Doc

Hi everybody
I'd like to replace my 1960A cab with a 4x12 greenback (25W each)
I'm also willing to buy a new head. Will a 100W jmp be ok? or will be better to buy a 50W one to avoid damaging the cab?
thanks

Doc

Marshall Amps

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PaoloJM

If you intend to crank the 100W head you will be beyond the capabilities of the greenbacks.
Remember that a valve will typically push about 150% of it's rated wattage when fully cranked.
IMO the 50W JMP would be a better option, the 1987X is a great amp and the DSL 50 has good rep 'round here too, and they're both damn loud too!!

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jtm-series player

In case you turn the 100w amp all the way up, the cab and
as a consequence of this the amp can/will take damage.

The 50w jmp would be the save way to go.

*EDIT*

paolo was faster then me! and he's exactely right!

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Doc

thanks, i will go for a 2204 or a 1987x
cheers

Doc

By the way, -only for my knowledge- a 100W jmp head wont damage a 4x25W greenback cab if the master volume is not pushed over 5, no matter how you crank the pre-amp volume, is that right?
I mean, is only with the master volume that i can do a damage, or the pre-amp can as well?
bit confused

PaoloJM

There's not a quick answer to this.
The power output is a given as the average power at the on set of clipping (Vrms X Irms, where Vrms equals the peak voltage X 0.707 and Irms = the peak current X 0.707).
This means that the power is 100W right at the point before the power section starts clipping.
The preamp volume is usually positioned after the first gain stage. It controls the level of the signal pushed through the preamplifier.
The Master Volume is usually just before the phase inverter and controls the signal level delivered to it and through the power amplifier.
In this way both controls affect the signal level delivered to the power valves and thus the power output of the amp.
With the preamp volume right down you would have to push the master volume right up to drive the power amp close to clipping (ie close to 100W output).
You should also note that Vrms = Vpeak X 0.707 when V is a sine wave only. However in a heavily distorted signal is effectively a square wave and Vrms equals V peak and so the power is higher for the same value of Vrms. The same applies to current in a power amplifier and this is why valve amplifiers can put out up to 150% of the rated output.

Anyways, in short after all that technical mumbo jumbo, both controls will affect the power output but I would agree that the master volume has a more significant effect.

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MonstersOfTheMidway

Something else that is important to remember: resonance. I have a JVM410H that I run through a 425B series cab that features celestion/marshall g12c speakers (the jimi hendrix speaker). My amp has a resonance control and a presence control. After carefully reading the JVM owner's manual, I encounterd this important bit of advice: "Care has to be taken to avoid over excursion of the speaker cones in high resonance settings." From my understanding, presence affects the high frequencies while the resonance affects the lower frequencies, so volume is not the only thing that can damage your speakers. By the way, I rarely find the need to push my amp beyond 1 o'clock, but I have "dimmed" it once and nothing bad happened other than the cops showing up.