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There is a bumper sticker that reads:
Audio Rule #1:
Use a Transistor
Go To Jail
It's The Law*
I own this bumper sticker, but not necessarily the viewpoint. An apostate to most one-and-only true paths, my viewpoint is closer to "whatever works, works." Yes, transistors and tubes can go together, if care is taken in the design. Thus, this month issue covers hybrid designs. Usually, the word "hybrid" evokes memories of tube front ends driving MOSFET output stages. But those designs past observed a sharp dividing point where the tubes left off and the solid-state began. In this issue we will erase that dividing line, mixing the two technologies in the same stage.
Remember, if you have a request or suggestion of your own for either an article topic or circuit, please e-mail:
Editor
* Copyright: Vacuum Tube Valley
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In general, like oil and water, transistors and high voltage do not go together well. High voltage is were tubes shine, as a tube will easily absorb an ultra high voltage pulse and keep on working, whereas in the presence of high voltage, solid-state devices seem delicate and fragile in comparison. Although recent advances in applying safety circuitry internal to solid-state devices have made them more robust, as little as a shuffle of leather shoes on a nylon carpet is enough to blow out many ICs and MOSFETs. So maybe the better cliché is, like a fool and his money, transistors and high voltages will be parted. Unless, that is, we take many precautions.
I have built many solid-state high voltage regulators, current sources, and DC servos that have worked perfectly up to the very minute one of the devices blew and took most of the circuit with it. (DC coupling has its advantages and its disadvantages and cascading catastrophes is one of them.) One result that was borne out of the electrical autopsy was that generic, low performance circuits were much more robust than the more expensive, tweaky, high performance versions. Do not fool yourself into thinking that this is somehow analogous to a race car versus a family station wagon, wherein the race car is actually much more reliable than the family car, but as it is required to work at the increased limit of reliability, it breaks down more often. The tweak circuit is not asked to do any more work (energy over time) than the stodgy circuit, both only needed to power a preamp or power amplifier, yet the tweaky, high performance version fails more readily.
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