How do I add an external speaker jack to my combo?

[Dante]

My lil' Marshall combo amp has no external speaker jack. But, I have an external speaker I'd love to use with it....oh, the dilemma!!

Can you guys steer me in the proper direction of how I get the speaker in the combo and the external speaker hooked up & running at the same time without damaging my amp? I'm thinking the result will give me a 4 ohm load, using two 8 ohm speakers. My concern is that they may not be loud enough, but who knows until we try?

[Harley Hexxe]

  Do you have any idea what the nominal impedance is in the amp?

[Kim]

Yeah, if it's rated for 8 ohms you don't want to show it 4 ohms with a Parallel wiring.    Wired in Series would show it 16 ohms though and live.

[Dante]

I happen to have a schematic right here, which I cannot read

Marshall 5010 Master Lead Combo Schematic

4 ohms?

The cabinet I want to use with it is 8 ohms. Can I do this without a switch? Like a jack that knows when something is (or is not) connected?

[MarshallJMP]

I just looked it up and it has (originally) a 4 ohm G12M70 celestion in it.So you can't add an extra speaker to it because and 8 and 4 ohm will result in a total impedance of 2.66 ohms which is too low.

But what you could do is add a jack input on the back and add a jack on the original speaker.This way you unplug the original speaker and plug another one in the marshall.

[Dante]

Dang...you're right, it's a 4 ohm load on the amp....makes no sense to drag the combo amp if you're not using the speaker inside it. Oh well, I still have a use for the single 1x12 as a standalone cab for tighter stages.

Thanks!

[rnolan]

If you wired the external speaker in series with the internal one you'd get 8 ohms (which I assume the amp would be ok with ? as is SS output). But 8 ohm load will give you less power, and having 2 speakers also gives a 3db boost (giveth, thaketh away...).  You could do a couple of jacks to wire it this way (like the ADA split stacks).

[Dante]

Actually, the stock speaker in the amp is a 4 ohm.

If I put an 8 ohm speaker in the combo, I'm sure I could wire up two jacks to make a 4 ohm load out of those two like this:

[rnolan]

Hey Dante, yes that's one way to go, but then if you didn't use the extension speaker the amp will see 8ohms from the internal and give you a bit less power. 
But there is another way I thought up last night (when I should have been sleeping  :facepalm: ) where you can use either jack and use 2 x 4ohm speakers using 2 x switching jacks.
So the switch bit connects to the +ve (tip) when no jack plugged in.  So with 2 of these jack sockets, you can wire it to be series with 2 speakers (both jacks; 4 + 4 =8ohms) or either jack will work on it's own at 4 ohms or 8 ohms.
T= tip, m = middle lug (connects to tip when unplugged), S = sleeve (earth). Note: for this to work you have to make sure the jacks don't earth to each other (i.e. not use a metal plate unless you insulate the jack from it as the plate earths them together.
On each jack wire the m lug to the S lug, join j1 S to j2 T, wire j1 T to amp +ve, and j2 S to amp -ve (series when both jacks used but each jack will also work on it's own)
so:             
amp +ve > j1 T
                  j1 m > S
                         j1 S >>>>j2 T
                                           j2 m > j2 S
                                                      j2 S >>>amp -ve
A. So with a speaker plugged into just j1, j1 T connects to amp +ve, j1 m is disconnected from j1 T, j1 S conects to j2 T wich is connected to j2 m (as no jack) which connects to j2 S connects to amp -ve, all good amp sees 4 ohms

B. One speaker plugged into just j2, j2 T connects to j1 S which connects to j2 m which connects to j1 T (as no jack in j1) connects to amp +ve and j2 S connects to amp -ve, all good amp sees 4 ohms

C. (series) 2 speakers plugged in, j1 T connects to amp +ve, j1 S connects to j2 T (speaker 1 -ve > speaker 2 +ve), j2 m disconnected from j2 T (coz jack plugged in), j2 S connects speaker 2 -ve to amp -ve, all good amps see 8 ohms.
-------------
D. (parallel) as you say just use 2 non switching jacks, wire j1 T to amp +ve and to j2 T, j1 S to amp -ve and j2 S, 2 8 ohm speakers amp sees 4 ohms (this option gives the most power).

Anyway, options.

MJMP, correct me if I'm wrong, but could Dante also wire a 4 resistor in series (inside the jack) for the 4 ohm speaker and make it 8ohms ??

[MarshallJMP]

MJMP, correct me if I'm wrong, but could Dante also wire a 4 resistor in series (inside the jack) for the 4 ohm speaker and make it 8ohms ??

Yes he can but you will loose 50% power to that speaker. So in total 25% of the power will be turned into heat by the resistor.

[Dante]

Richard,

Go to bed - lol!! Thanks for the detailed schematic in text, I do appreciate it. I'm leaning towards leaving it alone. No sense dragging the combo amp and an external cab when I have a 4x12 that will do a better job of filling the space. The 1x12 cab is awesome for smaller shows as a standalone cab with the same rack.

Dante