13A has a common ground terminal.  I have seen other jacks referred to as 'cliff' jacks that I am unsure about if the ground is common or not?  I have also seen 12A 'tip shunt' style jacks.  I might just order up a few different style of jacks, and see if any of them fit my intent of switching the hot and the ground when a plug is inserted.  My other question still remains though of whether or not a ground can be shared between amps.

Quote from: Loudthud on April 10, 2024, 03:06:02 PMNot sure what you want to do exactly.

Is the combo amp like a Fender where the internal speaker(s) just plug into the back of the amp chassis ?

Are either of these amps tube amps ?

Franken amps are notorious for poorly done grounding. Do you get any hum when you just connect a wire between the chassis' ?

You might risk damage to one or both amps unless they are both OFF when you plug the cable between them. Can you do that or do you need to make this idiot proof ?

Thanks for the reply.  This is a basic SS amp (Samick 10R) that is connected to a pair of vintage 6.5" alnico speakers.  The amp is picking up a ton of noise from the reverb tank, which is likely my fault for not using shielded wire after relocating it to the bottom of the enclosure (I will be trying to solve that eventually).  I want to leave the amp connected to the speakers in the enclosure, as well as being able to connect some of my heads to the vintage speakers inside this combo.  I want to 'idiot proof' it as I have nephews and sons that might also use this amp at times when I am not around.  I am confident that using the 13A jack will allow me to have one plug that will allow me to switch the hot (+) connection from the internal amp to the external 1/4" speaker connector; what I am unsure about is the ground.  I believe that these jacks have a common ground.  I do not know enough about electronics to know if sharing a ground with the internal speaker, internal amp, and the external speaker input is ok or not?  I am also unsure if the 'cliff' style of 1/4" inputs switch grounds, or if the ground is common.

Quote from: Tassieviking on April 10, 2024, 03:54:51 PMI think the best way to go forward would be to inject a clean signal and check the test-points with an oscilloscope.
Being a SMD build it gets a bit harder to check and replace components.

Thanks!

I would say the problem is with the Pre-amp, the reason I asked you to try the pedal is to see if you got the sound you expected.
I think the best way to go forward would be to inject a clean signal and check the test-points with an oscilloscope.
Being a SMD build it gets a bit harder to check and replace components.

The safes way to do this is to add a socket behind the speaker that the speaker goes to directly.
Also a speaker out socket on the back of the amp chassis, then you need a short speaker jumper cable with a plug on both sides to attach the combo amp to the combo speaker.
Edit: I have done this to some of my combo amps as it turns them into a separate head and separate speaker cab.
Sometimes I want to play the Marshall through the Fender speaker or the Sunn speaker, I just un-plug and plug into another combo for the sound I want.

Not sure what you want to do exactly.

Is the combo amp like a Fender where the internal speaker(s) just plug into the back of the amp chassis ?

Are either of these amps tube amps ?

Franken amps are notorious for poorly done grounding. Do you get any hum when you just connect a wire between the chassis' ?

You might risk damage to one or both amps unless they are both OFF when you plug the cable between them. Can you do that or do you need to make this idiot proof ?

Hey all, I am looking to add a speaker jack to a fraken-amp that I assembled, but not in the traditional configuration.  I am looking to use the combo-amp as an external speaker cabinet, as opposed to having the amp switching to an external speaker cabinet.  I am looking at a Switchcraft 13A (transfer circuit) jack to achieve this.  Is this the right part for the job?  Will sharing the ground from the external amp, the internal amp, and the speakers work?  Do I need something to completely switch circuits (hot and ground)?  I have seen other setups that use two standard jacks, and then use a jumper cable to mitigate, but I prefer to have one input jack to keep things clean and simple.

Thanks