External Impedance Switch for AD15?

[Crashcup]

I have an AD15 combo which has one 16ohm and one 8ohm speaker output. The jacks are difficult to reach when I want to unplug the internal 16ohm speaker and use an external 16ohm speaker cab instead (or use the external and internal speakers daisy-chained together out of the 8ohm output).
So I had the idea to build a switch box to allow easy switching between the two speaker outputs without having to reach up inside the cabinet to swap the speaker cables around. My question: is it safe to to have cables plugged into both the 8 and 16ohm output jacks running to the switch box so long as only one of the outputs actually sees a load on it at a time?

[Jondog]

As long as only one is connected at a time, it's safe. Make sure the unused output isn't connected to ground or anything else, in case you use switching jacks or something. I probably wouldn't even put a switch on it, just make a box with separate in/out jacks on it marked 16 and 8 ohm. No chance of accidentally hitting the switch or having it in the wrong position. Make sure you use speaker cable and 16 gauge stranded wire.

[Crashcup]

Thanks Jondog. Yes, I would be surely avoid using switching jacks due to the grounding issue you raised.

I had considered making (or buying) a simple jack box like this one:
http://procablesnsound.com/items/speake ... detail.htm

But I figured if I added a switch, I'd be able to leave both leads from the transformer as well as the internal 16ohm speaker plugged into the box all the time and then just flip the switch over to 8ohms whenever plugging my external PPC212 into the other jack on the box. No other replugging necessary.

[Jondog]

Nothing wrong with using a switch, just never know what accidents occur. I'm actually considering a mod to an amp with two switchable output transformers for different tube types. I'm weary on using switches for things like that just because things happen. If I do it, I think I'll try to make the switch physically lockable somehow, with a bracket and thumbscrew or something. Might be something to consider if you use a switch.

[Les Paul Lover]

Old Oranges and lots of modern amp still use a rotary 3 way switch to select the impedance.

It sounds proper enough - on the back of an amp head at least. I'm not sure if 8d be keen for that on a stomp box format selector.

I must say, owning an AD15 myself, I get where you come from re: difficult to reach output taps. I even labelled mine 8 and 16 ohms with a marker pen as they weren't even labelled.

Whilst they are awkward to reach, they still are simple enough to access and plug in though. I wouldn't consider bulidng such a box for my amp, as I'd be worried a cable could become detached and cause some serious trouble by the time you'd notice the speaker cable was the culprit.

[Sooner]

I must say, owning an AD15 myself, I get where you come from re: difficult to reach output taps. I even labelled mine 8 and 16 ohms with a marker pen as they weren't even labelled.

Yours isn't labeled on the back panel?

[bclaire]

Same thing with my AD30TC's - I used a Sharpie to write the speaker impedances and footswitch locations on the back of the cabinet....

[Les Paul Lover]

I must say, owning an AD15 myself, I get where you come from re: difficult to reach output taps. I even labelled mine 8 and 16 ohms with a marker pen as they weren't even labelled.

Yours isn't labeled on the back panel?

Call me daft...... I don't think I ever noticed that!!!!!!



Mind you, I didn't label them on the outside of the amp, but on the chassis itself. You can see it, but only when opened up.

[Crashcup]

Okay, I built out my switch box. I used a multi-meter to check that the A/B positions of the switch are isolated from the other. While I confirmed there is no impedance load seen on the circuit that's switched out, I noticed that the ground *is* seen by both circuits regardless of switch position since the input jacks all share common grounding to the box. Does this pose a danger to the transformer?

The whole idea of this project (and my original question) was to be able to leave both transformer taps plugged into the box and be able to easily switch between them depending on the speaker impedance I happen to be using at the time. I play my combo at home using just the internal speaker, but also take it to rehearsal where I add a PPC212 cab for a total 8ohm load.

Here are some pics so you can see how I wired it up and how I'm planning to mount the swichbox inside the cab, (assuming it's a safe design):

https://flic.kr/p/WMTVi4
https://flic.kr/p/VykJbT
https://flic.kr/p/VykHAV
https://flic.kr/p/VvEyZ7
https://flic.kr/p/VvECFq

[Jondog]

All the grounds should be connected, you just don't want the positive terminal on the unused jack connected to ground.
You're wiring will work fine. Only thing is the output side are not separate jacks, they are wired parallel so either one will work in either setting, which isn't really an issue as long you have the right impedance.

[Crashcup]

Cool. Thanks for the follow up answer!
The output jacks are intentionally wired parallel so that I can just plug my PPC212 cab in, flip the switch to 8ohm and have the internal speaker working as well.

[Jondog]

Cool. Thanks for the follow up answer!
The output jacks are intentionally wired parallel so that I can just plug my PPC212 cab in, flip the switch to 8ohm and have the internal speaker working as well.

Makes sense