Whats the real power output of a 50W plexi?

[Silvio55]

Hi!, I always wanted to know whats the real output power of a Marshall 1987 50 watt "plexi" at full volume. I'm gonna load a 2x12 cab I have with a pair of 25W Celestion Greenbacks, I always heard that a 50W plexi puts out more than 50 watt at full volume and would fry a pair of greenbacks, rated at only 25 watt each.
But then some people say it's imposible for that amp to put more than 50 watt at full volume, maybe just a little more if your plate voltage is high (I donīt remember exactly what's my plate voltage, I think it was around 420V). And others say that the rating of the Celestion speakers is rather conservative, and a GB should handle more than 25 watt just fine.
I e-mailed Celestion asking if it's safe to run my 50 watt plexi with just a two GB's, they said "no problem".
Well, I think the guys at Celestion know what they're talking, but now I have this doubt in my head. Does anybody know for sure how many watt this amp put out?
Thanks!

[Dave H]

No replies eh? I once replaced an OT in a 1987 and I measured it at 60W but that was a long time ago and my memory is shot so I'm not sure how accurate that is.

[Old Tele man]

1) Rule-of-Thumb: ALWAYS size speakers at TWICE the amps real power.

2) Lookup the power-content difference between a 100Vpp SQUAREWAVE and a 100Vpp SINUSOID.

3) Sinewave signals eventually turn into squarewaves when "overdriven" enough.

[Leo_Gnardo]

Early 50W I've worked on, plexis & early 70's, measured 30W at clip give or take a watt or three. B+ typically 395 to 420V. It used to flummox me because they're "s'posed to be 50W" right? Then one fine day I did the math, 30W is to be expected. I did post this result a couple months ago and got a nice fat "B.S." from another poster. No B.S. folks, that's what I measured. In fact I had a good jaw wag with John Suhr in the late 80's about this very subject, he was similarly stymied with the power figure, finding the same thing. We decided "if it sound good it IS good, so what the power."

Couple years ago a studio sent in an early 70's 50W JMP for repair. I'd fixed the amp for them about 15 years previous, was working fine, standard stuff. By the time they brought it back, it had a new power transformer and B+ was closer to 500V, rather stiff. Complaint was it was red-plating output tubes, 6550's they'd installed at some large expense. Nobody thought to adjust bias. That's about all it needed, and that amp measured 60 watts at clip. Back to work it went.

Of course amps will turn out more than clip measured wattage when you drive 'em hard. Old Teleman's post is spot on. 50W Marshalls sound great to me. Including when you schlam 'em hard! You may get away with a pair of greenbacks, but should carry a spare cab to every gig. And if you're going to do that, might as well have the 4x12 or two 2x12, share the power across 4 speakers, less likely to wreck 'em.

[Silvio55]

Thank guys!
There's an old engineer here in Argentina who makes tube amps and speakers, and a former employee of a big speaker factory, he once said in a forum that it's impossible for a Greenback to handle "only" 25W, Celestion advertises that way only to be "vintage correct specs.", but in reality this speaker would handle more than that just fine, since they're not made the same way they made them in the sixties, speacially since mordern adhesives are way better, and dry faster than old ones. He also said that's why an AC30 uses two 15W speakers for example, they know it would take the power with no problem.
I think he's right, and that's why the guys at Celestion told me that's there's no problem if I use a 50W plexi with only two GB's, or at least that's what I want to believe.
Well, I bought the speaker yesterday (I had another one already, that's why I a bought a GB), I hope they're right, man if I blow up the speakers I will never buy anything from them again!!

[nosaj]

I think it also has something to do with how they are wired series or parallel.
https://www.amplifiedparts.com/tech_...ing_and_wiring

nosaj

[R.G.]

Ah... we seem to be wading in the amp power and speaker power swamp again.

Audio power is easy to figure. Put a *resistor* load on the amp of the correct load resistance and enough power rating, run a sine wave into the amp and raise the output level until the sine wave is seen to just barely clip. Note the peak voltage. The RMS voltage of that sine wave is Vpeak/1.414.

The power into the resistor is then Vrms squared divided by the load resistance. 50Wrms into 8 ohms is 20Vrms sine wave or 28.28V peak. A square wave with a 28.28V peak produces 100Wrms because it's always full positive or full negative, no sloping sides running up to or down from full voltage.

Why a resistor? Because speakers are anything but a constant resistive load, and the actual power put into a speaker is highly variable, even if the speaker has the right load impedance number printed on it. An 8 ohm speaker is only 8 ohms at a few frequencies and in certain cabinets and on the Tuesday before a full moon. Resistor loads is how the amplifier people measure power output.

But you can tell what the amp really puts out by doing the resistor load and measure trick.

Now we get waffly. Tube amps do not have a constant output power. The power they put out depends in a complicated way on the speaker loading. If you have your amp set up so that the amp expects an 8 ohm load and you connect a 16 ohm load, the output power goes down. Likewise, if you connect it to a 4 ohm load, the output power goes down. This is unlike solid state amps that always increase power with decreasing load resistance until something burns out. So the output power on the amp depends on having the correct nominal speaker load (or resistor) for the output transformer tap. Get it wrong, power goes down.

There is enough variation in actual output with OT setting, speaker impedances, waveform, and phase of the moon that you could probably say that any given "50W" amp puts out anywhere between 20W and 100W without committing perjury. In fact, stating an amp's power output doesn't mean much unless the conditions it was measured under are also stated.

The mis-quotation "Lies, d@mned lies, and audio power specs" comes to mind. This whole morass is how you get car audio amps and speakers rated at multiple kilowatts.

[R.G.]

I forgot - even after you know the electrical power output of the amp into a resistor, when you put a speaker on it, there is the issue of how efficiently the speaker converts electrical power to sound pressure waves, which is presumably one reason we have audio amps instead of hair dryers and toasters in bands. Speakers vary tremendously. 12" speakers produce as low as 84db SPL to as high as 108db SPL from my memory of speaker specs (db reference to 1W at one meter on axis is a normal way to state that) so replacing speakers may cause as much as a 20db change in sound pressure, equivalent to a 10:1 change in amplifier power. And then the human ear gets into it, with a 20db difference sounding roughly "twice as loud".

[Enzo]

I have to add this. There is the whole run it up to clip and that is the power it can produce. That would be fine if the power tubes were clipping. But inevitably, they are running the test signal into the input jack, and so the signal runs through the preamp to get there. Clip the preamp and there you are. To use that hifi test, you need to inject your signal right at the PI.

But in the guitar amp world, the power question is about blowing speakers or not. So rather than what is the true hifi clean sine wave output maximum, it is how much heating power can this amp put into a speaker. You think anyone is going to run their amp up to clip and stop right there when playing? Not likely. You think anyone will say, "OK, my 25w speaker will be OK, because my 50W amp only puts out 20 watts when it starts to clip"?

[Dave H]

Couple years ago a studio sent in an early 70's 50W JMP for repair. I'd fixed the amp for them about 15 years previous, was working fine, standard stuff. By the time they brought it back, it had a new power transformer and B+ was closer to 500V, rather stiff. Complaint was it was red-plating output tubes, 6550's they'd installed at some large expense. Nobody thought to adjust bias. That's about all it needed, and that amp measured 60 watts at clip. Back to work it went.

I was surprised when I measured 60W. It wasn't an accurate measurement eyeballing it on the scope but it was up there. It had a replacement PT and I replaced the OT so it was hardly original. My home brew 5f6a only measures 35W with its tube rectifier. I did read a piece in Guitarist mag where they took an original JTM45 back to Marshall and had it measured. It came out at 28W. Marshall said 28W is correct. '45' is the model # not the power output.

[R.G.]

I have to add this. There is the whole run it up to clip and that is the power it can produce. That would be fine if the power tubes were clipping. But inevitably, they are running the test signal into the input jack, and so the signal runs through the preamp to get there. Clip the preamp and there you are. To use that hifi test, you need to inject your signal right at the PI.

But in the guitar amp world, the power question is about blowing speakers or not. So rather than what is the true hifi clean sine wave output maximum, it is how much heating power can this amp put into a speaker. You think anyone is going to run their amp up to clip and stop right there when playing? Not likely. You think anyone will say, "OK, my 25w speaker will be OK, because my 50W amp only puts out 20 watts when it starts to clip"?

Dead right Enzo. Guitar amps are odd ducks in the amplifier world. Some distort in the preamp, either accidentally or by design; likewise some distort in the PI before the output tubes are running full tilt, and some clip in the power stage.

There is even the question of whether the output tubes *can* clip. The PI may or may not have enough output swing to drive the output tubes to clipping, and the output loading may force a load line on the tube plates that can't be driven to clipping even with a proper PI swing, or may grid-clip early before the plates can swing far enough to limit by plate voltage swing.

I was synopsizing heavily enough that I see I even left out why someone would even want to measure sine wave RMS power.

[Leo_Gnardo]

I was synopsizing heavily enough that I see I even left out why someone would even want to measure sine wave RMS power.

Zinggg... OK I felt that! Go ahead call me a nut if you want to. Scarcely an amp I've seen won't pass a good sine wave. Maybe there are some amps out there that are incapable, by design, of doing that. Leo_Gnardo's term for them is "intentionally lame." That doesn't mean they're not marketable or capable of a sound quality that fits some player, just don't expect to ever get a clean sound out of them. Good enough for folks who like that. There's something for every taste in this world, like clothing, cars, food & everything else.

Old 50W Marshall, no worry about crunching the preamp, plenty of headroom there as long as components are healthy. When using a 12AX7 invert/drive tube, may see some premature collapse one side of output, part of the old Marshall sound. Swap in a 12AT7, no collapso, proves amp's healthy. Put AX back in so Johnny Guitar has the sound he expects, good to go.

[pdf64]

There is even the question of whether the output tubes *can* clip. The PI may or may not have enough output swing to drive the output tubes to clipping, and the output loading may force a load line on the tube plates that can't be driven to clipping even with a proper PI swing, or may grid-clip early before the plates can swing far enough to limit by plate voltage swing.

I wonder whether the power tubes in a class ABetc1 amp actually saturate / clip / overdrive etc, as at the amp's normal clipping point, they would presumably pass yet more current if only the preceding stage could pull the power tube control grids more positive.
Of course the power tubes may be at or beyond an average 100% plate dissipation at clipping, so it would be inadvisable for them to the pushed even harder, but the point still stands.
So clipping in a class ABetc1 amp may be seen to occur at the power tube control grids.

[g1]

I agree with what you were told about the modern "25W" greenbacks being able to handle more than that.

The JCM800 "1987" was officially rated as "in excess of 50WRMS @ clipping (3% average distortion), 90W at 10%THD."
That should give an idea of how much of an effect on power output clipping has.