Electronics > Projects, Designs, and Technical Stuff
Would it be possible to make a solid state vacuum tube?
magic:
I'm not very familiar with "toobs" and their characteristics, but IIRC whole point of tetrodes and pentodes was that they were supposed to have high output impedance. So you could cobble together some small signal circuit which generates the desired I/V curve by whatever means and cascode it with any convenient HV FET to get the desired "plate" voltage rating and high output impedance.
SK_Caterpilar_SK:
I dont think you really got that right.. Pentodes and tetrodes can both have high impedance outputs or low impedance just as triodes. 12AX7 for example is a high impedance output tube, but it has a even higher impedance input. A 6C33 is a very low impedance triod and so the sam applies to the input side.
The pentodes are here so we can controll with a very high input impedance a lower output impedance. EL34 has a high input impedance but the load impedance can be as low as 3kohm while its driven from a 100kohm input.
And you have many flavors and colors and shapes of tubes.. the big point of pentodes is their ampliffication factor. Its A LOT.
On the secondary thought it would be nice if I could put together something that could generate that I/V curve... I see it more as a V to I converter tho. I just dont really know where to start or what to look for
TimFox:
Triodes vs. tetrodes/pentodes:
All other things being equal (which they never are), the triode tries to be a constant-voltage device (for AC), with a relatively low plate resistance (output impedance), since both the plate and grid voltages affect the field at the cathode.
The added screen grid in a tetrode or pentode greatly reduces the effect of the plate voltage on the cathode field, with a constant voltage on the screen, thus making the plate current less dependent on plate voltage and greatly increasing the plate resistance. It is somewhat similar to a cascode series connection of two triodes, where the plate voltage on the lower triode (emulating a screen grid in the tetrode) is constant and its current flows through the upper triode (without much effect of the upper triode's voltage). Of course, there is current through the screen grid in the tetrode, but not in the grid of the upper triode in the cascode.
It is common to convert a tetrode/pentode to a triode by connecting the screen to the plate (preferably through a small resistor), since there are more power pentodes out there than power triodes.
The value quoted for an EL34 is not the output impedance, which is high, it is the recommended load impedance, which is much lower than the plate resistance for power pentodes. There is a rule of thumb for power triodes that the load impedance should be twice the plate resistance.
magic:
By output impedance I mean output impedance of the tube itself, i.e. how much extra anode voltage it takes to force additional current into the tube, i.e. the slope of the anode curve. I found a datasheet of ECL86 and it's pretty flat. The curves look quite similar to those of JFETs/MOSFETs but with less transconductance and higher grid voltages. It's probably a matter of a few resistors to adapt a FET to do the same job.
Maybe post the circuit you would like to plug it into?
TimFox:
Yes. Triode curves of Ip vs Vp at constant grid voltage slope upwards with a reasonable slope, while pentode curves of Ip vs Vp at constant grid voltages and a specified screen voltage are horizontal (high resistance) at higher voltages. There is a subtle difference between pentode curves and FET curves at the far left, however, where the pentode curves start from a common rising curve, while the FET curves come separately from the zero point (the effective resistance of the individual curves in that region depends on gate voltage).
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