Amp Head Switcher for One Speaker

[bcmatt]

I'd like to build a simple box for a friend of mine with 2 amps and only one speaker, but I'm trying to figure out if I can do it with a 3PDT toggle that I already got, or if it needs to be a 4PDT toggle.

Basically, I want it to be a single toggle that sends the guitar signal to Amp A or B, but also simultaneously switches the A and B outputs to either the speaker or the internal dummy load. The dummy load would never actually see signal (because it just loads the amp that is not currently selected), so the 100W 16Ohm power resister should be more than enough.

I figure the amp output/speaker side needs 2 of the Poles to toggle the positive side of their amp outputs to different amps to their load or speaker all sharing a common ground/- between their jacks.

However, my question is, on the input side, routing the guitar to amp A or B, can I do the same with just the positive side on a single pole, or do I need 4th pole so that the unselected amp input positive gets grounded out?

This was what I was thinking:

HeadSwitch4pDT.jpg

I assume that I just run extra risk of hum by keeping the unused amp input open...
Thoughts?

PS, I don't plan on using any relay switching, This is just meant to be a box with a simple 2 position toggle that chooses the amp you want to play through and it would sit on top of the stack of amps and speaker cab... so the cabling can all stay the same as he switches between amps.

EDIT: I replaced the relay switching schematic with my actual plan

[LOUDthud]

Sharing grounds between two guitar amps can cause unforeseen problems.

Go to http://geofex.com/

Scroll down to 12/8/04, the Hum Free A/B/Y pedal. It has transformer isolated outputs so there isn't a problem on the inputs of the amps. From there you can switch loads to the two amps, best if you can keep the grounds isolated between amps.

[Stevem]

As in the above reply you need to break the hot and the ground as I found out .

I tried having two amps share the same 4-12” cab.

2-12”s for one amp and 2-12”s for the other amp but I had both speaker jacks mounted on the same metal jack plate and it did not work right.

[bcmatt]

So, really, in order to keep all inputs and outputs with their signal and grounds all completely isolated, I should really be using some sort of 8PDT switch...
I should really draw this out...

[bcmatt]

Ok, so it's there any reasonable way that I can reduce the number of poles from 8?

17279661928177038929119002812799.jpg

Even reducing to 7 would allow me to use the 4pdt and the 3pdt toggles that I have already purchased together with a piece of wood tying them together.
Would using a seperate mini-toggle for a ground-lift on one of the amp inputs help free up a pole?

[LOUDthud]

I'm a little confused. In the first post you show a diagram using relays and a footswitch, but now you are showing a 8PDT switch. How are you going to implement this device ?

[bcmatt]

I'm a little confused. In the first post you show a diagram using relays and a footswitch, but now you are showing a 8PDT switch. How are you going to implement this device ?

The only visual example I had was the one schematic. I am not planning to build it like that but am using it for inspiration.
I want to build a completely passive device with no footswitch or relays.
My last drawing is how I actually plan to build it. I want it to be one switch to flip so he can have one setup with this box on top. He can decide whether he turns both amps on or not on any given day.
But I'm wondering if I can somehow get by with a 3PDT and 4PDT next to each other and the toggles physically attached so that it is still just one lever. Would a mini-toggle with a ground lift on one input allow me to not have to go out and buy another 4PDT to total 8 poles.

[bcmatt]

I guess I should have made this at the start and asked, can I do this and just use a 4PDT switch, or do I need to isolate grounds using the switch too?:

HeadSwitch4pDT.jpg

So far one response was that I need to isolate the grounds on the amp inputs.
The other response was that I need to isolate grounds on the amp outputs.

So, now I'm up at an 8PDT switch:

8PDT.jpg

Now I'm wondering if there is a low-compromise way to get it somehow down to a 7PDT switch, even if it means adding another spst that doesn't need to be switched every time the main toggle is switched.

[bcmatt]

If one of the amps always used a ground lift on it's power cable, would that potentially remove the need for isolating the grounds for either the inputs or the outputs at the switch?

[bcmatt]

As in the above reply you need to break the hot and the ground as I found out .

I tried having two amps share the same 4-12” cab.

2-12”s for one amp and 2-12”s for the other amp but I had both speaker jacks mounted on the same metal jack plate and it did not work right.

So, what sorts of issues did this cause when not isolating the speaker grounds from each other? Buzzing? or the speakers were affected in performance/phasing? Most amps should have their speaker jack ground also grounded to the star-ground their chassis, so if the amps are on the same circuit in the same building, wouldn't it be the same?
So I'm assuming this is a ground loop buzz problem that could be resolved by lifting the ground from the power plug of one of the amps, no?

[cdemike]

If one of the amps always used a ground lift on it's power cable, would that potentially remove the need for isolating the grounds for either the inputs or the outputs at the switch?

Yes, but that would also create a massive safety hazard since you've removed safety ground. In the event that a coupling capacitor fails, a tube shorts, the OT primary arcs, your chassis could become live with DC. Although you'd also be isolating yourself from that amp's ground from the input jack's side, imagine if you or someone else touched the amp during a failure like that (e.g., to change an EQ setting). Isolating grounds is the safer solution, but unfortunately I'm not really sure about a passive solution to your problem.

Is there a reason why you're looking specifically to use one speaker rather than two? A two speaker cabinet would solve basically all your issues and allow you to use a DPDT switch on the inputs.

If you're open to an active solution, an attenuator with a linear re-amp device would work since you'd be permanently sending both amps' outputs into dummy loads and re-amping line level signals which could be isolated and adjusted for volume or muted. Since that would happen prior to re-amplification but after attenuation, it'd also be easier on your output transformers.

[bcmatt]

Is there a reason why you're looking specifically to use one speaker rather than two? A two speaker cabinet would solve basically all your issues and allow you to use a DPDT switch on the inputs.

Yes. He only has a single 1x12 combo (a vibrochamp build with an Eminence Governor) that he also uses to plug in a second head (JCM800 2204 build) when he wants a different amp.
He lives way up north in the Yukon and it is very expensive to ship things there. If he was local, I'd probably find him another speaker cab... but for now, I want to get him an easy, compact and passive solution to just switch which amp he is using into that speaker without a bunch of unplugging and replugging. It's just for home use, so it would be able to just have this box sitting on top of the stack of combo and head.

Right now, he can run one amp into the other using this Zogomizor pedal with a dummy load in it which makes some unique tones, but it would be nice to just A/B the two amps as well.

20240810_183423.jpg

I think I'll try building it with a 4PDT at first, but leave space to be able to put the 3PDT right beside the 4PDT if I need to. I'll test it all myself.

[bcmatt]

I'm thinking I will use my 3PDT and 4PDT together but just use 6 poles (4 for input and 2 for outputs). I was hoping to do this for next to free, but I think I'm already into this for at least $50 in parts because I keep ordering one switch at a time.

Here is generally what I am thinking for layout. I am making an aluminum barrier to separate the inputs and outputs sections of the box down the middle. I plan to make a wooden stick on the outside that ties the toggle switches together like a single wooden paddle that toggles. That's why I will still use the 3PDT instead of a DPDT; the toggle physically matches the one from the 4PDT so they can hopefully be thrown together.

parts.jpg

[mhuss]

I did one of these years ago, but made it powered so I could use relays. I used high power DPDT relays to switch the amp input and low power relays to switch the guitar input. (I also included an attenuator in the box.) Fully double-insulated to avoid ground loops with one amp ground chosen arbitrarily to ground the box.

DSCN1311.JPG

[R.G.]

I just realized what was bothering me about this topic. Speaker switch/relay contacts need a different kind of material and physical setup than low signal, so-called dry contact inputs. High current contacts and low voltage, low current contacts are different animals.

I'm a little tempted to design up a 4PDT "relay" using MOSFETs back-to-back for the switching element instead of a relay. It would probably not be economically reasonable. But hey, much of what we do isn't.