|12AT7 Distortion in the LEAK TL/12 Plus|
The LEAK email list has been discussing the use of the 12AT7 in the TL/12, and its performance from a distortion perspective. Of interest was whether the 12AT7 would give less distortion if it was biased more towards the middle of it plate characteristic. As an alternative to the 12AT7, the 6CG7 type has also been investigated.
Using well known established formula for calculating second harmonic distortion from a graphical construction on the plate characteristics, the distortion of the 12AT7 and 6CG7 has been calculated with different bias points - one with 100k anode load, and the other with 22k anode load. See Radiotron Designers Handbook, Chapter 13, Section 2(i).
In circuits requiring higher bandwidth, a lower anode load (such as 22kOhm) is desirable as the output will be less affected by capacitive loading. It would seem that the penalty of driving a capacitive load well (such as large triode-connected valves in the output stage) is increased distortion in the driver valve brought about by adoption of low anode load resistance (22kOhm) and higher bias currents.
Note that the graphical constructions below rely on using relatively high output voltage swings (much higher than required in the TL/12 Plus).
Also, it was not possible to compare the two valve types and configurations at exactly the same output voltage swing.
However, the general conclusions above would likely remain valid if the harmonic distortion was calculated at the same output voltage.
Since valve non-linearity follows a three-halves power law, it is likely that the amount of second harmonic distortion is a good predictor of the
amount of higher-order even and odd distortion.The degree to which published plate characteristics actually reflect reality is a variable not yet investigated.