Featured VT: The 6AU6

 

6AU6

The 6AU6 isn't glamorous, and the wrap-around plate doesn't make it especially interesting to look at. Not the least bit glamorous, just a reliable work horse that does just about anything well: CCS duty, voltage regulator error amp service, small signal audio and RF amp. It has a generous V_HK= 100V (heater positive) / 200V (heater negative) rating that suits it for both error amp and CCS duties.

As a small signal amp, the linearity is very good, especially if you can get the passive plate load up, and run it at small plate currents, consistent with drive capability of the load.

The 6AU6 is a miniature sharp cutoff pentode primarily designed for use as a high gain radio or intermediate frequency amplifier. Its low grid-plate capacitance and high transconductance make it especially suited for high frequency, wide-band applications. -- GE Spec sheet

Given that the spec sheet doesn't mention audio, the included plate characteristic isn't especially helpful. For doing audio design work, the included plate and screen transfer curves are more useful. From these, one may pick off bias points, screen voltages, and plate current swings to calculate likely low distortion Q-Points. As this would most likely appear in a low level stage, linearity isn't hard to come by for small plate voltage swings. Though these curves were taken with V_PK= 250V_DC, this presents little problem as the plate characteristic is quite flat for voltages above 80V. Operating at lower voltages might require some adjustments to the screen voltage and/or bias point.

As with any other pentode, there are the usual considerations: microphony since there are three grids inside the bottle, as opposed to one, therefore, more parts that can jiggle. There is also the possibility of increased noise. This is especially vexatious at audio frequencies as such things as cathode flicker noise and screen partition noise decrease with increasing frequency.

There are two other ways to use the 6AU6: as a pseudotriode. This can be done in the usual manner: paralleling the plate and screen. The spec sheet even includes the pseudotriode plate characteristic. This gives a μ= 36, which falls nicely between the gains of types like the 6SN7-oids, and high gain types such as the 12AT7 or 12AX7. Convenient when you need gains that fall between those two types.

Triode Plate Characteristic

Whether to connect the suppressor to the plate or cathode is another gray area: some say better sonic performance is had one way or the other. There is no consensus for this. Changing the suppressor connection doesn't alter the plate characteristic all that much in any case.

The other way to make a pseudotriode is to connect the plate to ground, and use the screen as the triode plate. If the screen spec isn't busted by this, then the plate becomes an internal shield. This would prove useful in electrically noisy environments. It changes the actual characteristic very little since it's the screen, not the plate, which determines the actual operation. In any pentode, due to the isolation of the plate, the plate serves mainly as a pseudo anode that simply collects the electrons at the end of their journey.

Don't overlook the 6AU6 if you decide your design could use a pentode.