Marshall origin 20 watt combo with greenback 30 watt....risky?

[daz]

I have this combo amp with a 50 watt 10" speaker stock and the amp is rated 20 watts and not very loud. I emailed celestion and asked if thier 30 watt greenback 10 would hold up in this amp and they said yes. But heres the thing....the amp get WAY louder when i boost it because it's a old school design with only 2 preamp gain stages and remains almost totally clean with a strat or tele even with the master and gain both on 10. So to get some drive i boost the hell out of the input with a 26dB booster and the thing gets way louder than no boost. I can see how the rep told me the GB 10" would hold up if he is familiar with that amp because the amp is not loud at all by itself. Maybe not even loud enough to gig a small bar w/o micing. Boosted as i will be using it it'll be a lot louder and while i won't have to crank it, but in a band context i WILL be louder then the amp is capable of w/o the boost. So i'm not sure if i can trust the rep's reply.

Yes, i realize i could get a different speaker that'll handle it and yes, i realize i could use an ext cab to add a second GB or whatever. But i'm not asking for solutions like that, I already thought about those things. I just want to hear what others think about how risky it would be to use this amp by itself with a 30 watt celestion GB for band use in a average loudness classic rock band.(and yes, the 12" is 25w but the 10" for some reason is rated 30)

[Enzo]

Do you think a booster pedal will turn a 20 watt amp into a 50watt one? You can cram a dozen booster pedals in front of that amp and it is still just a 20 watt amp. Celestion said their 30 watt speaker would handle a 20 watt amp because the amp can make only 20 watts. It doesn't matter how loud some things sound. If the plain amp has to be on 10 to make the 20 watts and now it makes 20 watts on 3, that doesn't mean it makes extra watts. The wattage is limited by the power supply.

[daz]

No, of course i don't ! I'm no genius but I'm not THAT stupid. I'm just saying that this amp is absolutely old school like a plexi where unlike a modern cascaded preamps with 3 or more stages, the gain that gets to the PI and power amp is not totally smoothed out and homogenized where it doesn't present a big clean transient like the origin does.So when you boost it the amp the signal is a very clean sort of OD and gets friggin loud as hell. It's literally as loud with the boost and master on about 3 as the amp is straight in with master on 10. I never said it was putting out 50 watts, please don't put words in my mouth to make me sound like an idiot.

But the fact is technical specs aside, the amp full up stock is one of the quietest amps i ever played, THE most quiet tube amp easily. Yet boosted it presents huge transients to the speaker that sound like the cone is about to go shooting out the front with the master well under 1/2 way. At that point specs and theory mean little because it's dead obvious the speaker is getting pummeled far more then the amp is capable of by itself. I cannot overstate that. So while i have been told it will handle it i'm not so sure. I'm not tech but i've been playing rock in bands for decades and i know when a speaker is being pushed like hell and the speaker with the stock amp with gain and master on 10 isn't even dirty with a strat.....literally ! Play one some day and see. Never played a marshall like that but thats what the origins are like. It sounds soft and weak and l don't think it would blow a 10 watter. Boosted it would be far too loud for my band if i turned the master more than maybe 10:00-noon.

[Enzo]

Relax, no one called you an idiot. Excess power is what blows speakers. The amp has a 20 watt power amp. It won't blow up your 30 watt speaker. Preamp boosters will not increase the 20 watts. They just make the 20 watts at lower number on the control. I merely made up the 50 watts as what it would take to blow up the speaker.

If you don't believe that, fine, put a heftier speaker in it and enjoy the amp.

[daz]

My point is just that whether you call it wattage or what, the louder a speaker gets, especially when u r talking such a huge difference (wish u could hear it ! ) then i would assume all technical talk aside wouldn't that be riskier. The other thing is do we know the amp makes only 20 watts? I have read uncountable times that tube amps when cranked can make a lot more wattage than spec, probably i would think depending on the method the manufacturer used to determine it. In any case u seem certain and know this crap a lot better then i so i'll take your word.

[loudthud]

Several years ago there was an Amp show in Dallas. One of the exhibitors was Celestion speakers. Each "Booth" was a separate hotel room, so there was lots of isolation and no blaring guitars to interrupt conversations. I spent a lot of time talking to the Celestion rep. One topic of discussion was speaker power ratings. Contrary to my belief at the time, the rep told me that Celestion speakers are rated much the same as speakers of American manufacture. The rating is, simply stated, a heat dissipation rating for the voice coil. So many watts in, the temperature will remain in a range where failure will not occur.

Prior to this, it was my impression that Celestion speakers were rated such that a 25W amp could be safely connected to a 25W speaker. An example was four 15W 12 inch speakers connected to a 50W to 60W amp. With American speakers such a Jensen or Eminence, the speakers rating should be twice the RMS rating of the amp to be safe because a 50WRMS amp can produce a 100W square wave. Most of the time, a heavily over-driven amp will not dissipate double the RMS power into a speaker load except on single notes in the 500Hz range where a speaker's impedance is mostly resistive. Power cords typically contain lower frequencies where a speaker's impedance is high and reactive in proximity to the resonance region.

It is my opinion that the 30W Celestion will survive surprisingly well in the 20W Marshall, but it will eventually "burn out" depending on how loudly it is played.

[Mick Bailey]

The only way you'd know for sure what the amp is producing is to measure the output - ideally using the speaker instead of a dummy load.

[loudthud]

There was an article in Audio Express some years ago that actually used an analog multiplier chip to multiply Voltage and Current in a speaker. That part of the circuit was actually brilliant. What was totally incompetent was that the signal was then sent to an RMS converter chip. The author didn't understand that the RMS converter chip wasn't needed, just an integrator to accumulate the actual power and subtract the negative products where the load returns power to the amp. This would tell you how much power was actually dissipated by a speaker load.

[Mick Bailey]

I'm not sure how Marshall measures the output power of their amps, but it's perhaps unlikely to be with a fully clipping output. Maybe it's something like 1khz into X Ohms at 5% distortion. Pushing the output further towards a square wave increases the effective RMS, even though the peak voltage does not increase, and this increases the heating effect that the voice coil 'sees'. I've had quite a few blown speakers with bass amps that have used a combination of sub-octave and distortion with the amp turned up full. Despite the peak signal being limited by what the PSU is capable of delivering (and within the capability of the speaker's ability to handle a clean signal at that peak level), the speakers have still suffered with burnt voice coils.

You'd perhaps think that a speaker operated within the limits of maximum cone excursion (xmax) would always be safe, but this isn't always the case. Speakers have gone the way of output transformers. At one time it was fairly infrequent to replace them, but nowadays it's a more regular failure. I had a graveyard of speakers with burnt voice coils - mainly newer Celestions, mainly taken from amps where the rating should have been fine for the amp's rated output.

[pdf64]

The amp is going to be overdriven, probably very heavily. So the only sure thing is to measure its max squarewave output RMS power into a resistor of the rated nominal load impedance.
That may end up being somewhere between 30 and 40 watts (probably closer to 30), assuming that the 20W amp rating is genuine, eg 5% THD.

I get the impression that Celestion ratings are a genuine continuous type that all individual units of that model should cope with, ie even the lower quartile. A design centre kinda rating. From that, it may be that many / most units cope with a bit more, hence their rep.

Whereas I suspect that Eminence ratings aren’t for a continuous signal / applicable to all units, eg an absolute max kinda rating. Such that given the above, an Eminence speaker rating needs to be 4x the amp’s rating.

[Mick Bailey]

20W into 16 Ohms is approximately 17.89V RMS, 25.28V peak. A 1:1 continuous square wave would give an output of 39.95W, so given that a clipped waveform is likely between the extremes, then I agree that 30W is a likely maximum output figure. Given the nature of a guitar signal, duty cycle has to come into it. It would be very difficult for a guitar to provide a continuous signal of any kind into a speaker. The construction of the voice coil and how well it copes with an intermittent overload will have a bearing on the speaker's ability to withstand a signal close to its rated power handling. Some speakers hold up much better than others in closely-rated conditions. The early neodymium speakers seem to be particularly fragile. By comparison, I have an old McKenzie 12" 50W speaker in my bench cabinet and that really has taken a hammering over the years as a test speaker.

[daz]

Wow, lots of great replies, thanks ! My takeaway is it IS somewhat risky, especially in the long run. So what i think i am going to do it try a greenback and if i am totally in love with it i will put the stock speaker in a ext cab and see how i like them together then replace it with a second GB if i decide addin in the stock speaker ruins the greenback's tone. I will use the amp at home by itself and with the ext cab when using with the band. I really wanted a small grab and go amp but since i only play out now and then it's not a big deal. Thanks all.

[g1]

the amp full up stock is one of the quietest amps i ever played, THE most quiet tube amp easily.

Are you 100% positive there isn't something wrong with the amp? From the description that is what it sounds like to me.

[daz]

Are you 100% positive there isn't something wrong with the amp? From the description that is what it sounds like to me.

Positive. I played 2 others of the same model at GC before i bought it and they were really quiet too.

[The Dude]

IMO, you're not going to get even close to 30 watts out of any reasonably designed amp with 2 EL84 output tubes- Marshall or otherwise. The Celestion will be just fine.