TubeHead Johnny said:
On this site I often read about how bad 12A*7, and other 9 pin miniature preamp/driver tubes out there. (mostly due to their non-linear properties). And worshiping most things octal.
Yes, but what does "bad" actually mean anyway? In the first place, most of the 12*7 family were designed as small signal RF types. So far as 12AU7s are concerned, it's an RF tube, and most of the circuits using it are: oscillator/buffers, push-pull frequency triplers, push-push doublers, push-pull drivers -- all operating in Class C. You frequently see it used as a mixer/detector where linearity is definitely
not a good thing. The other big class of circuits is digital (logic gates, RS latches, flip-flops) or semi-digital apps like multivibrators, Schmidt triggers, and so forth. Audio was definitely an after thought. Does that mean you can't use it for audio? Not necessarily. You could probably use it for a cathode follower grid driver. If it produces mainly h2, then you could use it in a balanced topology such as an LTP, or SRPP, where h2 is largely cancelled. It just might work well in that situation.
My design philosophy is to "never say never", and I have used types like the 6BQ7 that don't even bother to make a pretense of being an audio tube, which don't necessarily have good loadlines, and which I wouldn't use in any conventional audio amp circuit. However, as an audio cascoded LTP, this type sounds just great. Use it in a conventional small signal stage, and it probably won't sound so swell.
As for other types, devices like the 12AX7, a high-u triode, these can produce more of the higher order harmonics, like a small signal pentode (large u-factor, and high r(p) ). The demand for this type arose during the later 1940s, early 1950s, when the Big Box manufacturers wanted to really horse up the gNFB in order to force impressive THD measurements to please the marketing depts. Throwing in 20+db(v) of gNFB requires an open loop gain that's considerably higher than that in order to preserve any sort of sane sensitivity. Many designs from previous times didn't have 20db(v) of open loop gain, and so the older octal small signal types were designed to be a good deal more linear (because they were used without gNFB at all, or only a small amount of gNFB to correct for the OPT and much smaller circuit nonlinearity) and to have lower u-factors. Types like the 6J5 and the 6SN7 have u= 20, and the 6SL7 has a u= 70. Compare to the 12AX7 and its u= 100. Doesn't mean that you can't use a 12AX7, but if you do, you also need to pay attention to the possibility of higher order harmonics, just as you would if using small signal pentodes.
