100W Non-MV JMP

[Ted]

Early 70s hand wired Marshall head. No MV. A couple of power electrolytics leaking, so replaced the lot. Revalved with Groove Tube EL34Ms

151W rms @ 5% distortion into a dummy load.

Oh.

[Leo_Gnardo]

Early 70s hand wired Marshall head. No MV. A couple of power electrolytics leaking, so replaced the lot. Revalved with Groove Tube EL34Ms

151W rms @ 5% distortion into a dummy load.

Oh.

Back in the good old days, you got more than you paid for with the 100W Marshalls, in wattage. 50W? they barely made 30W @ clip but the tone - wow, outstanding!

[Silvio55]

Back in the good old days, you got more than you paid for with the 100W Marshalls, in wattage. 50W? they barely made 30W @ clip but the tone - wow, outstanding!

30W at clipping? I thought that they put like 70/80W cranked!! at least thatīs what the Marshall service manual says.

[olddawg]

We used to call it "British RMS" back in the early 70s. I think England actually had different measurement criteria at the time.

[Ted]

Looking at my notes for the last five years worth of JMPs, the ones with 360V B+ struggle to put out more than about 40W at clip. The 440V ones are good for a touch over 50W.

[Leo_Gnardo]

30W at clipping? I thought that they put like 70/80W cranked!! at least thatīs what the Marshall service manual says.

400V power supply. Which sags 10-15% at clip. How ya gonna get 50W out of that?

Cranked. What. Does. That. Mean. Where does it say "cranked" in ANY Marshall service manual. Please be specific. I would like to know more about this new term "cranked." Do all amps now have to conform to "cranked" specifications? Where can I get test equipment that will guarantee I'm seeing the "cranked" figures?"

Rule of thumb for guesstimating power specs: For every 5% increase in distortion level, add 12% to the power level. Manufacturers do it all the time. Last year's 100W (at clip) amp is next year's 150W amp (at 20% THD), no change in design. "Crank" out a new specification, any time you want. No technical knowledge needed. Let the public know - the power figure. Never let 'em know - the distortion figure. "After all, you guitarists want distortion, don't you?" so think the sales and engineering departments.

After repairing his 70's 50W Marshall, a waggish repair recipient subbed in his own extra-tough extra-big PT. Then started melting output tubes. The PT he selected turned out northwards of 500V at idle, and hardly sagged at clip. Yes, he got 65W @ clip, after bias was set appropriately. Tone, large, loud & aggressive. Whatever charm it had @ 30W, gone. Now it sounded like a slightly anemic 100W. That's how the cookie bounces.

[Jazz P Bass]

You gotta love some of the specs.

Boom boxes rated at 250 watts output power!
Hmmm.
The little tag at the mains cord indicates 27 watts of power consumption.
Now that is what I would call 'Magic'.

[Silvio55]

I know "cranked" is not a technical term, I found the service manual, as you can see it says "90W RMS at 10% distortion", Iīm not saying thatīs the absollute true, I was just asking!

[J M Fahey]

[ATTACH=CONFIG]29741[/ATTACH]

I remember those service notes when they were freshly issued (ouch !!!! )

They were .... optimistic.

Speaking of 2 x EL34 tubes: *some* lucky amps just made the 50W RMS at the start of visible clipping, really just start rounding of waveform peaks, no idea of how much was that in %, but 3% quoted sounds reasonable.

Now 10% distortion is a little more, tops visibly flattened but just on the tips, not much more power available, *maybe* 60W RMS.

90W at 10% ??????????????????? Not even in your dreams !!!!
*Maybe* at 100% distortion, perfect squarewave so as to have the maximum area mathematically available below the wave.

And why not 100W then?

Because the PSU continues dropping voltage, so 90W instead of 100W is reasonable.

By the way, the same era datasheet for the Plexi claimed fantastic 170 or 187W or some other impossible value.

Personal opinion, the true muscle behind "British watts" lies in the crushing bone pressure created by 8 Celastions in closed back boxes, compared to similar rated Twins (which might have the same electrical power output) driving 2 puny weak inefficient Oxfords .

And Fenders used true Log volume pots while Marshalls , although also using log volume pots , bypassed them with ridiculous 10X the maximum recommended value 4700pF "bright" caps.
So a Twin on "2" was putting out, say, 10/20W while a similar rated Plexi on 2 was already clipping.

[g1]

So a Twin on "2" was putting out, say, 10/20W while a similar rated Plexi on 2 was already clipping.

This is further shown by those service note input sensitivity specs., all controls full: 1.5mV (overload level 150mV) !

[Silvio55]

I remember those service notes when they were freshly issued (ouch !!!! )

They were .... optimistic.

Speaking of 2 x EL34 tubes: *some* lucky amps just made the 50W RMS at the start of visible clipping, really just start rounding of waveform peaks, no idea of how much was that in %, but 3% quoted sounds reasonable.

Now 10% distortion is a little more, tops visibly flattened but just on the tips, not much more power available, *maybe* 60W RMS.

90W at 10% ??????????????????? Not even in your dreams !!!!
*Maybe* at 100% distortion, perfect squarewave so as to have the maximum area mathematically available below the wave.

And why not 100W then?

Because the PSU continues dropping voltage, so 90W instead of 100W is reasonable.

By the way, the same era datasheet for the Plexi claimed fantastic 170 or 187W or some other impossible value.

Personal opinion, the true muscle behind "British watts" lies in the crushing bone pressure created by 8 Celastions in closed back boxes, compared to similar rated Twins (which might have the same electrical power output) driving 2 puny weak inefficient Oxfords .

And Fenders used true Log volume pots while Marshalls , although also using log volume pots , bypassed them with ridiculous 10X the maximum recommended value 4700pF "bright" caps.
So a Twin on "2" was putting out, say, 10/20W while a similar rated Plexi on 2 was already clipping.

Yes, I thought that the amp at full volume was delivering more than 10% distortion, but 50W at clipping and 90W full volume (i like more the term "crancked" ) sounds reasonable to me, yet I donīt know for sure if thatīs the truth, iīm playing by ear with this, I know nothing about Watt measurement.

[Silvio55]

Here you are Juan:

"output power at 10% distortion 170W"