Is a 100w Plexi on 3 putting out 100w if it is just starting to break up?

[Axe-Man]

I was wondering about this as a 100w Plexi is very loud on 3.

I've read that a tube amps power rating is based on its ability to put out its watts before break up...hence a tube amp like the Plexi could put out 150 or 170 watts at 10 as it can continue to breakup and put out more power.

Is this correct and is it the reason that a 100watter is so damn loud when just on 3?

[diagrammatiks]

yes. should be if it's breaking up.

but you'd have to make sure it's not the preamp that's breaking up a bit first or the PI.

just checked the schematic. There's 2 stages with a volume between them followed by the tone stack...

it's possible that the pi is starting to break up first.

Most of the time the big 100 watt marshalls start to break up around 4-5.

[stump]

My 50 watt JMP puts out 51 clean watts at about 3 on the volume knob, 85 watts when dimed.

[Geetarpicker]

Another thing you need to consider is the taper of the volume pot in the amp. Original 60s Plexis typically came with nice volume pots with a pretty smooth taper that came on slowly. On some even half way up was still quite clean, and there was still a big difference between say 6 and 10, gain wise. Then in the 70s the metal face amps used a different pot that came on quite quick, plus having a bright cap on the volume pot added to the abrubt nature of the volume control. I've had some mid 70s Marshalls where 2 was almost as loud and dirty as 10. It's mainly a gain issue, but once a non master amp starts breaking up it's getting close to the full power rating regardless of where the actual "Volume" knob is set.

For comparison my Trainwreck Express has enough gain to develop 115db from a 4x12 with the volume only on about "1". With the amp cranked up to half or more it's only putting out about one db more. Point is once you get most non master amps into clipping territory you are probably already getting close to the same wattage as when it's fully cranked.

One way to fully limit the wattage potential is to run an attenuator. Each 3db of attenuation = 50% net wattage to the cab. So, if you attenuate a 100 watt amp by 9db that is cutting the wattage in half 3 times, or 12.5 watts.
I've used attenuators which much success when running large amps into small cabs. Hope this helps!

[Scumback Speakers]

I was wondering about this as a 100w Plexi is very loud on 3.

I've read that a tube amps power rating is based on its ability to put out its watts before break up...hence a tube amp like the Plexi could put out 150 or 170 watts at 10 as it can continue to breakup and put out more power.

Is this correct and is it the reason that a 100watter is so damn loud when just on 3?

My old amp tech tested a 68 plexi and said it was putting out 145w clean and 235w dimed/cranked/volume on 10.

JCM 800 50w put out "in excess of 90w with 10% distortion", the 100w models put out "in excess of 170w" at 10%.

Clean is considered 3% distortion according to Marshall.

So yes, at 3-4 your 100w could be hitting it's rated clean power of 100w.

Then there's that juicy area from 6-8 that most guitar players like, which is way more.

As has been stated above, it depends on the volume pot taper, circuit, bright caps, etc, etc.

[zzmoore]

My old amp tech tested a 68 plexi and said it was putting out 145w clean and 235w dimed/cranked/volume on 10.

JCM 800 50w put out "in excess of 90w with 10% distortion", the 100w models put out "in excess of 170w" at 10%.

Clean is considered 3% distortion according to Marshall.

So yes, at 3-4 your 100w could be hitting it's rated clean power of 100w.

Then there's that juicy area from 6-8 that most guitar players like, which is way more.

As has been stated above, it depends on the volume pot taper, circuit, bright caps, etc, etc.
235!!!!
Do you know how he measured the power out.?
Thanks

[todaystomorrow]

Pardon my ignorance, but how would you measure wattage like that?

[Axe-Man]

Thanks for the feedback!

It was in relation to a friends Metropoulos Plexi which was loud on 3 and pretty crazy when dimed.

My 50 watt poweramp needed to be on 8 to be as loud (preamp max'd) as the Plexi was on about 2 1/2-3.

235 watts...no wonder people can no longer push a healthy Plexi for natural gain without performing at a huge outdoor show!

[Scumback Speakers]

235!!!!
Do you know how he measured the power out.?
Thanks

Pardon my ignorance, but how would you measure wattage like that?

Sorry, I'm not an amp tech guru. I know the clean wattage is typically calculated with an 8 ohm load, 1khz signal (IIRC), no distortion, and there's an oscilloscope, waveform generator being used, and voltage being measured. That's as much as I remember, there may be more involved. To be honest, for me it's all about the tone, and enough volume for a particular gig, or intended use (bedroom/basement/small club, large club/stadium, etc) and getting that dialed in.

He built my line of amps and calculated the dual 6V6 single ended Class A Capt Crunch amp put out 10.8w clean and 19 dimed.

Everyone who hears that amp swears it's 25-30w, though. He's pretty conservative when it comes to specs, I know that much.

[solitaire]

I think that when the human ear is starting to hear distortion you're way beyond the rated power. Rated power should be somewhere between 2 and 4 on the gain/ volume.

[1moreknob]

Jim,

Kinda off topic, but I just purchased 2 of your speakers here from a GP member, an M75 and a J75, both 65 watters at 16 ohms. Loaded them in my Roccaforte 2x12, plugged in my 100watt Plexi with a Doug Hoffman board, and saw the 2d coming of Christ. Ive played this amp through a lot of cab's speaker combinations, and this my friend has blown me a way.

The Amp puts out 190 watts when dimed, which I don't do. It is LOUD

You are the man. Period..

[Dave_C]

All I know is that I haven't been able to get out of the -12dB range on the Alex yet on my Germino Monterey 100 with Loudness I around 4-4.5 (which is a clean tone with just a touch of breakup on a neck SC when hit hard). The tone controls also determine output level and how soon you break up. But, on that amp, I've heard it only puts out about 70-80W max "clean" power due to the high impedance of the OT primary.

The new Flukes (which I was just checking out) can log min/max/avg true RMS voltage/current of nonsinusoidal waveforms over a period of time and I wonder if measuring actual V and I (or just V and assuming speaker impedance) with an actual speaker while playing some steady state passages on guitar wouldn't be a more accurate way of determining output power. I think using just one test frequency could be very misleading.

[Scumback Speakers]

Jim,

Kinda off topic, but I just purchased 2 of your speakers here from a GP member, an M75 and a J75, both 65 watters at 16 ohms. Loaded them in my Roccaforte 2x12, plugged in my 100watt Plexi with a Doug Hoffman board, and saw the 2d coming of Christ. Ive played this amp through a lot of cab's speaker combinations, and this my friend has blown me a way.

The Amp puts out 190 watts when dimed, which I don't do. It is LOUD

You are the man. Period..

Thanks for the kind words. With those speakers you'll need to keep the volume under 6, 7 at the very most to not exceed their power handling.

Glad to hear the M/J75 mix is making you happy, though, thanks very much.

[Tone_Terrific]

Over 200W with a 100w Marshall is pushing each tube to 50+W, plus the power supply has to deliver the current/voltage to make that work.
I would expect things to sag out before that but I dunno:huh
Are Marshalls (with el34's) really THAT robust?

[Scumback Speakers]

Over 200W with a 100w Marshall is pushing each tube to 50+W, plus the power supply has to deliver the current/voltage to make that work.
I would expect things to sag out before that but I dunno:huh
Are Marshalls (with el34's) really THAT robust?
Lots of the old ones are. They can throttle that back with PT/OT voltages, circuit changes, etc. But back in the 60's they didn't have massive PA's to carry the sound, so that's why groups had multiple stacks...sometimes daisy chained together.:JAM

[diagrammatiks]

Over 200W with a 100w Marshall is pushing each tube to 50+W, plus the power supply has to deliver the current/voltage to make that work.
I would expect things to sag out before that but I dunno:huh
Are Marshalls (with el34's) really THAT robust?

you gotta remember that this was when tubes rained from the sky and were sometimes unloaded by the bushel full in exchange for goods and services.

Ritchie Blackmore's major was alleged to have blown a complete set of tubes every song.

hence why kt88s were not available for a long time.

[JTM100]

Lots of the old ones are. They can throttle that back with PT/OT voltages, circuit changes, etc. But back in the 60's they didn't have massive PA's to carry the sound, so that's why groups had multiple stacks...sometimes daisy chained together.:JAM

True, but 235w? I would have to say that is an exception and not the rule. I have owned several '68 SLP's along with several earlier and later Superleads and I have always run them pretty much on "10". They are all LOUD for sure. I can see 150w to 180w peak on averge for one...but 235 seems a bit high...not saying it cant happen though. Bet it was one hell of an amp!! Plate voltage and wall current will also play a role. The '68 12xxx heads are generally not as loud as the late '68 -'69 heads that followed. The filtering was increased and I beleive the transformers were beefed up a bit. Many of the loudest heads I have experienced were early metal panels. There was also this one '67 Supertrem that I played thru once that was insanely loud....one of the loudest Marshall's I have ever heard period!! The loudest was this Marshall Major that had the gain stages cascaded.....thru 4 4x12's that amp was the Hammer Of Thor!

[Scumback Speakers]

JTM100, 235w, and it was the loudest he measured, but he said the average was 180-210w.

Still, that's why Marshall sold TWO 100-120w cabs in a stack with each 100w head.

I don't know why people/players think they know better than Marshall did, I certainly don't think I do.

Every time someone emails me to see if I have a 25w pre rola G12M, or G12H30 to buy (just one or two) I ask them if they were running a single 4x12 with their 100w head.

Invariably the answer is "Yes."

And they all know they're not supposed to, but they just like the sound of their old speakers screaming for forgiveness...right up until they fry.

[zzmoore]

Sorry, I'm not an amp tech guru. I know the clean wattage is typically calculated with an 8 ohm load, 1khz signal (IIRC), no distortion, and there's an oscilloscope, waveform generator being used, and voltage being measured. That's as much as I remember, there may be more involved. To be honest, for me it's all about the tone, and enough volume for a particular gig, or intended use (bedroom/basement/small club, large club/stadium, etc) and getting that dialed in.

He built my line of amps and calculated the dual 6V6 single ended Class A Capt Crunch amp put out 10.8w clean and 19 dimed.

Everyone who hears that amp swears it's 25-30w, though. He's pretty conservative when it comes to specs, I know that much.
Yeah....just curious. I am not the expert on measuring Power Out. I think this is the first I have heard of a Marshall SL supplying 235 watts. But who knows. Output power is not a number we deal with much. That is, not in a real, measured, legitimate, manner. Maybe 200+ is more common than we think.
Thanks

[Sirloin]

All that being said....what did Blackmore's Major (200 watt heads) actually put out? Another thing, anytime someone talks about how loud an amp is or was, you ALWAYS have to take into account the efficiency of the speakers used.

[diagrammatiks]

All that being said....what did Blackmore's Major (200 watt heads) actually put out? Another thing, anytime someone talks about how loud an amp is or was, you ALWAYS have to take into account the efficiency of the speakers used.

probably something like 4 million watts. I'm sure the info on it was tested at some point.

That being said KT88s were very well built even then...they have a lot of clean headroom and not nearly as much non-linear room above that.

[Tone_Terrific]

EL34 datasheet

www.drtube.com/datasheets/el34-sed2002.pdf

I don't know know how the rated limits compare to actual outputs, whether duty cycle plays a role, or the wattage and distortion figures for Marshalls include peaks but over 200W is a lot from 4 little tubes.
Do we have a tube dude on board to explain?

[teemuk]

I've read that a tube amps power rating is based on its ability to put out its watts before break up...hence a tube amp like the Plexi could put out 150 or 170 watts at 10 as it can continue to breakup and put out more power.

This is a perfectly natural phenomenon and doesn't just apply to tube amps but in fact to all amps: The output signal clips and if taken to extremes a square wave output will mean a much higher output power than a sine wave output with equal peak amplitudes. Basic waveform math.

But it all depends on how large % of distortion you tolerate. Commonly amp's output power is rated at low percentage of THD and after that all bets are off as the output is considerably distorted and signal is no longer amplified with satisfying linearity - so output power measures become sort of meaningless. But yes, the output power has usually increased a lot at this stage.

[diagrammatiks]

tube data sheets are only relevant until the onset of non linear operation.
after that anything goes.

There's no particular relevance to any other industry for how a tube behaves when it goes into complete and total meltdown.

For old military, communications, and industrial use a thd of 1 or 2 percent already meant that the design was a failure.

[JTM100]

JTM100, 235w, and it was the loudest he measured, but he said the average was 180-210w.

Still, that's why Marshall sold TWO 100-120w cabs in a stack with each 100w head.

I don't know why people/players think they know better than Marshall did, I certainly don't think I do.

Every time someone emails me to see if I have a 25w pre rola G12M, or G12H30 to buy (just one or two) I ask them if they were running a single 4x12 with their 100w head.

Invariably the answer is "Yes."

And they all know they're not supposed to, but they just like the sound of their old speakers screaming for forgiveness...right up until they fry.

180-210w on average??? If that is true, I shutter to think what my 200w HIWATT's put out! BTW, have you ever thought about making an early 70's HIWATT era FANE type speaker?? I have always preferred 2 4x12's anyway. I agree with you about taking a full bore SL thru a single 25-30w G12M-G12H loaded cab. You are asking for trouble that can not only take out a speaker but something in your amp as well .....had that happen once .....and guess what I was doing!!! Lesson learned. If I go thru a single 4x12 I use an attenuator. I have an amp variaced down to 90v for the whole EVH thing, so far no trouble...it is more like a robust 50w when run that way.

[JTM100]

probably something like 4 million watts. I'm sure the info on it was tested at some point.

That being said KT88s were very well built even then...they have a lot of clean headroom and not nearly as much non-linear room above that.

Supposedly they were modded to produce about 350w- 400w if I recall correctly. Blackmore said that on thier own they were the loudest amps ever made at the time and that if he turned them up past 5 they would literally catch on fire. He also said that he blew speakers and O/T's constantly. It has been said that the amps were so powerful that when he would do a palm mute the birch back panels would bow out!

[Scumback Speakers]

Yes, 180-210 was the average of the 100w Plexis he measured. After you do so many you kind of figure that's what they put out. After about a dozen or so, you can pretty much surmise that's the output average.

Of course the 235w was special...high plate voltage...Mullard EL34's I believe he said, the biggest/most powerful PT/OT they put in any 100w Plexi he'd seen (wish he'd taken down tranny #'s, but whatever), etc.

There always going to be those that surpass the specs in any model, but the average was 180-210w.

As for the FANE speaker, I haven't gotten many requests for that, so I'm not pursuing it. Doesn't Weber do a Thames model that does this?

[JTM100]

Yes, 180-210 was the average of the 100w Plexis he measured. After you do so many you kind of figure that's what they put out. After about a dozen or so, you can pretty much surmise that's the output average.

Of course the 235w was special...high plate voltage...Mullard EL34's I believe he said, the biggest/most powerful PT/OT they put in any 100w Plexi he'd seen (wish he'd taken down tranny #'s, but whatever), etc.

There always going to be those that surpass the specs in any model, but the average was 180-210w.

As for the FANE speaker, I haven't gotten many requests for that, so I'm not pursuing it. Doesn't Weber do a Thames model that does this?

There are speakers like the Weber Thames that are considered a replacement but nobody really does an accurate reproduction that you can buy on it's own...without purchasing a cab that is. I really like your speakers and they sound really close to the original pre-rolas that I have compared them to. I was just wondering if you had ever actually considered working your "magic" in the FANE arena.

It is a shame that 235w Marshall couldnt have been spec'd out more.

[1moreknob]

Thanks for the kind words. With those speakers you'll need to keep the volume under 6, 7 at the very most to not exceed their power handling.

Glad to hear the M/J75 mix is making you happy, though, thanks very much.


Jim, just played the amp again through the 2x12, and the complexity of the tone coming out of the cab is truly amazing. the low end is exceptional, with all the mids and Highs being right without the speakers "mushing" out. I run the plexi at around 2, probably putting out around 30-40 watts. I'm thinking I may need to Mic both speakers so the PA doesn't miss anything!

I gig'd with this cab a week ago with a Weber re-coned G12M, and a G12H anniversary. Bogner Shiva was the head, and could hear the speakers were being taxed, (only a 55 watt rating) so went for yours due to the higher power handling. I did not expect the difference in tone with the Scumbacks, now I need to figure out how to get some more $ to load my 2 early '70's Marshall 4x12's with Scumbacks. For what it's worth, the video Pete Thorn did for you was what sold me. The way he recorded that, and the methodology behind it, was perfect

I wish I knew where people play where they can dime a plexi in a live situation, other than stadiums. I asked Doug Hoffman for a switchable gain stage on this Plexi, which gives me the best of both worlds, a great original Plexi sound, as well as heavy tone that's more musical and complex than my Shiva, at a useable volume.

http://i292.photobucket.com/albums/mm36/1moreknob/Plexiinterior2.jpg