Wednesday, July 24, 2013
Featrued VT: The 12BY7A
Even though the 12BY7A is considered a small signal type, it has ratings more in line with a small power pentode, such as the 50C5. This is due to the original design purpose: a video amp for color TVs. Given the 4.0MHz upper limit for TV video, it takes a lot of current to charge even the very small device capacitances of CRTs. For this purpose, the internal device capacitances are low, and the gm high.
The 12BY7A has also seen quite a lot of applications in ham rigs, either commercial or amateur, usually as oscillator/buffers. Unfortunately, this has made them rare and expensive. As for audio use, there really isn't much.
There are a couple of reasons for that: the 12BY7A requires a fairly high rail voltage. This wasn't a problem as there is lots of high voltage available in a color TV. The other reason is that you seldom need the gain the 12BY7A is capable of producing if your usual sources are CD changers, tuners, sound cards, or TV audio, as all of these produce outputs of 1.0VRMS or so. In applications where you need the gain for low level signals, you can get the same voltage amplifications with small signal pents that don't require the same still plate currents.
In those cases where you do have the rail voltage to support it, the 12BY7A can swing a high output while doing so with good linearity and low distortion. As with any small signal pentode in an audio amp, the best performance comes with the lightest possible plate loading. With this type, that usually means plate loads of around 20K or so, rather than the 100K+ plate loads of smaller types like the 6AU6.
12BY7A Plate Characteristic
As for most pentode plate characteristics, this one is more apropos to the design od vid amps and other wide band, high frequency circuits. The more linear region is to be found at the bottom, with smaller plate currents, where the gm doesn't vary as much with plate current.
For audio designs, the "plate transfer" curves are a good deal more useful, as you can pick off both screen voltages and Q-Point bias. Getting the screen voltage as low as possible, consistent with input requirements, always helps improve linearity, as you don't want the plate swinging below the screen.
It isn't a type you may ever have to use that often, but when you need gain and large voltage swings, it's a type to consider.
Tuesday, July 23, 2013
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 VHK= 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 VPK= 250VDC, 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.
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