Speaker load on tube amplifier...

So, lower plate load is better, from what I'm hearing you say, and this is because it's easier on the amp, less current to control, less heat to wield?

@listening99


Not in this case- the reason I would go with the 7581/6L6 is that apparently the output transformer is set up so that it presents a reasonable load to a variety of power tubes, each of which work best with different impedance. Since the KT88 wants to see a higher load impedance, this means that the transformer is probably loading the 6L6 at that higher impedance too.

Now transformers are called that because they transform impedance, but that works both ways. So if you put a lower impedance than 4 ohms on the 4 ohm tap, the power tube will see a lower impedance too. But if the power tube is normally seeing something that is a bit higher than is optimal (for power output reasons) when the speaker load is lower the amp will respond by putting out *more power* which is actually what you want it to do when that lower impedance (3.1ohm dip) is encountered. So the 6L6 will allow you to do that, while the KT88 probably will be less likely to do so (and also will make higher distortion).

The KT88 needs about 6Kohms as a plate load (plate to plate). The 6L6 needs about 4.5K (the 7581 is a variant).


So on paper it appears that the 7581 would be more suitable although there would be a bit less power.


But what is an unknown is what the actual plate load really is. At any rate, you won't be doing any damage to the amp; at worst you'll go through tubes a bit faster if the low impedance proves to be too adverse. And it may not sound right.

One thing to keep in mind is that to get the best out of any tube amp, its in your amplifier investment dollar's best interest to use a speaker with a higher impedance! This is simply because you get lower distortion and wider bandwidth (mostly on account of the output transformer's efficiency) driving a higher impedance. In fact even with solid state while you get less power, you also get less distortion; in solid state amps this will mean its less harsh and bright. With tubes it usually means it won't be as 'fat'.