Triode Pass Devices
   Triodes or triode-connected pentodes/beam tetrodes are the most common vacuum tube pass devices. They are simple to use, rugged, and are often available in dual packages. Their electrical characteristics are less than ideal, though. The main problem is shown in fig. 4. This is approximately the triode-connected characteristics of the pentode shown in fig. 2. Instead of pushing up towards the upper left corner, the Eg = 0 curve bends gently to the right, leaving points G and H completely uncovered. If the regulator tried to handle this condition, the triode grid would try to go positive, but the driver would give up. The voltage across the tube would exceed the available headroom and the output voltage would drop out of

regulation.
   A solution to this problem is to use a triode with lower plate resistance -- really just a triode that can pass more current for a given grid voltage. Fig. 5 shows such a triode.
   All voltage and current combinations can be handled. The down-side of these low-rp triodes is that they tend also to be

is that a floating screen supply is usually needed, which complicates the circuit. The other is that at low plate voltage and high plate current, the screen current starts to rise dramatically. If the pentode sat at point G for any length of time, the screen dissipation could easily be exceeded -- leading to bright yellow glowing screen grids and eventual failure. If point G is reached for just short periods of time (a low duty-cycle) then the heat doesn't have a chance to build up and operation is practical. This is precisely the situation for TV horizontal sweep amplifiers and Class-C RF amplifiers. In these applications, beam tetrodes and pentodes are very effective.
    In summary, bipolar transistors and MOSFETs work well as pass devices as long as maximum ratings are carefully observed, transients are avoided, and adequate drive is available. Pentodes can be used if the maximum screen dissipation rating under worst-case conditions is observed.

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