So I have been seeing questions about impedance of a speaker when it comes to tube amps, mainly if the impedance of a cabinet when not matched to an amp of the same impedance, will it damage the amp. Another question I commonly see is if you need to half the impedance on the amp's selector switch if you pull out two of the tubes. Well I plan on explaining everything.
For some basic electronics theory. All semiconductors or amplifiers need to have a load on them. A load is resistance that limits maximum current and controls gain. With power amps, the load is the speaker, however most speakers have a very low impedance. If you were to look at the load required for a 6L6 for a typical 120 Watt amp, would be in the ballpark of 1042 ohms from plate to center tap. There is a slight incompatibility as described below:
Tube: High Voltage, Low Current
Speaker: Low Voltage, High Current
Since high current would destroy a tube and high voltage would destroy a speaker, a transformer is used to match the signals. A transformer can convert voltage to current, and current to voltage depending on the setup. A step down transformer takes the high voltage of the vacuum tubes from about 350v down to about 44v. The stepdown ratio is thus in the ballpark of an 8:1 stepdown. For every 8v on the input, you get 1V at the output. For current you get the opposite, for every 1mA available at the input, you get 8mA available at the output. As a result, the load that is placed on the transformers output will have a multiplication effect on the load.
If you have a 16 ohm speaker plugged into a the output of the transformer then to the input of the transformer will be 1042 ohms. So to two 6L6 tubes in parallel, when the amp is operating in class B, the speaker actually looks like a 1042 ohm speaker and to the speaker it looks like the amp is pumping out about 44v out and 44v into a 16 ohm speaker is indeed about 120 Watts.
The impedance selection taps allow the user to plug into different cabinets. Each tap has a different winding ratio so that you can use different impedance cabinets but still maintain that 1042 ohms to a parallel set of tubes. Due to the winding ratio change when a different impedance is selected, the voltage decreases. This is good because with a lower impedance cab, you need less voltage. The lower voltage combined with the lower speaker impedance also causes an inverse increase in current, keeping the power dissipation to 120 Watts. Keep in mind the effective speaker load to the tubes will as a result be left unchanged.
So what happens when say you have an amp set to 8 or 4 ohms being plugged into a 16 ohm cabinet? It is simple, the load to the tubes looks larger and since the voltage to the tubes never changes, the current has fallen and so has power. An easy way to get less power out of your amp is simply to have the amp set on a lower impedance than the cabinet your are plugging it into. With a 16 ohm cabinet, the 8 ohm setting on the amp you get your half power, and the 4 ohm setting will get you quarter power. How much quieter is quarter power? Well in order to get half the perceived volume from your amp you need to reduce the power by one-tenth, or 12 Watts. Even at quarter power, or 30 Watts, the amp will still be way loud. The benefit to this though is that you achieve power amp distortion at much lower volumes. As a rule of thumb the cabinets total load should not be more than 4 times higher than the impedance the amp is set for, so for the different impedance settings, don't go any higher than this:
Amp -> Speaker
4 ohms -> 16 ohms
8 ohms -> 32 ohms
16 ohms -> 64 ohms
Anything higher can cause the inductance kickback voltage to exceed the winding enamel's breakdown voltage and cause destruction to the power transformer and power tubes. This is the same failure when an amp is played with no load or rather an "infinite load".
What happens when you go the opposite direction? While it is not advised to do so as the amp will over dissipate, due to the pentode's constant current characteristic, which limits the total amount of current regardless of voltage, the increase in power is not linear like the decrease of power when you use a larger load. So if you had the amp on a 16 ohm setting and plugged in an 8 ohm cabinet, the amount of power increase will only be slightly higher than if you used a 16 ohm cabinet. Keep in mind thought that this will cause a shortened life of the tube and transformer and most likely the transformer will be over saturated which will cause overheating which mitigates the working life of all transformers. Not only that, but when you over dissipate the tubes, they actually loose gain due to their load line characteristics, so you will notice a drop in output volume as the efficiency of the amp drops dramatically. More and more of that power is just wasted as heat and not sound.
What about pulling tubes, will that effectively reduce power? Not a chance. While if you do pull a tube from each side will cause a very drop in total power due to tube resistance and tube current limiting, the drop is too small to notice and over dissipates the two tubes left in the amp. Even with two tubes pulled, since the load of the transformer has not changed and remember it is the load that determines how much power is pulled, the two tubes will be trying to pull 120 Watts, remembering though that the tubes do have a current limiter characteristic, there will be a decent power drop, but the amp will still be dissipating over 30 Watts per tube reducing tube life. Transformer life is not reduced in this scenario because it won't be over dissipated, just the tubes will be.
So what is the moral of the story? Within reason an amp can have an impedance mismatched, even if it means the amp and tubes are being over dissipated. While the amp or tubes won't blow up, the life of the tubes and possibly the transformer will be reduced, so users be warned. If less power and a tighter amp is desired, pulling two opposing tubes and setting the amp to 4 ohms when using a 16 ohm cab will turn a 120 Watt beast into a two 6L6 amp 30 Watt little jam amp.
Until next time!
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