OHMs law on S.E.T. Amps


A quandary I’m unable to find an answer on, even asking brick and mortar dealers is: why do SET tube amps output wattage decrease with lower impedance loads (speakers) in comparison to SS amps wattage output staying the same or even increasing with lower impedance loads? I have a rudimentary knowledge of OHMs law, but by no means claim to be an electrical guru. Any explanation would be appreciated. Layman’s terms explanation is just fine for me. 
sdguyer87
I had this page bookmarked a while back: https://forums.stevehoffman.tv/threads/can-an-set-amp-serve-as-a-long-term-main-amp.244368/

Typically, single ended Class A SS amps are significantly more inefficient than SET amps. Could be a difference in topologies - although inefficient, (that’s the point) a pure Class A amplifier that is solid state can provide consistent high current (into let’s say a 4ohm speaker) - that’s why you have monoblocks with huge capacitors that drive flagship speakers.

Ohm's Law: Volts equals current times resistance, V=IR.

Power in watts equals volts times amps. W=VA.

Ohm's Law says when speaker resistance decreases (lower impedance) then voltage will drop. Unless current increases enough to make up for the impedance drop. If impedance is cut in half going from 8 to 4 ohms then the amplifier will have to deliver twice the current (amps) to make it up. Otherwise voltage will drop. 

You can see that since W=VA then one way or another current must increase to make up for impedance, or else power (watts) will decrease.

That's the math. In layman's terms, think of it as tube amps are designed as more of a voltage source. Not current. So when impedance drops, resistance is less, the tube amp does not increase current to compensate and so as a result power (in watts) is lower. Solid state amps are designed differently. As impedance drops the current increases and so power in watts doubles from 8 to 4 ohms. 

So why is it there are all these tube amps that sound like they have way more watts than they do? Good question.


why do SET tube amps output wattage decrease with lower impedance loads (speakers) in comparison to SS amps wattage output staying the same or even increasing with lower impedance loads?
@sdguyer87  The short answer is that an SET has a high output impedance that is not that dissimilar to the speaker it drives, while most solid state amps have an output impedance that is a very small fraction of the speaker load.

The latter allows the amp to behave as a 'voltage source', meaning that it can make the same voltage regardless of the load it drives. So if it is making 1 watt into 8 ohms its 2.83 volts; it will make the same voltage into 4 ohms, thus it is making 2 watts.

An SET typically operates without loop feedback and the tendency is thus to behave more like a 'power source', although it will only be an approximation. This does mean though that it will try to make the same power into 8 ohms as it does into 4. In reality, because the output transformer transforms impedance, into 4 ohms the power tube will have a load that is half of what it should have, and so will make far less power, unless the 4 ohm tap is used which restores the proper load for the output tube. However, output transformers are typically less efficient on their lower impedance taps, so its very likely that it will make slightly less power into 4 ohms with the 4 ohm tap, and likely also will be more restricted in the bass region- a loss of an octave of response is commonplace. The distortion will likely be higher too.
What I wanna know is why It's always referred to as "an" SET amp instead of "a" SET amp? "S" is a consonant! Sheesh :)
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Well SET stands for single ended triode, no? Unless it's used as an acronym.. "(an)" ess, ee, tee...phonetically. But then that would still be wrong since the first letter of each word is only simply spoken but not as a "word". Weird
Okay, who deleted the post answering my question with "an means one"?:)
What I wanna know is why It's always referred to as "an" SET amp instead of "a" SET amp? "S" is a consonant! Sheesh :)
I refer to an SET by saying 'ESS EEE TEE'. If mentioned any other way its usually 'single-ended triode'. I hear 'set' far less often.
I figured. Gotta be in the loop, huh.