monty

Hi, I have a JCM900 50w valve combo amplifier, with an 8 ohm speaker in it

i have just got an extension cab which has two 8 ohm speakers in it wired in series, meaning the resistance of the cab is 16 ohms

on the back of my marshall amp is a switch to choose between 4 and 8 ohms. my JCM900 has two speaker output sockets.

i was wondering, is it possible to leave the 8 ohm speaker already inside the combo plugged into one of the marshall speaker sockets as normal, but use the spare speaker socket on the back of the marshall amp to plug in the 16ohm cab? and if so, do i set the switch on the back of my amp to 4 or 8 ohms?

finally, assuming i can use this 3 speaker combination in this way, looking around the internet i believe that the single speaker inside the combo would be louder than the cab (because it would receive more power from the amp (about 2 thirds)) ..the cab would receive the other third of the power) so is there any reason why i cant use my attenuator (power brake) with the speaker inside the combo to lessen the volume a bit on the single speaker and even things out?

or, is this whole idea bad and i could damage something, eg. my JCM900 or the speakers..?

would appreciate any advice, best wishes, monty

Marshall Amps

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JohnH

Hi monty - unfortunately your arrangement is far from being ideal. There are plenty of 8 ohm 2x12 cabs, such as a 1936, that you could use with that amp, either on their own (amp set at 8 Ohm) or with the internal speaker (amp set at 4 Ohm), which would balance the power and match the Ohms properly.

But your 16ohm cab, on its own, does not match any settings on the amp so youd be risking the amp.

Together with the internal speaker, the combination is 5.3 Ohms, but as you noted, 2/3 of the power will come from the single combo speaker, which will probably mean that you dont hear much benefit from your cab. You might be lucky, if your cab has very efficient spaekers such as V30s though. Theres still an impedance mismatch at 5.3 Ohms. You are probably (possibly) OK to run this at your 4 Ohm setting. No promises, but probably OK at least to try it at low to moderate volume for a test.

Other options:
1. Wire your cab with speakers in parallel, to make a 4 Ohm cab and use it on its own without the combo speaker.
2. Get two 16 Ohm speakers, wire them to make an 8 Ohm cab and Ebay the old cab speakers
3. Take out one cab speaker, and blank off the hole with plywood, now you have a large 1x12 cab, at 8 Ohms (great bass with this, with twice the cab volume per driver)
4. Add a switcheable 8 Ohm resistor in series with the combo speaker, to make it 16 Ohm total for when you use the cab also. run the amp at 8 Ohms. The resistor will soak up 25% of your power. Make it out of a group of high power resistors in series and/or parallel with a total power rating of at least 20W and 8Ohm combined.

John

__________________
DSL401
8412 cab
Crate Powerblock
1 x 12 Vintage 30 cab
'76 Shergold Masquerader
'93 Les Paul Studio
'83 Roadstar

monty

hello John, thanks for the reply and the great info

well, i think i might have to go with another option, and that is, i have another two 8 ohm speakers which i can put in another cab. then i will have four speakers in total, in the form of two 16 ohms cabs. so, then, i just plug each cab into the two jcm900 speaker sockets, and set the switch to 8 ohms and im done, is that right?

and could u help with one more question please. can i actually link up both the cabs together (so as to form a series-parallel wiring setup) and then have the total output of the two cabs joined together equalling 8 ohms, and then only have to use one of the speaker sockets on the back of the JCM900, switch set at 8 ohms still, and therefore be able to use my attenuator/power brake for the two cabs at once?

best wishes, monty

JohnH

Monty - yes both of those options are fine. To the amp, they will feel the same and sound the same. The second option just needs some form of Y connector, with three legs so that all the tip connections are joined and all the barrel connections are joined. eg It could be a box with three jack sockets in it, two go to the cabs and one (via the attenuator if required) to the amp. It wouldnt matter which sockets go to what since they are all joined.

Another good option is to have a second jack socket on one of the cabs, wired in parallel with the main jack socket, then you can daisy chain the second cab by plugging it in there, with a single cord from Cab 1 back to the amp. Some commercial cabs have this feature and I just put it on a cab that I built.

Just for grins, you could plug the combo speaker in, and the combined pair of cabs, and run the whole lot with amp set at 4 Ohms. 1/2 the power would go through combo speaker and 1/4 from each cab.

John

__________________
DSL401
8412 cab
Crate Powerblock
1 x 12 Vintage 30 cab
'76 Shergold Masquerader
'93 Les Paul Studio
'83 Roadstar

Webslinger

Some months ago. I also had the same issue and had the same idea about installing an in-line resistor to raise each speaker load to 16. I posted the question and I remember you and some other memebers thought this idea wouldnt work very well? I did some other research and read something about speakers being somewhat of a variable-working load, as opposed to a static impedance of a solid resistor that would just draw-off heat & power. Did you or anyone ever try this since? Id still be curious how it would work out?

(I wound up doing suggestion number 2...just swapping the 8ohm Celestions for a pair of 16 Eminences, but I would like to have kept the 8s since they sounded great. They were old G12-80s)