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Discussion Starter #1 (Edited)
I recently got a Marshall JCM 800 4210 combo but have only used the combo speaker for practicing at home at a fairly low level. I play in a very loud rock band and when we rehearse I'm running the Marshall through a 1960a 4×12 with the gain at about 6, channel volume at 9 and the master just a little over 3/4 of the way up.
I'd like to try the combo by itself at some point but I'm worried about running the amp that loud through the stock G12M-70 since it's only a 70 watt speaker.
I do have a 2×12 with 120 watt eminence legend V12's in it and I was thinking of putting one of those in the JCM 800 and using the Celestion in the 2x12.

What do you guys here at GC think? Can the G12M-70 take a beating or would I risk blowing the speaker running the Marshall that hot through it?
Would the Eminence be worth trying even if wattage isn't a concern with the other speaker? As far as I can tell from the Internet it's a V30 ish or "british" voiced speaker so it should go well with the Marshall Id guess
 

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Unless you use the neck pickup, crank the bass, and play choppy chords in first position I would not be worried.
 

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Discussion Starter #4
Upon those recommendations, I just put some earplugs in and gave it a good go at my rehearsal settings. The speaker seemed to handle it just fine. Sounds surprisingly full even with just the one speaker doing all the work. I guess I can save myself the trouble of a speaker swap.
 

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Discussion Starter #6
I just had the first rehearsal using just the 1x12 combo on its own. Sounded great! I'm really glad I grabbed this amp when I did. It's nice to have a small combo that can hold up so I don't always have to be lugging cabs around
 

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Speaker voice coils can behave like fuses (and go poof) under a number of circumstances. It helps to understand the inner workings of speakers to be able to second guess such circumstances.

The efficiency of the speaker is the extent to which it can translate the current passing through it into movement of the voice coil (and the speaker cone it is attached to). In conventional dynamic speakers, the voice coil is wound on a coilform that sits around a magnet. The strength of the magnet plays a role in determining how the coil responds to fluctuations in the signal voltage coming from the amp. But because we're talking about magnets, you can imagine that the distance that voice coil is from the magnet also determines how much the magnet can influence the movement of the coil (not unlike the way that distance between strings and pickups influences tone, sensitivity, and response). Higher-quality, and especially higher-efficiency, speakers will be designed to have as small a voice-coil gap between magnet and voice coil as possible. Smaller gaps will rely on precision engineering such that when the coil moves back and forth, its physical coupling to the cone allows it to move like a perfect piston. Remember the children's game Operation? It's a bit like that. You need that voice coil to move along flawlessly so that it never touches the side.

Why is touching the side bad/wrong? Well, not only does it distort the tone, but since the voice coil is moving back and forth, if it touches the side of the gap that means it is rubbing when producing sound. And what happens, dear cub scout, when two objects are rubbed together at high speed? Why, HEAT of course. But you also likely know that rubbing two objects together slowly results in very little heat buildup. It's not just how hard you rub the sticks together, but also how fast and persistently, that determines the heat buildup and likelihood of generating enough heat to burn.

Okay, let's bring this back to the realm of speakers. If a speaker has to take occasional big bass transients, AND has a big voice-coil gap, that is less of a risk than if a speaker with a narrow voice-coil gap is being asked to handle sustained distorted notes, since the fuzzed-out sustaining notes represent potentially rubbing the voice-coil against the side of the gap at steady high speeds. A coupla swift rubs every now and then won't get your campfire started, but steadily rubbing two sticks VERY quickly probably will.

How does this translate into what you should be careful with, as regards your amp and speaker? You don't need to worry so much about the occasional power chord (unless it makes the cone move SO much that the bit of slack in the speaker's internal leads is exceeded and it snaps). You likely DO need to keep a lid on playing steadily at high volumes with a lot of high-frequency content. Not just for your ears, but for your speaker as well.

All of that said, I think the recommendations of folks above are pretty much in the ballpark. If the speaker is what the amp came with, chances are very good that Marshall anticipated this concern.

If you are pondering other speakers, do be mindful of their respective efficiency. Loudness of speakers of identical size, fed the identical wattage, can vary substantially, sometimes by as much as 6-8db, which is a lot.
 

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Honestly, with ordinary guitar playing it would be a bit of a challenge to blow a 25 or 30 watt speaker with a 50 watt amp. Unless you are deaf, or wearing earplugs or have the cab in another room and you're actually trying to blow it up, it will get REALLY loud while still in the safe range even with say a Greenback.
 

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Quite likely true, but do keep in mind that power ratings are not as consistent across brands as efficiency ratings or magnet size. I'm sure some of you have seen cheap (as in $10) 3" computer speakers rated at 180W PMP. I'm pretty confident that major manufacturers do try to make their specs honest and comparable to competitors, if only because reputations are at stake. But it pays to keep one eye open for the peculiarities of the spec, just in case.
 

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Discussion Starter #10
Thanks for all that info! In rehearsal we do use earplugs and the amp is turned 3/4 of the way up on the channel and master volumes with a boost on most of the time. I use an almost "metal" amount of gain. I'm the only guitarist in the band so I go from playing power chords to single note runs to solos all in the same song. I'm pretty confident in the amp now after testing it at a rehearsal. I had it up on the cab pointing directly at my head and didn't hear any noticeable stress on the speaker. Everything was coming through clear as a bell.
 
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