JTM1C impedance question (using ext cab with combo)

[MarshallInTown]

sorry if Im a bit slow but if I want to use an external cab with a JTM1-Combo I have to do the math between the stock speaker impedance and the ext cab one ?? or plugging an ext cab automatically switches off the internal speaker ?? the manual doesnt say anything about

thanks x clarifyin my issue

[Voodoo Chile]

When using an extension cab, the internal speaker is disconnected and not heard. Just make sure to match up the proper impedance from the back panel with your cab. Master of the obvious: use SPEAKER CABLE!

[MarshallInTown]

thanks x the info now I'll try it with a 4x12