🙄 , 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.

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 😳

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 🙂

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 Marshallamps.com 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 😀

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.