What Was The Real Net Wattage Of EVH's SL w/ Variac?

Blanket Jackson

Every day is like Tio's birthday
Gold Supporting Member
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16,147
If EVH decreased the voltage feeding the amp, but biased the tubes to spec under the decreased voltage, did that also decrease the real wattage of the amp (and just push clipping earlier) or was the wattage the same as 110V because the tubes were biased to spec?

I am not an electronic whiz, so apologies if this is an easy answer - I genuinely don't know (and am curious).

Thanks!
 

HotBluePlates

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11,302
... decreased the voltage feeding the amp, but biased the tubes to spec under the decreased voltage, did that also decrease the real wattage of the amp (and just push clipping earlier) or was the wattage the same as 110V because the tubes were biased to spec? ...
TL;DR - The power output would assuredly be less.



Idle Dissipation (due to bias) ≠ Power Output

Power = Volts x Current

The "Volts" available is set by the power supply. We get to use the power supply Volts minus 40-100v, because there needs to be some left across the tube at peak output to keep the tube functioning.

"Current" is totally a function on the tube, and the Volts applied to it. The tube receives a voltage-input, and attempts to pull Current from the power supply (through the output transformer).

Fewer Volts on the tube (and especially its screen grid) from the power supply (due to the variac) means the tube cannot pull as much Current.​

We're left with:

Less Volts x Less Current = Less-Less Power​

Doesn't really matter how you re-bias the tubes after the supply voltage is dropped. (Bias only directly affects the direct-current condition at idle, not the alternating-current condition when you play the amp). That only governs whether the tubes tend to overheat when you're playing as loud as they get.
 
Last edited:

macmeda

Silver Supporting Member
Messages
1,541
TL;DR - The power output would assuredly be less.



Idle Dissipation (due to bias) ≠ Power Output

Power = Volts x Current

The "Volts" available is set by the power supply. We get to use the power supply Volts minus 40-100v, because there needs to be some left across the tube at peak output to keep the tube functioning.

"Current" is totally a function on the tube, and the Volts applied to it. The tube receives a voltage-input, and attempts to pull Current from the power supply (through the output transformer).

Fewer Volts on the tube (and especially its screen grid) from the power supply (due to the variac) means the tube cannot pull as much Current.​

We're left with:

Less Volts x Less Current = Less-Less Power​

Doesn't really matter how you re-bias the tubes after the supply voltage is dropped. (Bias only directly affects the direct-current condition at idle, not the alternating-current condition when you play the amp). That only governs whether the tubes tend to overheat when you're playing as loud as they get.
@HotBluePlates

So does this mean you don’t need to re-bias when lowering voltage with a varic?
 




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