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.
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