Space Charge and Low-Voltage Tubes

Posted on Feb 5, 2014

The best way to get a sense for space-charge tubes is to study some real circuits. Look at the online circuits listed below, and see if you (or one of your friends) have any of the older print articles listed after the online articles. That done, choose a simple circuit for a `learning project` and just have at it! That said, here are some tips: B

Space Charge and Low-Voltage Tubes
Click here to download the full size of the above Circuit.

e aware that these tubes (with the possible exception of the type 8056 nuvistor) draw a lotof current at 12V, sometimes almost half an amp. (Virtually all of this goes into heating the cathode. ) If you put three or four in a circuit, you had better be able to supply 12V at two or three amps. This is easy in a car; not always so easy on a test bench. I don`t recommend making up space-charge portable radios using AA cells! Avoid grid confusion! The first grid (the grid physically closest to the cathode; what in conventional tubing is called the control grid) should be tied to 12V. If you`re using a conventional tetrode or pentode in space-charge mode, make sure you put 12V on the control grid, and use the screen grid as the control grid. Pentode suppressor grids are almost always tied internally to the tube cathode, but if the suppressor grid comes out separately to a pin, try putting 12V on the suppressor as well. This may allow greater gain in some circuits, as you have two grids accelerating electrons toward the weakly charged plate. Biasing is not anything like biasing when your B+ is in the 150-200V range. Don`t just drop a 12V tetrode or pentode into a circuit designed for a 180V tubes. Try to find some of the articles I list below and study the circuits. It`s not that they`re more complex. (Generally, you`ll find them simpler!) They`re just different. If you possibly can, find a copy of an early 1960`s RCA receiving...

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