[Radio Wrangler]

Ah, that's good, and tells it like it is. Designing a sure-fire circuit with an FET in it that will work properly with any in-spec FET in it is one heck of an achievement.

Sometimes you see mention that the FET may require selection. The danger here is if you buy a number of FETs at the same time, they likely all came out of the same batch!

For a preamp with a guitar pick-up feeding it, the source impedance will be a better match for a typical bipolar transistor than for a typical FET, so the FET is going to be noisier than can be achieved with plain old bipolars. The same goes for magnetic record-playing cartridges.

It's often said that FETs 'have a valve-like characteristic' and this gives people a warm and fuzzy feeling. So they get used in places sometimes to make a marketing point, sometimes because constructors think they feel less culture shock. If the former, then if overload characteristics are important, as in overdriven guitar situations, then the benefit gets shot down by the variability and not all units will have the 'good' sound. So purchasers and users have a hard time finding a good unit, and manufacturers don't get much fun either. Reputations suffer, and the pricing of the good units sold has to carry the costs of making and rejecting the bad units somewhere along the line.

Working for a manufacturer there was a certain horror felt when a design needed an SOT (Select-On-Test) part, and you had lots in stores, all from a batch you discover to be unsuitable.

I came across a case where hundred-quid crystal oscillator units were averaging only one out of ten parts good. some months. PCB pads were getting worn out by the production line folk frantically changing oscillators until a noise problem stopped. A spec change to the oscillator fixed the issue. It pushed the problem back to the quartz manufacturer who changed the design and things were dramatically improved.

So, it's easy to fall into the "They're like valves" mindset, but the variability of FETs is nothing like the dependability of parameters you get with valves. So there is a much bigger learning curve than many people think.

David