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RE: Using 6L6s in Altec 1568a

They are very different tubes. The 6L6 has twice the Plate resistance and puts out half the power as the EL34

Not true,at least when it comes to half the power.The KT66 has a much lower plate resistance than a 6L6gc and even tho it's still higher than an EL34,on measurement in a circuit that is optimized for EL34s the power loss is measurable but not that noticeable under listening.
I have done this test on a Marantz 8B and when I swept the amp,the response was very flat to 60k with the KT66s and I was down about 6 watts overall but the distortion was lower in the sub 20 cycle region with the KT66s and this was at 1 watt,5 watts,and 10 watts.In the 2khz to 10khz region,the distortion numbers would literally flip between the two tubes at various frequencies but at no time was it ever over .16% in the 1 watt and 10 watt range from either tube.
I understand where you are coming from with the point of logic that a lower resistance tube should perform better especially in a circuit that is optimized for one tube vs the other. This just isn't always the case.
If you look at the 25 watt curves of an EL34 as opposed to a 6L6gc or a KT66,it will explain a lot why many people prefer the 6L6gc and KT66 to the EL34.This is provided the EL34 circuit can swing enough extra voltage to drive either of the tubes..I have two pair of rebuilt and upgraded Altec 1569s and I can run four KT88s in them and make 106 watts at 1 khz.
I think a lot of people like the imaging,focus,and detail at the upper and lower extremes of the 6L6 family of tubes.Where an EL34 takes about 70 volts of drive to reach full power,the 6L6gc and KT66 takes about 90v volts of drive to reach full power.You just need the drive to make it happen and there are very capable EL34 amps that can do this.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken


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