# Is Power Additive in Series And Parallel?

Discussion in 'Amp Tech Center' started by HD28, May 16, 2021.

1. ### HD28Tele-Meister

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It seems like the interweb answer is yes and no.
In guitar and amp circuits, regarding resistors, is power (watts) additive in both series and parallel?
In guitar and amp circuits, does Ohms law E=IR equally apply as does P=I^2R?
If you use E=IR the answer appears to be no if voltage remains constant.
If you use P=I^2R, the answer appears to be yes if amperage remains constant.
The question stems from trying to build a 10% power switch using https://robrobinette.com/AB763_Modifications.htm#10%_Power_In_External_Box
I wish to use two 3ohm 100W and two 4ohm 100W resistors in series to make a 6ohm and 8ohm 200W 10% power switch, suitable for a 4ohm output 10% power switch for a dual showman, twin reverb, or similar 4 power tube fender amp.

2. ### RLee77Friend of Leo's

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Yes. In both cases, your 3 + 3 = 6 ohm resistor and 4 + 4 = 8 ohm resistor are the equivalent of a single 6 ohm and 8 ohm resistor, with a power rating of 200w each.
Used as shown in Rob's circuit, they will give you the 10% power attenuation, with a total load resistance of 4 ohms. (8 + 4 = 12 ohms, in parallel with 6 ohms, is 4 ohms.)

Last edited: May 16, 2021
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3. ### beanlucTele-Holic

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Power is voltage divided by current.

Voltage is additive in series, not in parallel.

Current is additive in parallel, not in series.

So a power switch logically (to me, I haven't looked at this type of circuit) could be additive in series if it affects voltage, or additive in parallel if it affects current.

In other words, there's no single answer, and whatever answer makes sense for your application is probably not even related to my over simplification.

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4. ### LowerleftcoastFriend of Leo's

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You would not question this when adding two speakers together.
Two 25W speakers in series = 50W of power handling.
Two 25W speakers in parallel = 50W of power handling.

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5. ### HD28Tele-Meister

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Power is additive in both series and parallel.

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Thank you.

7. ### RLee77Friend of Leo's

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Just to show how the power splits up in your attenuator, here's a rough sketch, showing watts for each resistor and speaker with 100w total going in; the total load on the amp is 4 ohms. Since the two 4 ohm R's in series with the speaker won't see as much power, you could scale those back to two 50w R's if you wanted. I'd leave the two 3 ohm R's at 100w each for safe power dissipation.

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and power corrupts
absolute power corrupts absolutely

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9. ### RLee77Friend of Leo's

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Bass players are therefore more corrupt than guitar players.

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10. ### SRHmusicTele-AfflictedSilver Supporter

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Power = voltage x current = V x I

Also P = I^2 x R = V^2 / R, from substituting V=IR or I=V/R.

Power always adds; when looking at a circuit the total power dissipated is the sum of the power dissipated in each element.

One method: first sort out how much current flows in each branch, then use P=I^2 R to find the power in each resistor.

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and drummers are pure evil

12. ### DaciousPoster Extraordinaire

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Singers are the worst - they get thousands of watts in dedicated foh and foldback,and still complain....

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Thank you.

14. ### beanlucTele-Holic

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yeah, I blew it.

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15. ### kboldFriend of Leo's

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Well ..... maybe not.
Bassist requires 10 times more power than the guitarist.
So, it only takes one tenth of the power to corrupt a guitarist (compared to a bassist).
Therefore the guitarist is 10 times more corrupt. Allow a guitarist to play through a bass amp ...............

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16. ### kboldFriend of Leo's

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. duplicate

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The problem is how you calculate the perceived power rating of the amp and resistors.

First of all, a 100W amp is rated at that clean, so if you're cranking it you'll get something like 160-200W.

Second, the ratings of the resistors are understood to be with heavy heat sinks, and a fan might help as well. For example, here's one rated at 100W:
https://www.tube-town.net/ttstore/high-power-resistor-3r9-4-ohm-100-watt.html
as you can see, the rating without a heat sink is 40W. So if you just use two of those in the circuit with a Showman without heat sinking, you'll likely fry them and possibly damage your amp.

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18. ### the fatchTele-Meister

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Actually, power is voltage multiplied by current. Voltage is shared between resistances in series, and is the same across both when in parallel. Current is shared by resistances in parallel and is the same in both when in series.

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19. ### dsutton24Doctor of TeleocityAd Free Member

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In DC circuits, and AC circuits through the audio range, power is pretty well behaved. Once you get into RF, though, things can get weird.

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