Power tubes & wattage ratings?

cherrysunburst00

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DAMN, getting old SUCKS--insomnia.

So, I've got a question about power tube & wattage ratings: How can (for example) an EL 84 have so many different wattage ratings? All of the following are EL 84 amps with different wattages (sic):

Carr Raleigh: 3 watts
Laney Lionheart & Dr Z Mini Z: 5 watts
Phaez Sibly: 7 watts

Then, you have amps with 2 EL 84s (I'm only familiar with 2):
Orange Tiny Terror: 15 watts
Dr Z Monza (a Rock 'n' Roll BEAST) 20 watts.

SIMILARLY with EL 34's (I only know these 2):
Carr Mercury: 7 watts
Phaez Sibly (mine) 10 (?) watts

So, why the wide range? The answer(s) can feed into another thread idear I have :fingersx:
 

Marshall & Moonshine

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Class of operation, different voltages applied, maybe biasing??
They have a finite ceiling, although it's usually well above the spec sheets. Push them too hard and they'll fail faster than they should. Push them harder still and they could fail instantly.
I always think it's weird when you see a SE EL34 doing 8w in something small and then 50w from a pair in a Marshall. They have a list of limiting factors, but Plate Dissapation is usually the one we think of as THE limit for tubes.
 

ARandall

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Well, its not like the tubes HAVE to be running flat out in a design.......there is no 'tube percentage police' running around fining amp designers for not having a circuit that runs the tubes less than the absolute maximum they can put out.

The Blackstar HT 20, 40 and 50 all run a pair of EL 34's for example.
 

Iron Broadsword

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IIRC the new mini jubilee by Marshall is running a pair of EL34's at 25w.. what the heck. 2 EL34's in a marshall usually means 50w. And then there's the SL5 that runs 1 EL34 at 5w. Idk man, I have to assume the lower wattage circuits are just running them at a lower voltage on purpose to reduce headroom.
 

cybermgk

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In simple terms a tube takes a signal in, and amplifies it. Adjust the amount of power fed to said tubes to accomplish this (aka plate voltages), you adjust the amount of that amplification. Bias affects this, fwiw, but also the rest of the amp circuit, some MV circuits etc. Power scaling is some form of adjusting these voltages to the output tubes.

Additionally, how many EL84s are in those amps, 1 or 2? More tubes, more potential output volume. Doesn't have to be, but it can be.
 

V2

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Most 15-18W EL84 amps stick to the recommended 300V limit on the plate and are cathode biased. But a pair of EL84s will deliver more power with 380V and fixed bias. Generally fixed bias is more efficient than cathode bias and thus will deliver more clean power. Similarly, high plate voltages and an "efficient" primary impedance on the output transformer will deliver more power. EL34s, for example, will deliver more power into a 3k5 OT than a 10k OT.

You can drop power in other ways too, like running a power tube in pseudo triode mode.
 

cherrysunburst00

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Thanks, All. This begins to explain some stuff to me: for example, why my Carr Raleigh (3 watts) needs to be rebiased, and why it is well-known for its shimmering, beautiful cleans (oddly enough, I NEVER use that aspect of it!)
 
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Liam

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Just to add to the good replies, the power tubes are just one of the limiting factors in how much power you can get from a tube amp. Output transformer, power transformer and rectifier also need to be capable of giving the power tubes what they need.

Liam
 

dspelman

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There's also this: Wattage ratings are usually quoted at a specific level of THD (Total Harmonic Distortion), but with guitar amps, those specifications can be...uh...approximate.
 

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