Speaker load on tube amplifier...

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'.

Because of the impedance dip of the speaker, the KT-88's were recommended. Why is this the case?

Good question. At first glance it would seem tubes are totally unrelated. Tube amps have different speaker connections for 4 and 8 ohm speakers. But there is no right or wrong. They both come off the same output transformer. The difference is one comes off a transformer tap with more turns, which changes the power ratio of the transformer. Which one is "right" is totally up to you. Use the one that sounds the best. 

Same goes for tubes. There are differences between tube types that are easy to hear. KT88 does sound different than 6550C which does sound different than EL34. Within each tube type there are also differences. 

Here's where the KT88 advice may be coming from. The sound difference you hear with different speaker impedance can in some ways be balanced out or tuned to your preference by tube selection. There is no right or wrong it all comes down to what you like. So what they are really saying is a flat-out guess that KT88 might be more likely to work in your situation. 

The main thing to learn is this is a flat-out guess. They know your amp, and they know the tubes, and they might even know your speakers- but no way they know you or your room or anything else. So KT88 is a good safe bet. 

Personally I would forget about impedance and when you get these and listen a while maybe think about how you would like it to sound better. Then if you want a little bit more punch and drive you might try 6550C. If you want a little more liquid midrange warmth EL34. Or whatever else your amp can use. Forget technical stuff like impedance, power, etc. Focus on descriptions of sound quality. Liquidity, soundstage depth, that kind of thing. Those kinds of things tend to hold true regardless of which amp they go in or which speakers are used.

@atmasphere Super helpful, thank you. I have read your comments on speaker ohm rating many times. I comprehend your perspective, and I find it interesting that the engineer of my speakers praises the 4ohm load, for other reasons. It seems what is lacking here is a "whole" approach to the various designed decisions, which seems to be much more complex.

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? 

@millercarbon Do you have a good resource for describing the sound quality of various tubes? Maybe there is a breakdown here on A'gon.

@freediver Thanks for your informative post... the speakers are either 95db or 98db at 4ohms. There's some confusion in reading the stat. These are Tekton Moabs. So, I think the amp could fit... 

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).