Speaker switch box wiring help

ultrataco

Member
Joined
Sep 21, 2019
Messages
25
Reaction score
37
Hi. I have 2 amps that I use with 1 cab. I don't like having to move the speaker cable back and forth between amps. I also don't want to pay hundreds of dollars for some load box thing that I don't need. So I decided to make a simple switch box.
I have it mostly assembled but not sure exactly how to wire it up so that's where I need your help. (I know how to solder.) And yes I know I can blow an amp with this if used incorrectly. I already run that risk if I forget to switch the cable before using an amp.
Anyway, I'm using an on-off-on switch. I want the middle jack to go to the speaker cab and the 2 on the side to come from the amps. Is this the correct way to connect everything?

Edit: I got all the parts from mojotone, and before buying I asked if they would work for what I want and mojotone said the parts "should work fine". I didn't ask about wiring though.

Edit 2: just to be clear: I will only ever have one amp turned on at a time and will only flip this switch with both amps turned off.

IMG_20220619_190028260_HDR_2.jpg
 
Last edited:

fitz288

Well-Known Yinzer
Gold Supporting Member
Joined
Apr 7, 2020
Messages
6,076
Reaction score
16,427
Location
the Burgh
I think that looks right.
I've been thinking of doing a similar thing with 2 cabs to one of 2 amps.
I would use just an on-on switch, but if you already have an on-off-on, I don't see why that wouldn't work also.
I've heard warnings about a shared ground to 2 amps on one cab, but the switched ground should solve this issue.
BUT - I'd also be interested in anyone who thinks I'm (we're) just plain nuts, and why this may be a bad idea.
 

ultrataco

Member
Joined
Sep 21, 2019
Messages
25
Reaction score
37
Well I haven't been able to find any good info so I've sent an email to mojotone with that image so hopefully they can help.
 

fitz288

Well-Known Yinzer
Gold Supporting Member
Joined
Apr 7, 2020
Messages
6,076
Reaction score
16,427
Location
the Burgh
Well I haven't been able to find any good info so I've sent an email to mojotone with that image so hopefully they can help.
What's the amp rating on your switch?
And you never mentioned the amps that you are using.
 

ultrataco

Member
Joined
Sep 21, 2019
Messages
25
Reaction score
37
What's the amp rating on your switch?
And you never mentioned the amps that you are using.
According to the mojotone website:
Current Rating:5A 125Vac or 28Vdc
Current Rating:2A 250V

The two amps are a Marshall SC20h (usually in 5w mode) and an EVH 5150III 50w Stealth (at low volume). Both at 8ohms because that's what the cab is. (Marshall 2x12 with V30s)
 

Chris-in-LA

Well-Known Member
Joined
Jul 19, 2018
Messages
3,311
Reaction score
3,420
Location
Los Angeles
According to the mojotone website:
Current Rating:5A 125Vac or 28Vdc
Current Rating:2A 250V

The two amps are a Marshall SC20h (usually in 5w mode) and an EVH 5150III 50w Stealth (at low volume). Both at 8ohms because that's what the cab is. (Marshall 2x12 with V30s).
Marshall 1936v cabs usually have two 70 watt, 16ohm speakers connected to 2 jacks. Is that what you have? Keep it simple, plug one amp into each speaker.
 

ultrataco

Member
Joined
Sep 21, 2019
Messages
25
Reaction score
37
Marshall 1936v cabs usually have two 70 watt, 16ohm speakers connected to 2 jacks. Is that what you have? Keep it simple, plug one amp into each speaker.
No, it's a Marshall SC212 (matches the head) with 1 jack. And I want to use both speakers with each amp. (One amp at a time obviously)

Eventually may get another 2x12 for the evh but this Marshall cab sounds good and I don't feel the need to spend all that money.
 

ultrataco

Member
Joined
Sep 21, 2019
Messages
25
Reaction score
37
I got a reply from mojotone:
Yes, that is correct for switching either side, but you only need to switch the positive (tip). All 3 of these jacks share the same ground (box).I wouldn't use this setup on any amp over 25w. If it is over that, you need to have load on the output if you are hot-switching it.

I replied making it more clear I would not be hot switching ever. I'm not sure if the 25w limit recommendation is just for hot switching or because of the switch or something else.
 

Chris-in-LA

Well-Known Member
Joined
Jul 19, 2018
Messages
3,311
Reaction score
3,420
Location
Los Angeles
BTW, it takes some time to blow an amp. I switched cables the other day and left my 2203 running for at least two minutes without a load while I played another amp. The 2203 seemed to be unaffected. Obviously not a recommended practice.
 

vtrain

Well-Known Member
Joined
May 15, 2020
Messages
273
Reaction score
474
Location
Chicagoland
BTW, it takes some time to blow an amp. I switched cables the other day and left my 2203 running for at least two minutes without a load while I played another amp. The 2203 seemed to be unaffected. Obviously not a recommended practice.

I am not an amp tech, but my impression is that it’s less of an issue to idle an amp without a load compared with running a signal without a load. Running a signal through it is almost guaranteed to damage the amp in a few minutes, idling it is much less certain. Neither is recommended practice, obviously.

Anyway, I’m happy to be corrected by someone more knowledgeable.
 

paul-e-mann

Well-Known Member
Joined
Dec 7, 2009
Messages
19,220
Reaction score
20,738
I stack 2 heads on top of each other on my cab, they share the same power cord and speaker cable so no chance of turning an amp on without swapping the cables first. This is the safest way to do it, no room for mistakes. :yesway:
 

JivRey

New Member
Joined
Sep 23, 2019
Messages
7
Reaction score
5
If it was me, no matter the amp wattage, I would never use such small wire gauge and a wimpy switch for a utility box that handles signal between the amp and the cabinet. Plus big fat switches look much cooler :p

This is a small box I did to be able to run two cabinets in series or parallel. My JCM 800 is 4/8 ohms only and my Sovtek MIG 100 is 8/16 ohms only.

IMG_3244.jpg

IMG_3245.jpg
 

Old Lex

New Member
Joined
Dec 27, 2021
Messages
1
Reaction score
2
Hi. I have 2 amps that I use with 1 cab. I don't like having to move the speaker cable back and forth between amps. I also don't want to pay hundreds of dollars for some load box thing that I don't need. So I decided to make a simple switch box.
I have it mostly assembled but not sure exactly how to wire it up so that's where I need your help. (I know how to solder.) And yes I know I can blow an amp with this if used incorrectly. I already run that risk if I forget to switch the cable before using an amp.
Anyway, I'm using an on-off-on switch. I want the middle jack to go to the speaker cab and the 2 on the side to come from the amps. Is this the correct way to connect everything?

Edit: I got all the parts from mojotone, and before buying I asked if they would work for what I want and mojotone said the parts "should work fine". I didn't ask about wiring though.

View attachment 110078
Are both amps fixed to ground or does either amp have a floating ground. If either amp has a floating ground you will have to replace your jacks with insulated types. Otherwise, looks to good to go! Always turn the volumn down completely before switching.
 

ultrataco

Member
Joined
Sep 21, 2019
Messages
25
Reaction score
37
If it was me, no matter the amp wattage, I would never use such small wire gauge and a wimpy switch for a utility box that handles signal between the amp and the cabinet. Plus big fat switches look much cooler :p
Yeah I wanted bigger wire (I got 18ga) and a heavier duty switch but I ended up on mojotone site for some reason and that's what they had. I think the stuff is for pedals actually.

I'll look for heavier duty wire and switch.
 

ultrataco

Member
Joined
Sep 21, 2019
Messages
25
Reaction score
37
Are both amps fixed to ground or does either amp have a floating ground. If either amp has a floating ground you will have to replace your jacks with insulated types. Otherwise, looks to good to go! Always turn the volumn down completely before switching.

Well... I don't actually know what a floating ground is. They both use 3-prong power cables.
And I will always have both amps OFF when switching. I'll only use whichever amp the switch is set to. If I want to switch, I'll turn that amp off, flip the switch, turn on the other amp.
I never need them on at the same time. (Rarely ever play them on the same day even)
 

JivRey

New Member
Joined
Sep 23, 2019
Messages
7
Reaction score
5
Yeah I wanted bigger wire (I got 18ga) and a heavier duty switch but I ended up on mojotone site for some reason and that's what they had. I think the stuff is for pedals actually.

I'll look for heavier duty wire and switch.

Check on Digikey.com
 

lshindel

Member
Joined
Nov 19, 2020
Messages
32
Reaction score
1
Hi,
Something doesn't quite look right to me. In the center position either amp would not have a speaker connected. It looks like a double pole double throw would be safer.
 

ultrataco

Member
Joined
Sep 21, 2019
Messages
25
Reaction score
37
Hi,
Something doesn't quite look right to me. In the center position either amp would not have a speaker connected. It looks like a double pole double throw would be safer.
Yes you are correct. I went with an on-off-on switch because I thought it would be safer and avoid the 2 amps sharing any connection, but that doesn't really make sense and I guess everything is grounded together anyway. I don't know much about electronics obviously. But that's why I made this thread before soldering everything together and trying it.

I'm going to look for a heavier duty dpdt switch.
 

mspart

Member
Joined
Oct 17, 2020
Messages
21
Reaction score
35
I just saw questions regarding wiring size. I can tell you my cab sounds great and the wiring was quite thin in my opinion. The wire going from my amp to the cab carries bigger wires I'm sure. If you look at the actual output, the amperage is not that high. I would think that wiring size that your cab has would be sufficient for your switch. You could go larger (smaller gage) but I don't think that is necessary. Wire gage is a function of amperage. I did some calcs using 8 ohm cab.

VoltspowerResistanceCurrent
2.828427​
1​
8​
0.353553​
6.324555​
5​
8​
0.790569​
8.944272​
10​
8​
1.118034​
12.64911​
20​
8​
1.581139​
14.14214​
25​
8​
1.767767​
17.88854​
40​
8​
2.236068​
18.97367​
45​
8​
2.371708​
20​
50​
8​
2.5​
28.28427​
100​
8​
3.535534​


So the gage of the wire does not have to be too big to carry that amount of current. I used Power = V^2/R. You know the Ohm (R) of the cab which is 8 or 16 or 4. So V=(Power*R)^.5. Once you have the voltage you can then use it and the Ohms to find current. Current = V/R. I don't know if these are correct but this is what I came up with.

According to wiring standards a 20 gage wire can carry 11 amps. 24 gage wire can carry 3.5 amps. See https://learnmetrics.com/wire-gauge-chart-amp-wire-sizes/

So a 20 or 22 gage (7 amps) wire should be sufficient. I believe my cab has 22 gage wire, it was really small. A 16 ohm cab would draw even less current.

Now I don't know much about all of this, but from your wiring photo, it would look like to me your stomp box switch should work. Hopefully this was helpful.

mspart
 

maarvold

New Member
Joined
Feb 12, 2013
Messages
1
Reaction score
0
Just glanced at this topic. I believe your switch should incorporate some type of dummy load for the amp that is unloaded if they are both intended to be on during the switching process... seems to me it is hard on the power tubes or output transformer or both to be on and unloaded.
 


Top