Marshall JTM60 and C212 [2x12] extension cabinet

Hi guys - I'm a complete newbie and need help with connecting my extension cab to my combo. The extension output socket on the combo just doesn't work and my local amp guy had it for for two weeks and can't figure it out. Plug the extension speaker in and it screammmmms big time. This is what I've done - tell me if it's OK or not! I've put a splitter into the internal output socket [one male, two female] and then connected the combo speakers to one side and the extension speakers to the other side of the splitter. I figure [?!*?] that's OK as they will be connected in parallel and the handbook for the JTM60 says "The JTM60 combo will deliver 60 watts into either one or two 16 ohms speakers, ie. 16 or 8 ohms total impedance. The unit shold not be driven into a load of less than 8 ohms." Seems to work fine but I want to check it out in case it may be doing some damage. Thanks for your help. Ken

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Rocken, be very careful when connecting extension speakers and cabs, if you kill the output transformer then it could prove to be very expensive, first of all what exactly have you wired to where?
we need to know what speakers are in the combo, how they are wired to the amp (series or parallel) and what this splitter is? also we need to know impedance of all speakers.

A drawing (done in windows paint programme) would be very useful.

We have a resident expert who goes by the name of 1bassleft who will be chomping at the bit to get hold of this one, i can hear his engine revving up now.

PS we are hoping to get an amp section up and running on here soon.

Hi - Thanks for the reply. There are two 12" speakers in the combo with an overall impedance of 16 ohms. Same for the extension cab - 2X12" with impedance of 16 ohms. Normally, the combo speakers are connected to the internal output socket and an external output socket is provided for the extension cab. That external output socket is faulty so I've connected both the combo speakers and the extension cab speakers through the internal output socket. I've just used a simple splitter to give me two sockets - one for the combo speakers and one for the extension cab speakers - and they are both connected to the internal output socket with the splitter. hope this clarifies the points you raised. Thanks again.

Hello Rocken. I've been thunking about this one for about 24hrs (not continuously, obviously :) ) and had a look at the schematics for the JTM60 combos. The short answer is, what you've done will work (you've found this out already) and, even in the worst case scenario, is unlikely to harm your amp, but there is the possibility that you're not getting the full benefit of your extension cab. I'd ask for a bit of clarification from Marshallamps. com, who're usually pretty good at tech support. Here's what I'm thinking...

From the schem, I can see that the output sockets are mounted directly to the PCB (printed circuit board). I have some sympathy with your tech, here, because PCBs are fine when everything's hunky-dory but tricky to suss when things aren't right. I can also see from the manual that the JTM60 can handle 8 or 16ohm but there is no selector switch for the differing impedances. Now, with a solid-state amp, that's not unusual and you can use cab(s) with lower impedances within the lower limit (8ohms in the case of the JTM60) and get an increase in output Watts and volume. Valve amps are usually a bit different, though, and you need to "tell" the amp, via the impedance selector, what load your cab(s) are in order to match things up and get the best efficiency from your rig.

Just using the combo speakers, (a pair of 8ohm wired serial to make an overall 16ohm) connected to that "internal" jack is exactly what all the amp's components are expecting. By using a splitter, you have a nice solution for connecting the C212 cab but there is a problem. You now have the C212 (16ohm) connected in parallel with the combo's internal speakers (16ohm) to give an overall loading of 8ohm. But, without a selector switch, your amp has no way of "knowing" this. As far as it's concerned, you've just plugged in the internal (16ohm) speakers. This possibly creates a mismatch that makes the transformer and the valves work harder (although it's not as amp-deadly as sometimes made out) and you don't get the output you could.

The thing I don't know is: does the output transformer (OPT) automatically handle/sense the different loading or does sticking a jack into the external socket "jump" the circuit into 8ohm? You have tried sticking the splitter into the ext socket have you? Does it still scream? If not, that's where I'd plug it all in. I've got a feeling that this ext socket "tells" the OPT that you are now using an 8ohm load. It might best be solved by sticking a "dead" (+ insulated) jack into the ext socket, and putting the splitter into your normal jack as you do now.

I'd feel better if the boys came in. You could email them and link this thread in your email. Otherwise, perhaps I could yak with them and do likewise. Either way, it would be great if their expertise could be added to this thread or else put in their FAQ.

Great Q-thread, rocken :D

Hi 1bassleft - Thanks so much for the superb reply. Yea - what you say really makes sense and fits my feeling of unease about using the splitter like that. I was sort of moving in the same direction with my thinking and I was trying to figure out how the amp could "tell" that I'd chaged the impedance to that socket. This evening, I tried sticking the splitter into the ext socket as you suggested and - hey - it worked! .... but only for a while. It sounded great and then when I switched on my Rat - boom! it started roaring - not screaming this time but low deep down roaring. If I keep the volume down it seems to hold it but as soon as I crank it - off it goes again.

Then I also tried putting a "dead" (+ insulated) jack into the ext socket, and putting the splitter into the normal jack like you said and that seems to have solved it. It is definitely louder, clearer and faster - sounds great. By comparison, without the "dead" jack, it's really muffled and a lot quieter. I've turned it up as far as I dare - loud! - and it's still OK - no screaming, no roaring. Brilliant! :D Thanks a lot! I just want to check what you mean by a "dead" jack + insulaled. I've just used one end of a lead - should I insulate it in some way?!

I've also contacted guys. I'll let you know when I hear back. Thanks again for your detailed and considered reply - much appreciated. You guys sure do provide a great service on this forum.

Yowza! Not only is it very rewarding to hear a suggn tried and reported on (your compliments are much appreciated), but it's also a huge help to others out there wondering the same thing and trying a search engine.

I'm no wiser from the PCB schem (and unlikely to be) but, from your descn, it seems extremely likely that the dead jack in the ext socket "tells" the OPT that it's now playing with 8ohm. The symptoms of a mismatch (OPT thinks x-ohm, actual loading = x/2 ohm) are a loss of potential output volume and a rolloff of the treble frequencies. From your reply, it looks exactly like the difference between using the splitter with or without the "dead" jack in the ext socket. The Marshallamps solderheads will know for sure better than I do.

Finally, here's what I mean by "insulated" dead jack. Putting an unplugged "extra" lead in doesn't harm the amp; I just don't want it to harm you. I can only explain by telling you my "when 1BL was young and thick" story :oops:

Back in my teens, and still now, I'd check a "bfft, bfft, durrr, durr" noise by pulling the jack out of my guitar and sticking my thumb on the jacktip. My body, grounding the miniscule difference, would silence the noise and tell me I had a bad earth problem. One time, though, I was still getting the noises so I pulled the jack out of the cab connected to my 100W valve head. Not a great idea anyway (running a valve amp with no load = internal smoke), I then checked the lead by sticking my thumb on a 1/4" tip carrying the sort of current designed to turn the magnets on my speakers around enough to push the cones in and out.

My arm and legs did a pretty good impersonation of MC Hammer's U Can't Touch This breakdance. In fact, I looked about as silly as the video, and I wasn't wearing his taste in trousers. It didn't kill me, but it wasn't my favourite experience.

So, bottom line, if you just stick a spare, dead lead in the external socket, do wrap some insulating tape all over the other end's metal bits. Saves you making an eedjit of yourself at some point during setup, soundcheck or teardown :)

:roll: , I'm such a dumbo, the obvious solution has only just occured to me. Get one of those really cheap and nasty 1/4" jacks in a moulded, plastic mount. They're maybe 70p/70cents from the likes of CPC Farnell (UK) or Radio Shack (US). If you have to, go the whole hog and pay a coupla quid for a cheap and nasty moulded lead, and pull out the cable.

All you want is the male plug, telling the PCB you're now in 8ohms territory. The moulded cover (without any wire going anywhere) provides perfectly good insulation against forgetful fingers. Just a bit of black tape over any exposed end is all that's required.

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