How to use Marshall 2x12 1936 cab with Marshall JTM 60 1x12

Hi! I got combo JTM 60 and want to use it with 2x12 cabinet. Unfortunately, I don't know how can I plug it in without taking any risk of damaging my combo. I read similar topic on this side and I hope to recive also such professional and helping respond. Thing is that I want to use all three speakers in the same time, so without unpluging internal speaker. I know that impedance is important, however I don't understand it at all I have to admit. I hope that someone can help on that forum. Is there any possibility to use all 3 speakers, and what lead I should use?

Thx for your time!

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First thing we need to know is what is the impedence of the 3 speakers individualy?

combo 16ohm, cabinet 16 ohm each, however if I use mono its 8 ohm. Here is how it looks in my cab:

Problem is that I don't understand anything about impedance.
Here is manual for the amp: â€¦ %20Eng.pdf

Is it possible to use 2x12 cab for 1x12 combo?

The JTM60 has an internal speaker of 16ohm impedance. If the external speaker socket is connected to a 2x12" cab of 16ohm impedance (the JTMC212 was designed for this combo) then the output impedance of the amp changes to 8ohm and this is within the combo's safety limits.

However, connecting to the mono socket of your 1936 will give the cab an overall impedance of 8ohm (the two speakers are connected in parallel) and that, together with your combos 16ohm speaker (which will also operate in parallel with the cab) gives a final impedance of 5.33ohm. This is lower than Marshall's recommended minimum. Now I, personally, don't spend too many sleepless nights over a fairly small mismatch like this but it does put a strain on the output transformer (OPT) and Marshall's are known for tolerating this less well than other amps.

I'd say your options are like this:
1) Plug the ext spkr into the mono 8ohm of the 1936 and take the risk of overworking the OPT.
2) Plug the ext spkr into the "right" channel of the 1936. This means that only one of the cab's 12" speakers will actually pump out the volume but the overall impedance will be 8ohm.
3) Disconnect the internal speaker and use the mono socket. Again, only two speakers working and the overall impedance will be 8ohm.
4) Get hold of a JTMC 212 and sell off the 1936. That way, you'll have all three speakers going and be operating at 8ohm.

ps, you must use an unshielded "amp to cab" cable and not the standard, instrument lead lying around in your bag. Your local guitar shop will be able to supply you an unshielded cable and really ought to know the difference (avoid the spotty 17 year old on Saturdays). Using an instrument cable to connect an external speaker is not a good idea.

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